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63viking

Idle problem

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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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Have you guys found the cause for the high idle when stopping?

My '88 started doing the same thing today. Coming to a stop the rpm stays about 1100 and slowly comes down to about 700 while sitting still.

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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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Thanks for your advice John. I agree that it is running to cold for driving in the cold weather we currently are having here.

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