Does anyone know why my 90 sse vin c decelerates at 1500rpm and won't
come down until you stop the car?
This only happens during deceleration while the car is cold.
It idles down fine while the car is parked in driveway worming up.
After it reaches 195 deg. and you stop the car and let it idle down it works fine?
Change / swapped most sensors except cam. changed purge and vss.
talked to dealer drivability guys and no luck.
Could it be something in the tranny electronics?
Thanks

Sounds like the PROM program to me. There is subroutine for Decelleration Fuel Cut Off (DFCO) that is designed to keep a puff of HC from coming out the tailpipe and the car from shaking if in TCC lockup when you take your foot off the gas. It is not supposed to be noticable.

One of the things I did was install a used ECM from an olds with my prom and
then the olds prom. They both did the same thing.
What is so baffling to me is the car somehow resets once reaching 195 deg.
and you stop the car. Then all works fine.
In other words if you get on the highway cold it will hold 1500rpm like cruise
control on deceleration for as long as the car is moving. Get off an exit. Stop the car
and let idle down to 750 or so then everything is ok. Idles down fine on deceleration.
I guess computers are great until you have a mysterious problem.
Thanks

Just a long shot but check the gear switch connector on the transmission. About a year ago a freind and I converted a "C" 3800/440T4 to a manual trans (Getrag) which does not have the gear swich connector. Until I blew a new PROM that had the DFCO dialed out, it would do the same thing, hold the idle way up until it came to a complete stop.

Thats what I'm thinking. Its something to do with the tranny connections as
it only does it while the car is rolling. Thats the only thing I did'nt check so far.
I'll disconnect batt and unplug connector and check the terminals.
Any way to check the switch?

On OTC 2000 scan tool can tell you if in p/n or a gear and if the TCC is locked. Think it will also show what gear you are in.

Yah I've been looking for one, they are expensive.
Would save a lot of guess work though.

Can find OTC 2000s on eBay for under U$50 if you are patient. Not worth much to a professional because the last update was for 1993 models & was obsoleted by the 4000. Just make certain you get a GM cartridge covering your year (1993 3in1 is the most desirable) and at least the GM two row cable adapter. There is even a cable available (but rarely seen) to plug the 2000 into a PC.

Would have to check but think the 88 cartridge I use for Fieros worked in our 90 Bonneville (C engine had same data stream as in 88 )

I tape the adapter connectors together so it won't get lost.

Pickings are slim right now but have seen 8-10 at a time.
http://search.ebay.com/search/search.dll?query=otc+2000

I can think of two things....1 when your engine is cold it idles a bit high so that the battery is sure to get recharged after being drained from starting, ( a temperature sensitive voltage regulator also helps to do that). OTOH when you say "until the car stops", that reminds me of the way my engine behaves while driving. Saying your going 65 and decellerate moderately for an offramp....the way i understand it to work is that (keep in mind an automatic transmission is ALWAYS connected to the engine while in gear) you hit the brakes, the TCC unlocks, and the comp is controlling the idle of course but only drops it a few hundred rpms such that if you were to hit the gas again and accelerate there wont be such a thump or jolt to the drivetrain and car, making for a smoother performance. Only when you get down to <5mph or so does your car go to its lowest programmed idle 700-900rpm at operating temp for most. That'* my .02 but this is what ive observerd, and my bonny pulls like a motha and shifts like a dream.....