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Idle problem


jonlabree

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My engine speed does not come like it should down when I let up on the gas. Even when I pull up to a stop the idle stays at

around 1200 to 1500. When I completely stop it drops to 750-800 rpm. If I am running say at 50mph and let up on the throttle

and shift to neutral the RPM stay high. I am at a loss trying to figure it out.

Any ideas Please?

Jon..

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My engine speed does not come like it should down when I let up on the gas. Even when I pull up to a stop the idle stays at

around 1200 to 1500. When I completely stop it drops to 750-800 rpm. If I am running say at 50mph and let up on the throttle

and shift to neutral the RPM stay high. I am at a loss trying to figure it out.

Any ideas Please?

Jon..

Does this occur all the time or only from the time you start the car (cold) until the car is running at normal temperature (190*)?

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had same problem last year-----replaced MAF,IAC,had throttle cables pulled and cleaned----problem persisted------then replaced TPS and that did the trick----good luck.

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a) can monitor the tps on the dash - should be .38-.42 at idle

B) DFCO - decelleration fuel cut off - keeps some fuel going to the engine when you take your foot off the gas at speed to prevent a burp of HC.

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I would guess this is a new issue, since you know your car inside and out. It sounds like a small vacuum leak except for the normal idle completely stopped? I haven't driven mine for a while but I think it always remained well over 1000 rpm when decelerating until stopped. I think I remember the S/C engine have a bypass of some kind to unload the S/C while cruising. Maybe this is hung open? The others sensors can be monitored to see if something stands out.

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Guest Corvanti

i had the same problem on mine a few weeks ago. i cleaned the MAF & Idle Air Control with the proper cleaners. also cleaned the butterfly valve. that helped a lot - it brings the idle down to normal, but takes a bit longer than it should. next up is the TPS and (gasp!) checking the electronic codes.:)

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Well, I fixed it. I don't know what I did but it's fixed.

I took all of the sensors out of the throttle body and cleaned then including

the holes they came out of and low and behold the idle is back at 800 rpm.

It idles a little faster because of the cam.

Thanks for all the ideas guys.

Jon..

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Guest Corvanti

thanks for the update "63..."!:)

i did some more intake/sensors cleaning today and mine is now back to "normal" idle.:cool:

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My idle is now normal, but I still have the original problem. Which is, Slowing down to a stop the RPMs stay high even if I take it out of gear.

Only After I come to a complete stop the RPM come down to idle. Anyone, Please it is eating up my brakes.

Jon..

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Guest Mc_Reatta

Have you used diagnostics to monitor values of the TPS and MAF sensors to see if they return quickly to idle readings when you let off the gas or also seem sluggish?

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  • 3 months later...

I have made the following observations with my car. 1 after the car reaches operating temp if I come to a complete stop two or three times tne idle will drop in a normal fashion. 2 If I drive no matter how many miles with out a stop when I do come to a complete stop the idle will be high. I am frustrated by this problem and I hate just throwing parts at it. I think I am going to try a new VSS sending unit first when we get a warm day again.

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I have made the following observations with my car. 1 after the car reaches operating temp if I come to a complete stop two or three times tne idle will drop in a normal fashion. 2 If I drive no matter how many miles with out a stop when I do come to a complete stop the idle will be high. I am frustrated by this problem and I hate just throwing parts at it. I think I am going to try a new VSS sending unit first when we get a warm day again.

I have not read the whole thread but if your speedometer is working then there is no reason to replace the VSS, as it only has one job to do. Everything else other than sending a signal is done in the ecm.

I would look at vacuum lines. If you use your cruise on your longer drives, I would start there. If you have swapped to a vacuum assisted booster then that would be an other (or main) area I would look. Last idea that comes to mind is the vacuum modulator and its hoses.

How fast is the high idle and does it drop off under any circumstances?

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The idle stays around 1100 and takes a long time to come down to 625-650. This only happens until I have come to a complete stop a few times than is normal. I have done the intake manifold gaskets, replaced all vac lines, pvc and gromit and hose. The cruise works fine. My neutral safety sw has a vac line if that makes any difference.

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When it's idling high write down the reading for ED22 (IAC). Then do it again when the idle comes down to normal. Then compare the readings.

