90 Coupe Overheats

tjenkins · July 17, 2001

Status as of today... Dealer indicates he's 'perplexed' and can't find anything wrong. Says water pump is good, with good output, and no orifice restrictions. Anyone have any ideas?Note details:Initial: Over last 3 years, temperature gauge has been solid straight up (84F-99F) during both cold and hot weather. Over last month, temperature rises steadily. Dealer can't find anything wrong.Symptom: Measured ED04 computer readout, coolant temperature rises normally to ~90C/195F, then, either after 30 minutes or in hot(er) weather, the coolant temperature slowly but steadily rises to over 119C/246F (a differential of nearly 30C/50F). After >10 minutes on freeway, temperature slowly drops back to ~88C.Background: 1990 Buick Reatta (same as Riviera), 3800 engine, bought used at dealer in 1997 with 33K, presently 77K. Driven daily with AC on all the time. Routinely flushed and changed the coolant with Prestone every year. Water pump changed on 16 June 1998, at 41,869 miles.Over these last three years, my analog temperature gauge has quickly warmed up and has always pointed straight up at 12 oclock (9 bars, with 4 on the left, one straight up, and 4 on the right). The temp gauge has ALWAYS exhibited this straight up condition during times of cooler as well as hotter weather, either on the highway or idling for long periods in town. Based on measure temperatures, a straight up analog condition ranges from 84C to 99C - well within the 90C +/-5C range.I watch the instrument panel all the time.Details: Since early May, while at a stop light downtown after running for 45 minutes, the temp gauge climbed to 1, then, 2 oclock.I changed the existing thermostat (which I assumed was an OEM 90C/195F but cannot now confirm that) with an after market 90C/195F NAPA unit, but no change. Upon starting up in the morning to go to work, it quickly rises up to normal 12 oclock position, and stays there until 30 minutes later when I pull into the parking lot. However, in the afternoon, when the outside temperature is higher, upon arriving back home from both highway and city driving, the gauge reads 1 bar to the right at 1 oclock, then, 2 bars, etc.Entering in the on-board computer readout (ED04), the temp bounced from 88C-93C or so during the first 30 minutes, then, in the afternoon going back home, the temp would rise to 95C, 100C, 105C and then, up to 119C. Fans have not gone to high-speed mode, since the temperature apparently didn't exceed a predetermined value.This is/was a change in the normal operation for this particular car.18 May 2001 - Brought into dealer: R/R Radiator cap R/R coolant R/R coolant sensor Checked/swapped the ECM and BCM computers Indicated that the Instrument Panel Cluster was bad (reading too high). I bought a used IPC from PA and had them send it to me. I installed the IPC and it provided the same temperature gauge readout as the original IPC. Wanting to prove the coolant sensor/ECM/BCM/IPC combo were good, I went to Radio Shack and bought a 5K ohm potentiometer. Warming up the car, I removed the plug on the coolant sensor, and took several measurements, substituting the potentiometer for the coolant sensor. Readings confirmed factory settings and values: Temp Ohms 85C 261 90C 233 95C 197 100C 178 105C 164 110C 126Bar 'switch on analog gauge': 84C 12 oclock 99C 1 bar (1PM) 102C 2 bars (2PM) 105C 3 bars (3PM) 109C 4 bars (4PM) It does appear that the coolant sensor through both computers to the IPC is good. Subsequent trip to Kuroda Radiator confirms correct coolant temperature with infrared pyrometer aimed at the coolant elbow on engine block. See attached spreadsheet.8 June 2001 - Thinking that the radiator was bad, or blocked, I had a radiator shop R/R the complete radiator assembly. Although the original aluminum core radiator was not plugged, it did have some cracks in the plastic tanks on the side. Good preventive maintenance I assume, but did not change the situation.12 June 2001 - Changed the NAPA 90C/195F thermostat with a Stant Premium 82C/180F non-linear thermostat. Currently, the car quickly warms up to ~85C, then again, in warm weather slowly climbs to 109. Again, this is a temperature differential of nearly 30C.Parts confirmed good by either replacement or testing: Radiator Radiator Cap Coolant sensor Thermostat Both pusher and puller fans (confirmed high speed when manually jumpering relay) BCM/ECM/IPC combination Wiring13 July 2001 - brought into dealer, who says they confirmed no abnormalities. Coolant good, water pump pressure good, and can't find any restriction. Dealer is "perplexed".

