OBD I Buick check engine light on 90%

[Guest alleyyooper]

The 1985 buick LeSabre check engine light is on about 90% of the time. it has the OBD I system so you short out the connector to flash a trouble code. 307 Oldsmobile V8 with 4bbl carb.

 

It flashed a code 13 for me which is a O2 Sensor fault, the trouble code chart I found says I should get from 350 to 5 milivolts from thre ecm. I did have that so I bought a new O2 sensor and installed it.

 

Problem didn't go away, still have the code interment. I don't have a trouble tree to see what else is involved I should check.

 

Car seems to run just fine. No running rich smoke out the exhust and no residue on the spark plugs or O2 sensor.

 

Any Ideas?

 

            Al

[61polara]

Many times a O2 Sensor fault code is not a failed sensor, but an engine performance problem, such as the engine running too rich resulting in the check engine light coming on because the sensor is working.  OBD I won't tell you any more.  It's time for old time diagnostics.

[joe_padavano]

The CCC system on those 307 motors uses a pretty stupid ECU.  There is little processing power and few useful telemetry points.  There are MANY failure modes that require good old fashioned troubleshooting instead of just replacing the part that the code points to.  In this case, O2 sensor "failures" can be caused by a number of other system failures that will not show up in ECU telemetry.  I learned this the hard way when a failure in the A.I.R. diverter valve caused fresh air to be blown at the O2 sensor during closed look operation.  This tricked the ECU into thinking the carb was too lean, causing the mixture control solenoid to run full rich and setting all kinds of codes.  This may or may not be your problem, but there are a lot of vacuum hoses and wires on this motor that can cause false positive codes in the ECU.  From what I've seen the Buick factory manuals are not as good at describing the CCC system and troubleshooting as the same year Oldsmobile manuals.  I strongly suggest that you get a 1985 Olds Chassis Service Manual for a Delta 88 and read the CCC section in detail.

[Bloo]

Onca upon a time I worked on CCC systems all day. Listen to joe_padavano. O2 codes are NEVER (well, almost never) the sensor in these cars. Yes, O2 sensors fail. On old systems like this you absolutely must troubleshoot. If you go by code descriptions, you may wind up throwing $600 worth of parts at it with no change. Cars like this used to land in my service bay all the time.

 

An O2 code (or any other code for that matter) means the ECU saw something that it thinks cannot be. For instance, the O2 voltage is stuck either low or high. reporting dead lean or dead rich.

 

The most likely scenario is that the sensor REALLY IS seeing excess oxygen (lean) or extremely low oxygen (rich) and reporting what it sees to the ECU correctly. Joe outlined some possible reasons.

 

P.S. you cannot have any vacuum leaks on this car. Every vacuum hose must fit tightly, with no splits in the ends, looseness, tape, etc. There can be no leaky diaphragms in any of the vacuum systems, This includes not only the engine controls, but also power brakes or anything else that might use vacuum (heater controls, etc.)

 

P.P.S. It is never the ECU. I can count the bad ECUs I have seen on one hand. I have seen a few hundred cars with replaced ECUs, still with the same symptom someone was trying to fix.

Edited April 5, 2018 by Bloo (see edit history)

[Ben Bruce aka First Born]

Bloo, did that system change much by 1990?  Using a '90 Chevy pickup system on my straight eight Buick. Have not had any problems,

 but would be good to know of any potential ones.

 

  Thanks

   Ben

[Larry Schramm]

The fix for the car would be relatively easy if you had a Tech 1 scanner by GM.  The above comments about something else being the problem are correct. 

 

Examples could be a rich fuel system. Rich fuel could be caused by things like a bad spark plug wire, bad plug, carb mis-adjustment, bad coolant system which could tell the computer to run rich, bad/poor connection at the oxygen system which could give the computer a signal that it is cold and the car would run rich.  A bad engine ground could possibly cause the engine issues.

 

If someone replaced the valve cover gaskets and used high silicone gasket sealer it might cause the oxygen sender to become contaminated.  I said might , not would.  Lots of variables.

 

You should get the idea that there are many things that could cause the issue.  The easiest thing is to have a scanner to see what the engine computer values are in real time with the engine running.

 

Where are you located?