If ED22 changes when the idle speed drops you will know the ECM is commanding the high idle for some reason. If not the high idle speed is caused by something besides the engine controls... usually a vacuum leak. I would cap off the vacuum ports on the intake as a test to see if it makes a difference. Drive easy with the modulator valve vacuum line plugged. It will make the transmission shift really hard.

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I think that this has already been mentioned: I have an '88 98 with the 3800. I have noticed at highway speed, when I let off the throttle, it seems to coast exceptionally well. I have guage pack so I got to watching the tach and it never returned to base idle until I stopped. To test it, I would slip it into Neutral at speed and the tach still hung at fast idle, until I stopped and quit registering speed from the VSS. I surmised that the ECM was commanding fueling so I quit worrying about it. It NEVER required unnecessary braking or seemed to pull against braking. I never got the feeling that I was throttling and braking at the same time. Just my observations, I don't know if that info is of any use troubleshooting except that the ECM does command some fueling on decel when there is road speed input to the ECM.

PS: The vss has been mentioned and probably discounted as not plausible BUT, I should mention here that I have seen VSS sensors that had metal shavings or cast iron powder stuck to the magnet sometimes cause problems, as well as vibrations causing a "dirty" signal and sometimes a signal while not moving. These instances were observed on heavy duty diesel trucks, not on Oldsmobiles. The point is, with electronics, I have learned not to disregard anything out of hand just because it did not make sense. Testing with another VSS would be easy enough when nothing else has done any good.

Edited by TexasJohn55 (see edit history)
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Well, I fixed it. I don't know what I did but it's fixed.

I took all of the sensors out of the throttle body and cleaned then including

the holes they came out of and low and behold the idle is back at 800 rpm.

It idles a little faster because of the cam.

Thanks for all the ideas guys.

Jon..

Jon, if the idle speed is ECM controlled, how does a cam change make any difference?

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There are a number of "maps" in the computer for controls, several relate to the idle speed and one of the sensors to monitor when acting up is the coolant temp - nomarl operation is over 146F for some, 158F for others.

High idle all of the time is usually a small vaccuun leak. High idle that comes down slowly is often a dirty IAC (one of the "sensors" on the throttle body. Unfortunately the 3800 is a MAF rather than a MAP engine so there is no manifold vacuum data point though you could use a vacuum gauge. A small leak will affect the O2 reading and the computer might kick up the injector pulse width (something else good to monitor) to match and this would raise the idle.

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  • 3 weeks later...
Guest Corvanti

Ronnie, my similar problem was a dirty MAF, but with the colder weather - i'd also check for brittle/cracked/loosened vacuum lines. just a "WAG".:)

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Thanks for the tips Kerry. After a little investigation here is what I've found.

1. There are no codes stored in diagnostics.

2. The ECM is commanding the IAC to keep the idle up when coming to a stop. ED22 shows the IAC is at 65 when coming to a stop and then it slowly drops as the idle drops while sitting still.

3. ED04 (coolant temp) is 71c* (`159f*) going down the road in less 40* weather. I have a 160* thermostat so that is about what I expected. The Gauges screen also shows about 159f*.

4. Diagnostics shows the green "LOOP" triangle (Closed Loop) is going on and off randomly and the green triangle for the Oxygen Sensor is going on and off at the same time which is expected.

Although I've not confirmed it yet, I think the cold weather is keeping my engine cold enough that the ECM is seeing the engine as not being warmed up enough to stay in closed loop. That would account for it keeping the idle high by commanding the IAC to stay open more. The ECM is thinking the engine is still cold and is controlling the engine accordingly based on data tables instead of feedback from the oxygen sensor. The engine is staying in closed loop only about 30-40 percent of the time. The borderline for the engine being warm enough to go into closed loop must be about 159*.

More to come later after a little more testing.

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I have replaced all the vac lines did the intake manifold gaskets, tried 3 different mafs, replaced air and water sensors. I think it maybe the ECM or something in the trans (which has been serviced) all the ecm tests were normal. This has been very exasperating and I am at a loss as to what else to check. My temp gauge goes from 197 to 207/208 and falls back. At 60 mph I put it in neutral It takes for ever for the revs to fall, normal idle is 625/650. I hope a smarter brain than mine can figure this out.

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Have you looked at the diagnostic screen while driving to see if is indicating your engine is going into closed loop operation and if the oxygen sensor is cycling on and off? Your temperature shouldn't be the problem but maybe something else is preventing closed loop operation. You should also look at ED22 to see if IAC reading is high when the engine is idling too fast.