Reatta Man · July 17, 2001

Here's a few more places to look:1. Have the coolant tested for exhaust gas, or do a compression test. Possible SMALL head gasket leak could allow super-heated gasses to enter the coolant passage in small quantities, raising the temp. 2. Scan engine, checking for an ECM that is advancing the timing way too far. Possible bad knock sensor or bad ECM. A bad O2 sensor could be telling the ECM it is running rich, causing an extremly lean condition, which would raise the temp. On the older cars with a distributor and vacuum advance, the timing being too far advanced would show up when the car would crank very slowly when hot. On newer cars, a bad sensor could cause the ECM to advance the timing under certain conditions, but not all the time. If you don't want to have it scanned, try swapping out the ECM with one from a junkyard or a rebuilt unit; see if the condition changes. Anyone else have any suggestions?

July 17, 2001

have the cooling system checked for exhaust gasses, which could indicate a bad head gasket. I have a slant 6 which had a bad head gasket and displayed the same symptoms you describe. Changed the gasket and the problem went away.

padgett · July 17, 2001

I have three vehicles with essentially the same drivetrain - 88 Reatta, 90 Bonneville, 92 TranSport. Of the three, only the Reatta exhibits a similar low speed climb: sits on 179-183 (180 thermostat) is under 85F or over 45 MPH. In heavy traffic over 90F will slowly climb to 197-203 depending on how long in heavy traffic.My V8 experience leads me to think that a good engine/cooling system will not run more than 100F over ambient, have crossed the Monteagle pass in Tenn. In a station wagon with 400 cid V8 pulling a tandem trailer with a 3800 lb car on it in mid-July and never broke 200F.Have found that the 3800 is usually a very cool running engine so the excursions should not be happening. Have sat in wife's car for 45 minutes with ac on and themp never budges.Now at this point the only real answer is to instrument several cars both with temp rise probelem and not with something that can capture all of the sensors in a sequential manner (DIACOM software which I have can do this), make a few runs and study the results. (e.g. real engineering tests).My problem is that while I have the equipment, am on almost constant travel (in Calif at the moment) and have a problem putting the time together.Meanwhile we need to figure out a way to do something the factory does not make easy - measure the initial timing setting. Factory uses the cam sensor to do this, I do not even know if there is a mark on the balancer or flywheel.Do not know if needed but is an important data point.Then we compare the readings from several cars in a similar condition (unfortunately we cannot get to the CRT diagnostics with the a/c on but may be able to jumper the fan relay.Secondary problem is that I am sure that when the ALDL connector is pulled to connect a recoding device, the BCM will go bananas so have no idea if readouts are possible.Meanwhile I hope the set of 88 tech notes I ordered will be at home when I get back - the fan system on the Reatta only was changed in 1989 to turn both fans on at low speed while my 90 Bonnie still has the ballast resistor and one fan on like the 88 Reatta. That was done for a reason.

ductune · July 17, 2001

Check the AC condenser for air flow restriction. Stone damage or other debris will make a difference in air flow to the radiator. My Reatta was running hot in heavy traffic last summer, but now is fine after replacing the condenser.

July 17, 2001

...i know this probably won't help...but food for thought. i know you replaced the thermostat...first NAPA then Stant. The last time i had a car respond like that was when i completely removed the thermostat. First normal for a period of time then a slow rise to over heating. It would't hurt to check and make sure the thermostat is seated properly with the spring end down (even though the manual pictorial shows otherwise). Maybe even drop it in a pot of hot water to test it. If it's positioned in an obscure location ...then remove the parts blocking your visability to make sure it's seated and you've included all the "o" rings and/or gaskets. My gut feeling would be that it's a thermostat blow-by problem. If the coolant flow isn't regulated by the thermostat....the coolant won't dwell in the radiator long enough to be cooled and will return into the engine....hot. i offer this post only because i see you've been very thorough and maybe it's something less complicated. Good luck and hang in there.

grm · July 17, 2001

Check to see if you put the thermostat in up side down. I had a car that the shop put in wrong. Run hotter than normal for years until I changed the thing myself. grm

padgett · July 18, 2001

Know that this will not be easy considering where you live but what happens if you let it get hot (over 100C/210F) and then can drive at 45+ for a mile or two ? Does the temp continue to climb, drop, or stay the same ? Is there a difference between stopped/idling and slow traffic ?If the temp drops at speed and the idle is smooth then the problem almost certainly must be in the fan circuit (anyone have a schematic for a strobe light ?)Know the problem has been going on for a while and is getting frustrating. Just for S&G try instrumenting the DC voltage being applied to the fans (not the relays but at the top of the fan string) at idle and again at 1750 rpm.Am wondering about a resistive circuit or possibly a defective alternator which would show up in this test (however a failing fan would not but said were r&r'd). Low voltage could mean the fans are running slower than spec and not moving enough air to cool.Know mine has some tendancy to do the same thing (but is not as extreme as yours & cools quickly at speed) and have noticed the alt is only putting out 13.3-13.4 vdc.Think we have checked about everything else.