Edited April 8, 2018 by Larry Schramm (see edit history)

[Bloo]

On 4/5/2018 at 4:38 PM, Ben Bruce aka First Born said:

Bloo, did that system change much by 1990?  Using a '90 Chevy pickup system on my straight eight Buick. Have not had any problems,

 but would be good to know of any potential ones.

 

  Thanks

   Ben

 

Back when I was working on these, people were not putting them on other engines. They were just starting to reverse engineer and make aftermarket chips for performance use. Most techs who did not specialize in electronic engine control hated the CCC systems and were downright scared of them. looking back that seems silly, but its true.

 

GM kept improving them of course, but it changed surprisingly little. GM's computer controlled carbs actually ran well, so there was not a quick push to get rid of them.

 

The basic plan is this, when the engine warmed up, the system will dump or divert any air injection that might be blowing on the O2 sensor, and look for a signal from the 02 sensor, and if it is there, the system goes into "closed loop"

 

In the carb are some metering rods. Unlike the old vacuum controlled ones, which ran all the way down (leanest) at cruise, these are designed to run halfway down. They are attached to a solenoid. The solenoid is controlled by a "duty cycle" signal from the ECU. The signal is going on and off at a high enough frequency that the solenoid cannot respond at the switching frequency due to its own weight. The result is that the ECU can move the solenoid up or down however much it wants, and just keep it there. For instance, if the signal is on 50 percent of the time, the needles just hang at 50%. On 75% of the time? 75% down, and so on.

 

The signal from a hot 02 sensor is less than 0.5v for excess oxygen (lean) and more than 0.5v for no oxygen (rich). The curve in-between the two is very steep.

 

In closed loop, the duty cycle of the solenoid is normally 50%. The ECU looks at the O2, If it is lean, it richens it up a bit by changing the duty cycle. this causes the 02 sensor signal to change to rich, so the ECU leans up the duty cycle a bit. Back and forth and back and forth and....... it goes on like this forever if it can. The 02 sensor responds quickly because it's curve is so steep. The ECU reacts quickly, and the result is that the engine runs with a 14.7 to 1 (by weight) air fuel ratio, and the variation between rich and lean is so slight that the driver cannot detect it.

 

Back in those days, few people had duty cycle meters, so dwell meters were used. 30 degrees on the 6 cylinder scale is 50% duty cycle. There is a wire hanging out of the harness somewhere to connect a dwell meter. 30 degrees and varying back and forth is normal. Since the solenoid is halfway up, it can go equal distances either way to correct fuel mixture if necessary. The more the system is "correcting", the further away from 30 degrees (50%) the signal will be.

 

At full throttle (low vacuum), the ECU simply takes the signal away from the solenoid. The needles come all the way up (richest position). This provides the function of a "power valve" for full throttle enrichment.

 

(Hint for Ben and anyone else running a non-stock system: Deliberately running the closed loop duty cycle at something other than 50% changes the relationship between the cruise mixture, which is always 14.7 to 1 if the system works, and the full throttle mixture. If the metering rods are not halfway down (as designed) when the system is in closed loop, then the change when the signal goes away will be either more or less than it should be. For instance, if the rods were running a little lower than normal at cruise, the system would have less travel to compensate for a mixture that is too rich, and more travel to compensate for a mixture that is too lean. It would also result in more than normal enrichment when the signal goes away at full throttle)

Air injection leaking or not shutting off? The 02 will report lean, and richen the carb as much as it can trying to compensate. Vacuum leak or misfiring plug? The leftover oxygen will make the 02 sensor report lean, and the ECU will richen to compensate, until it runs out of solenoid travel.  Carburetor running over or choke not open? The 02 will report rich, and the ECU will try to compensate by leaning the carb until the solenoid runs out of travel.

 

All of these things (and many more) will set an O2 code without a bad sensor. Nearly always, a car with an 02 code will also be found to have the dwell (duty cycle) stuck at one end or the other, maybe 6 degrees or 57 degrees, and not moving.

 

On a normal system that is not broken, you should see about 30 degrees (50%) and varying back and forth. You should be able to see it react, for instance if you force the car rich, by maybe pushing the choke plate shut, the dwell (or duty cycle) should change. If you force it lean by pulling a vacuum line off, the dwell (or duty cycle) should go the other way.

 

 

2 hours ago, Larry Schramm said:

The fix for the car would be relatively easy if you had a Tech 1 scanner by GM.  The above comments about something else being the problem are correct. 