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School me here. I thought that the O2 sensor determined closed loop operation and that engine temp affected idle rpm and inj pulse width when cold. Can temps below 159 kick system into open loop? I have noticed that temp on my '88 below appx 145 will disable lockup. If the system is switching back and forth into closed loop with the O2 sensor maybe it has an intermittent signal? Of course I would think it would throw a code. Hmmmmm....

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I'm not qualified to do any schooling. I don't know enough about the logic written into the ECM prom for open or closed loop operation for that. As I've said many times before, there are many Buick Reatta experts on this forum but I'm not one of them. I just know enough to get by

The way I have to troubleshoot these types of problems is by forming a hypothesis about what the cause of the problem might be and then through trial and error try to prove the hypothesis right or wrong. If I'm wrong I move on to something else.

I'm not certain that 159* is the magic number for going into closed loop but I believe I remember Padgett suggesting that it was in one of his posts. I do believe around 147* degrees is the temperature the TCC goes into lockup because my TCC is working as it should even when the engine isn't in closed loop.

What I intend to do to see if the temp being low is causing my high idle is to let my car idle in the garage until the temp gets up to about 180* degrees and then drive around to see if the problem with the fast idle disappears. Then drive around enough for the temp to drop back down to 159* and see if the problem returns. I may get time to do that tonight. Right now we have company over.

Edited by Ronnie (see edit history)
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Thanks for the tips Kerry. After a little investigation here is what I've found.

1. There are no codes stored in diagnostics.

2. The ECM is commanding the IAC to keep the idle up when coming to a stop. ED22 shows the IAC is at 65 when coming to a stop and then it slowly drops as the idle drops while sitting still.

3. ED04 (coolant temp) is 71c* (`159f*) going down the road in less 40* weather. I have a 160* thermostat so that is about what I expected. The Gauges screen also shows about 159f*.

4. Diagnostics shows the green "LOOP" triangle (Closed Loop) is going on and off randomly and the green triangle for the Oxygen Sensor is going on and off at the same time which is expected.

Although I've not confirmed it yet, I think the cold weather is keeping my engine cold enough that the ECM is seeing the engine as not being warmed up enough to stay in closed loop. That would account for it keeping the idle high by commanding the IAC to stay open more. The ECM is thinking the engine is still cold and is controlling the engine accordingly based on data tables instead of feedback from the oxygen sensor. The engine is staying in closed loop only about 30-40 percent of the time. The borderline for the engine being warm enough to go into closed loop must be about 159*.

More to come later after a little more testing.

... What I intend to do to see if the temp being low is causing my high idle is to let my car idle in the garage until the temp gets up to about 180* degrees and then drive around to see if the problem with the fast idle disappears. Then drive around enough for the temp to drop back down to 159* and see if the problem returns. ...
OK, I tested as described above and low coolant temperature is what is keeping my engine from going into closed loop. When the ECM is not is closed loop is has the high idle rpm when stopping. As soon as the ECM goes into closed loop the high idle problem goes away and the idle is normal. Really there isn't a problem. The ECM is seeing the engine as not being fully warmed up and is adjusting the engine accordingly.

Here is what I found by driving around in a parking lot at slow speeds to keep the engine from cooling off too much. If the coolant temperature drops below 71c the ECM drops out of closed loop. Above 73c it stays in closed loop. The crossover from open loop to closed loop occurs somewhere between 72.1c and 72.9 because when the reading is 72c (no decimal is displayed in diagnostics) it can be in open or closed loop.

Something must have changed with my thermostat since last year. It is a 160* thermostat but I don't recall the coolant temp getting below 165* last year when it was cold. The only other explanation for the coolant temperature going down to 159* this year is because I added a transmission cooler to my car over the summer. Maybe it has added to the overall capacity of the cooling system.

The fix for my high idle is to install a 180* thermostat om the winter but I probably won't do that. I don't drive my car enough in the winter to justify going to that much trouble. At least now I know what is causing it.

I hope this my experience with the high idle will help someone else who is having a similar problem.

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You have reduced the heat load on the cooling system, that explains the cooler temps. To each his own but that engine is running too cold, needs the factory stat put in if the cooling system is good.

When the heater core flow becomes restricted from old age rust and scale, you will need that extra heat when it is 20* outside.;)

Edited by TexasJohn55 (see edit history)
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