Gary Frank · July 18, 2001

I also have a 90 with the same condition and have replaced the high speed push and pull fan relays with NAPA replacements. I also have purchased but not installed a 180 Deg thermostat. I have not been able to test the results due to the mild weather and lack of time for a joy ride, but I did disassemble the old relays and foud the contacts and enclosure very burnt. I would think this condition would decease voltage and fan speed . I would replace the relays on a maintenance procedure if you have not already done so. I will post the results after the next warm day of leasure driving. Note. Last summer climbing a montain the motor got uncomfortably hot and it has my attention since.

padgett · July 18, 2001

Gary: you meant the LOW speed relays didn't you ? Unless the ECM has been altered, the HIGH speed circuit is not supposed to kick in until around 225-234F (forget exactly where & not at home so can't look-up but remember it is hotter than what we have been seeing). A resistive realy could certainly account for both overheating the relay and slow fan speed.Problem is that I do not know how fast LOW speed is supposed to turn. Do know that the difference is not something you can tell just by putting your hand in the airstream.

Arrallen · July 18, 2001

This problem seems to plague several of us, I just discovered I have the same hot weather raise in temp when I had to use my car for out of town trips. When someone come up with the solution please post it for all of us.

July 18, 2001

.....this sounds silly...but i've poured this stuff into the crankcase called Tufoil. It is a PTFE lubricant along the line of Slick 50. Two bottles of this stuff reduced the temperature of the engine about 10F and greatly improved hi-rev quietness. Please do not share this information with non-Reatta owners. For $8-$16...it could help.

tjenkins · July 18, 2001

Thank you ALL for the various recommendations. It seems that this is not a 'normal' problem in one sense, but rather common for the 3800.In light of that, after having the dealer now re-replacing the thermostat with a factory OEM 195F unit, and the R/R of the lower hose to factory as well, I'm going to watch the temp over the next 2-3 weeks to gather data.The dealer also confirmed good pump pressure as well as non-leaking water pump.In the meantime, I'll R/R the 3 relays for preventive measure. I've also looked at, and am convinced that there is no blockage on/near the A/C condensor coil(s).In short, the vehicle experiences this symptom:In cold/warm weather (for what that's worth in Hawaii smile.gif" border="0 , the temp is 90C/195F +/-10C.In hot weather, the temp slowly jumps to 25-30C higher than the nominal 90C/195F temp. Symptom is exacerbated by in-town and stop light driving, yet drops back to normal with a 5-10 minute 60MPH speed.Confirmed yesterday at the dealer than the FSM does *NOT* indicate when the fans go from low to high, but lead mechanic indicates normal threshold is (get this) 240F!!!I've have yet to see the high speed fan relay kick in.I'll take the above comments to my non-Dealer mechanic and have him concentrate on engine internals (leaking head gasket etc)."You can lead a horse to water, but you can't make him float on his back."--Tom

July 19, 2001

Several owners have been discussing this and other overheat problems by email. Below I will describe the temp patterns of my recent two 89 models and the newly aquired 91. Outside temp 90+. Both 89's have a tendency to run a little higher than their 195* thermostats. One about 197* the other 203/4*. Both would climb to 208* after an interstate run, upon slowing down. I decided to replace the hotter one's thermostat w/ a 180*. Afterwards there was absolutely no change in behavior. Not even a hesitation at 180* Today I pulled the new thermostat, a Stant, sure enough, it read 195*! Mispackaged! I replaced w/ another ( read 180* on bottom) and the temp rises to 185* and immediately returns to 181*. Have not driven on a trip but will soon. The 91, during my 1st. and only drive, climbed to about 3 o'clock momentarily but on the highway and in city traffic, stayed at 12:30.

