'90 thru '96 RWD Broughams --- idle speed interference

[Maynard Krebs]

Because I own two '89 RWD GM sedans w/ EFI (not Cad) with Chevy engines, and both have the same very annoying "artificially slowed return to normal idle speed" for emission purposes, I am wondering if owners of '90 - '92 Broughams with Chevy engines.. have the same problem or similar.   RSVP

What happens.. is that the drive wheels are fighting the brakes when the operator is slowing down or trying to stop the vehicle.   It's frankly dangerous.   I can demonstrate this by coming to a stop sign, putting the car into neutral and noting the idle speed.   If I shut off the engine and quickly re-start, an immediate normal idle speed takes place.   It must be a drop of 350 to 475 RPM!

I do not believe that the problem is caused by the "lock-up torque converter".   No, it's caused by the GM emission control system... based on the idea that most car pollution is caused when under deceleration (high intake manifold vacuum condition).   [For the record, my similar vintage RWD FoMoCo does not exhibit such behavior----it must be a different emission control technology.]

I'm not anti-emission control, nor want 'dirty air'... but I despise this GM monkeying with the idle speed when decelerating.

[Eric DeVirgilis CLC# 8621]

Were the engines changed in your cars? 1989 RWD Brougham came standard with the 307 Olds 4 bbl only. There were no other engine options offered in the Brougham from 1986 - 1989.

To answer your question (for what it's worth), I once had an '89 Brougham whose idle speed increased as the engine warmed. This is not normal operation. I never got it addressed but I was later told it was a carburetor issue.

[TMoore - NTCLC]

I am not an expert on this vintage, but some other, more knowledgeable members will be able to chime in and correct me -

I believe there are some known issues with the electronic torque-converter lockup not releasing during de-acceleration, which might cause the symptoms you are describing - hopefully you will get some confirmation or better information from other members.

[Maynard Krebs]

Thank you, gentlemen.   Both of these '89 RWD GM sedans are Chevrolet Caprices, for the record.   They both have the original 305 'small-block' Chevy V-8.  Interesting is.. that one still has only just under 80,000 miles... and the other has 221,000 miles.... and BOTH exhibit the same "delayed return to normal idle speed interference"---which I say is related to emission controls.

I'm aware of the not-unusual problem of the lock-up torque converters sometimes becoming unwilling to 'release'.   In fact, I had my lower-mileage one repaired for this very problem.. a few years ago.   My high-mileage Caprice, bought from the original owner a few months ago, only rarely exhibits such behavior---and only very briefly after coming off the freeway.   My theory on them as a result... is that they're more likely to get that way if the car "does a lot of sitting".   For the record, my high-mileage one still has the original transmission --- which has never been touched or opened for repair.

So, I'm wondering about owners of Cadillac Broughams with these Chevy 305 V-8s... figuring that they're very likely to have the same emission controls.   If the behavior that I get from mine.. happens to Brougham owners with the same engine also, that would be a significant 'clue' as to the cause.   Of course, what complicates this needed diagnosis.. is the infernal on-board computers.   My GM shop manual is of little help.

Stop at a stop sign, put it in neutral, estimate the idle speed.   Then shut the engine off, re-start, and note the idle speed THEN.   I will not be the least surprised that the idle speeds are different.   [Shut off the climate control for this 'test'.. so that is not a potential "curveball" in diagnosis.]