 

I agree with Larry Schramm that getting a scan tool attached would help a lot. GMs of this period have "live data", meaning you can look at all the sensor signals, you can see what the ECU sees. This often clues you in quickly to what the trouble is.

 

In addition to the Tech 1, the OTC Monitor 2000 and Monitor 4000 can do this, as well as the Snap-On Scanner (and a few others I have forgotten the names of) , as long as there is a cartridge installed that covers the year the car was made, and a GM ALDL connector.

 

I hope there aren't any mistakes here. It's been a long time....

 

Edited April 8, 2018 by Bloo (see edit history)

[Ben Bruce aka First Born]

Thanks for responding, Bloo.   I actually misinterpreted what you were talking about.  I have an EFI, not CCC. Sorry. I really like it, just would like to KNOW more.

 

  Thanks

  Ben

[joe_padavano]

10 hours ago, Larry Schramm said:

The fix for the car would be relatively easy if you had a Tech 1 scanner by GM. 

 

Sorry, no.  I have an OTC Genisys, which does everything a Tech 1 does (and more).  The CCC system on the 307 just doesn't have enough useful telemetry back to the ECU for it to be useful in troubleshooting. Newer OBD II vehicles have many more telemetry points, so a Genisys or Tech II actually does provide useful info.  Not the case with these limited OBD I systems. The error codes only trigger for very gross problems (ie, a voltage that is supposed to be about 5 v is zero).  That pretty much tells you if you have a broken wire, but if the voltage is incorrect but credible, no error code triggers.  As I pointed out, problems with leaking vacuum lines or failed actuators that are not monitored by the computer will frequently trigger false positive error codes, leading one to replace perfectly good parts in a failed attempt to correct the problem. Unfortunately, "mechanics" today only know how to read the code and replace the part that the computer says to replace.  Any failure that is not caused by that part will be a mystery to them and result in expensive part replacement "troubleshooting".  Of course, eventually even a blind squirrel finds a nut.

[Bloo]

I wouldn't call them "false positives". Generally speaking, if you have a code on one of these cars, something is broken, and probably really broken. You have to troubleshoot. You cannot read a table and replace that part, (well ... you can, but it wont work). I think we all probably agree on this and are just stating it differently.

 

One thing to keep in mind, and NEVER lose sight of, is that a code does not necessarily mean a problem with the electronics or sensors. These cars get all the same vacuum leaks, burned valves, ignition misfires, bad carburetors, etc. that happen on cars with no electronic engine control. Don't get tunnel vision about the electronics.

 

I don't consider the scan tool useless on these either. There may not be many data points, but the ones you see are the same ones the ECU sees and makes it's decisions with. I doubt I ever let one of these cars roll through my bay without plugging in the scan tool.

 

Don't think "oxygen sensor", think "what is wrong with the signal?" too high? too low? not crossing over? Ok, now look at the other values (in the scan tool) that the ECU is looking at. Are any of them screwed up? Which ones? What could cause the readings I am seeing?

 

[Bloo]

5 hours ago, Ben Bruce aka First Born said:

Thanks for responding, Bloo.   I actually misinterpreted what you were talking about.  I have an EFI, not CCC. Sorry. I really like it, just would like to KNOW more.

 

  Thanks

  Ben

 

GM's fuel injection of the time is almost the same system as CCC. You can ignore all my comments above about duty cycle and dwell. You use the same scan tool and the same troubleshooting methods more or less. All the hardware looks about the same. Ignore what the code says it is, and investigate why the code set. It is all inputs and outputs (sensors and actuators), just some different ones. The values can be looked at on the scan tool just like CCC. For instance, to make a decision on how much fuel to inject, once warmed up, the ECU needs either how much air (mass airflow systems) or the Manifold Absolute Pressure (MAP) and the engine speed (speed density systems). These systems move the mixture rich to lean and back quickly in closed loop just like CCC does, they just have to vary the width of the injector pulse slightly to accomplish it.

[joe_padavano]

2 hours ago, Bloo said:

I wouldn't call them "false positives". Generally speaking, if you have a code on one of these cars, something is broken, and probably really broken. You have to troubleshoot. You cannot read a table and replace that part, (well ... you can, but it wont work). I think we all probably agree on this and are just stating it differently.

 

 

Yep.  We do and we are.