padgett · July 19, 2001

Hokay folks, home again so ran some checks between 88 Reatta and 90 Bonne. Both code C engine, both have ACC. Both have resistor/one fan on at low. Reatta has slow rise observed by many. Bonne does not budge.Difference #1: Reatta radiator core is 28"x19" (approx). Bonneville is 31x19. 10% larger.Difference #2. Both engine running in park. 92F ambient Reatta (from cold but see no difference hot) a/c off - no fan a/c on - low fan, 8.1vdc on "puller" onlyBonneville coolant at 181-183F a/c off - low fan, 7.95vdc drivers side fan only. a/c on - low fan initially, within 30 seconds fan switches to high & both fans on full temperature 183F steady.Note Bonneville converted to 134A two years ago. Will freeze you out. Do not know what high side pressures are.Note 2: nothing in Bonneville FSM accounts for this behavior.Am tempted to design a circuit to turn both fans on full with a/c on & TCC off & see what happens.

July 20, 2001

HI Tom! Cant believe You are still fighting the overheat. I thought You had it straightened out. I kind of wonder with some others here if You dont have a slight head gasket leak. Napa sells a kit that checks for combustion leaks in the coolant. It is very accurate and very easy to use. You just take your radiator cap off when the engine is cooled of and it has a tester that You put a chemical in that changes color if it detectect combustion gasses, and hold it in the top of the radiator while it runs. We use one at work and have had good results with it. I believe it can be bought for under 50.00 and You can do it yourself. Good luck.

tjenkins · July 20, 2001

Outstanding... this is one of the things that I still need to check.In fact, after 3 days at the dealer, I had asked him about a cylinder head leak. His response "yeah, I was thinking about that".Gosh, if he WAS THINKING ABOUT IT, WHY DIDN'T HE INITIATE IT? Oh well, film at 11.I'm going to R/R the thermostat back to a 82C/180F Stant unit - will ask about the test kit when I get to NAPA.--Tom

July 20, 2001

I tested my 180* thermostat today (95+*'s outside). Temp rose to 181 held, then during slow city driving rose to 185. I then parked, let engine run for some time. 190 was max before temp drop to 188. Then at 45mph the temp dropped back to 185.

July 21, 2001

Oddly enough I was looking through an RV magazine and there was a tech. section that was discussing cooling problems, (always a concern with the loads they carry), and there was one guy that discovered the side of his air conditioning condensor, on the radiator side, was partially blocked by bent fins. He had spent considerable time trying to improve his cooling and discovered this by accident when the radiator was pulled for service. Completely invisible when it's all together. Just a thought. By the way, my '90 has been running straight up, 12 o'clock, with low 90 deg. F. temps. and high humidity, and it will drop back to 11 o'clock at anything over 30 mph. This is with a 160 deg. t'stat. Fans are set to engage low speed at 175 deg.F. with a program change in the chip.

tjenkins · July 21, 2001

Three things:How did you get the PROM reprogrammed to enable both fans (on high speed?) at 175F?I went to NAPA today - sure enough, they offer a $50 kit that can test for exhaust gases via coolant. However, this NAPA also has a motor shop, and for $17, they'll do the same one-time test. I'll let you all know how it turns out.And, someone please confirm 90 fan rotation. My two fans, on slow speed, seem to turn in opposite directions from each other. The blades for both fans do seem to 'pull' and 'push' correctly, but it's odd that both fans turn in different directions???In the meantime, Stant's 82C/180F thermostat is going in tomorrow.--Tom[ 07-20-2001: Message edited by: tjenkins ]

July 21, 2001

Tom,No where in these comments can I find any verification that your high speed fan operation is kicking in (although you jumpered the relay to confirm fans are OK). I only have an 89 and 91 service manual, but both indicate the high speed fans should engage at 108C (226F). What isn't given in the manual is when they turn off.

July 22, 2001

In the 91 manual, the fan diagnostics (and temp info) is in Section 6E3-C12. You will notice that the fans can also be triggered by the A/C high side temp. If the A/C high side temp connector is disconnected, the fanms will function based on coolant temp only (and probably give you an error code about the high side temp). Fans also do not work above 45 MPH. Using your Radio Shack pot you might be able to connect to the coolant sensor connector and test fans, or just let it idle in drive with hood closed.

July 22, 2001

The PROM change engages the low speed, not the high speed. I would think the high speed could be changed as well. I do not know if the high speed engagement is changed or not, the car simply doesn't get that hot. Just tried it today, 92 deg.F. ambient temperature and high humidity, low speed fans come on just as it gets to 12 o'clock and then cools down to 11 o'clock and they shut off. Generally runs at 10-11 o'clock when moving, air conditioning on or off.

July 22, 2001

Oh, I forgot, the fans actually turn the same direction, they simply face opposite directions so the fan blades turn opposite each other when viewed from a single perspective.

tjenkins · July 22, 2001

Ok, so per the 1990 FSM, the fans go from low to high with either condition being met: 1) Coolant temperature exceeds 108C/226F, or, 2) High side AC temp exceeds 65C/147F.Since I run the AC on all the time, I wonder if the fans were at high over the last three years, and somehow, the fans are now on low speed?My AC doesn't seem REALLY cold, but it's no warmer than it used to be...--Tom

tjenkins · July 22, 2001

Outstanding...Stan, please tell me on what page did you find that info?--Tom

July 22, 2001

LeBra just arrived and wit installation, instant 10F increase - was running 197 on freeway. Opened license plate hole up and dropped back to almost normal 183-186 but ambient is only 89F.Was conserned about the much reduced area of the grill but thought maybe was mostly a bottom breather. Wrong. Now am wondering if front end cover is worth it. Thinking about increasing the front air dam size but definately indicates that air flow is the key to cool.

July 22, 2001

Tom, Since it appears you have a service manual. You can use the diagnostic function to monitor A/C high side temp (BCM BD27) or Coolin Fan Relay #1 and #2 (ECM EO05 and EO06). You can also use the ECM Override function to command fans on (ECM ES05 and ES06). In the 91 manual this is all shown in Section 8D-10, Fig. 4 & 5.

padgett · July 22, 2001

Can anyone please provide a coolant fan schematic (really just need the relay part) for a 90 or 91 Reatta) from the FSM ?My '88 just brings the "puller" fan on at low and uses a ballast resistor. In 89 or 90 (think 89 but not sure) the factory changed this so that both fans are in series for LOW and parallel for HIGH. Since I am seeing a mild rise in summer Florida weather, am curious to see if this is a fix. However having only the '88 FSM I can only guess.Just looked in the FSM for my '92 TranSport also with 3800 (code L). Puller comes on full at 212F/100C or with a/c. Pusher comes on at 226F/108C or when a/c pressure reaches 210 psi (lowest I've seen, '88 is @250 and '90 seems to be @275 - any a/c guys know what is typical high side pressure for R-12 @ 95F ambient ? Is 200-250 psi normal or high ?)

tjenkins · July 23, 2001

Good idea on checking the AC high side temp. I'll watch it over this week.Btw, is there a way to manually switch both fans from low to high?--Tom

July 23, 2001

Ground Pin #2 of Relay "C" and Relay "D" (Pin # GC3 at PCM or Lt Grn / Blk wire going to relays). Low speed fan is turned on by grounding Pin #5 of Relay "G" (Pin # GC4 at PCM or Dk Grn wire going to relay).

tjenkins · July 23, 2001

I need confirmation and the location of AC high-pressure temperature switch.Per the 1990 FSM, page 1B1-5, Figure 3, the *HIGH* side temperature switch is located by/at item 3, toward the middle portion of the dual row of AC piping underneath the black plastic cowl on the top of the firewall.My car has 3 sensors and two sizes of piping.The right side sensor (drivers side), attached to the smaller of the two pipes, has BLUE and BLACK wires out of the connector.The middle sensor is connected to the larger diameter pipe, and has WHITE and BROWN wires from the connector.The left hand side is also connected to the smaller pipe, and (hard to see) has black and white (?) wiring.Could someone confirm that the FSM is wrong? Is the larger pipe the high side of the AC system? If so, then the above and manual do not match. If the small size, then my car matches.Note page 8A-31-2 for wire colors.In fact, I opened the right hand side (drivers side) sensor and checked ED27 - sure enough, it was a -34C (or some close value). When I shorted the sensor with the BLACK and PINK wires, the temp rose to 240C (or close). At this point, both fans turned on to HIGH. However, the sensor was on the smaller of the two pipes.If true, and both opening and shorting of this right hand side sensor (drivers side) causes ED27 to go low and high, with the resultant fan operation changing from low to high, this would account for the stabilization of the motor coolant temperature.In other words, if the above is true, my problem may indeed be a lower-than-normal AC high side temperature. As the temperature increases outside, if the AC high side also does not increase, my fans would never go from low to high in hot weather.I'll watch ED27 over the next week.Again, can someone confirm the exact location (smaller or larger pipe) for the AC high side temperature sensor?Thanks VERY much in advance.--Tom

July 23, 2001

I had exactly this problem with my '90 Reatta coupe and the Buick dealer replaced the entire computer three times which did nothing. It turned out that the engine was not overheating. When the temperature rose a few degrees, the gauge pegged nearly all the way to red hot. Finally after weeks of wrangling, Buick downloaded a new program into my computer while it was in the car. This actually solved the problem!!!

July 23, 2001

Tom, The simpelist way I can tell You which is high and low is the high side will be hot with a/c on and the low side will be cold.

tjenkins · July 23, 2001

Anonymous,Could you pass on the details here? I'm EXTREMELY interested in how and why the dealer (?) downloaded a new program into your computer.Please *REPLY* via here or via email to:tjenkins@hawaii.rr.comThanks in advance.--Tom

July 24, 2001

Tom, After you install the 180* thermostat wonder if results will be similar: Drove the car on the road for the 1st. time w/ 180, today. previously it ran at 203/4 w/ 195. O.A.T. 96* F. Drove through town and car warmed to 183, immediately dropped to 181 and held until I began cruising on 2 lane at 63/68 MPH. At this point it was 185* 10 or so miles later it ran at 190. Temp. varied several degrees either side of 190 as I slowed down a little uphill or coasted downhill. After 25 miles as I stopped for a light it hit 203, recovered as I attained 60MPH. Arriving at the next town required me to do a lot of stop and go, saw 212* once and car ran around 200* in slow traffic. It certainly likes ram air! W/ the 195 it used to see 213( a fan must kick to high at this point on this car, as temp has always recovered at 212/213. I think the new thermostat will work fine this summer and car will run between 188/195 on the interstate. But 32* above the thermostat opening? Guess Buick isn't concerned w/ temps around 215/220.

tjenkins · July 24, 2001

Did you have the A/C on? I am beginning to wonder if in fact the high side temperature sensor doesn't contribute to the low --> high fan speed, albeit without higher than (what you would expect) normal temperatures in the coolant.In other words, even with a non-hot engine (~105C?), with hot ambiant weather outside, the AC high side would rise above the predetermined threshold of 65C and turn the fans from low to high.--Tom

tjenkins · July 24, 2001

Today, with the ambiant temperature around ~92F, the ED04 coolant temperature rose steadily.Sure enough, the AC high side TEMPERATURE reading (BD27) hovered around 30-40C. After 20-30 minutes this afternoon, in traffic, the AC high side temp rose to 64C, one degree shy of the factory low-high fan turn on. I would have let the engine idle to check, but really didn't have the time. At this point, the ED04 temp was at ~109C.To simulate turning the fan(s) on from low to high, I pulled the two wires to the AC high side temperature sensor. Sure enough, the temp rapidly lowered to 93C. I drove with the fans on high for 3 miles, at 10-15 MPH, and the temperature hovered around 93C. So, lesson learned is that if I could enable the low-high fan transition below 109C, or lower than 65C AC high side temperature, my problem would be solved.I'll attempt to repeat this tomorrow, and let the engine idle up to, and hopefully passing, the 65C AC high side temperature. If this circuit is acting normall, I should be able to see the low to high transition at that temperature.I'm also going to verify exhaust/no exhaust gases in the coolant tomorrow.--Tom[ 07-23-2001: Message edited by: tjenkins ]

padgett · July 24, 2001

Tom: if you get a chance, let the car sit for five minutes after turning off when hot and then recodr the BD027 and BD028 (low and high side "temperatures" - is actually a pressure reading fudged to temp) Should be about the same once the system equalizes unless a sensor is failing.GM sensors can gett pretty far out of tolerance before a trouble code is set.

tjenkins · July 24, 2001

Today, I confirmed that there are no exhaust gases in the coolant.Quick and easy test. While warm, the mechanic turned on the engine, and used a fat syringe bulb device, half-filled with blue fluid. Opened up the pressure cap, and sucked in the gases from the green coolant.Since the blue fluid did NOT turn yellow, a rather sure-fire test, apparently I do not have a head gasket leak.I did email, and then call, the main Buick Technical Support team. Spoke to a Rick there, but after I had explained what I had done so far, he had no other suggestions.I also asked him about a flushing methodology, but apparently Buick doesn't recommend anything more than the fluid flush.I'll watch and record the above BD27 and 28 values over the next day or so, however, I'm running out of ideas.An afterthought - can I coerce my AC hight side to go >65C earlier, causing my fans to go from low to high (and then recooling the engine back down)? Why would the high side remain cooler than 65C, even with hot weather outside?--Tom

90 Coupe Overheats

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