engine speed heavily-controlled by ECM ?

[Maynard Krebs]

I know that '90 - '96 RWD Cad FWBs have Chevy small-block engines.   My two '89 RWD Chevy Caprices have 305CID Chevy small-blocks... yet said car is borderline unsafe and difficult to drive.   Many times said engine will not slow down upon taking foot off accelerator.   If one comes to a stop sign and places the transmission in neutral 20 feet before said stop sign, the idle speed will be much higher... than if one were to shut the engine off at the stop sign, and re-starting it.   I figure that it must be the infernal ECM.. that messes with the natural engine reduction in RPMs upon deceleration.

Since a fair number of RWD Cadillacs in the early 1990s had the same Chevrolet engines, I wonder if Cad owners of this vintage have a similar 'drivability' problem.   Anyone??

[TJ Hopland]

Which fuel system does the car in question have?   The ECM is rarely at fault, most likely its getting bad info for one reason or another and or is unable to respond properly because of a mechanical condition out of its control. 

If its a carburetor for idle speed they used an external plunger like thing that was a motor with a switch built in.  When it sensed that you had stepped on the gas it would extend out to a high idle position ready to 'catch' the throttle when you released it.  It would then retract as needed to achieve the desired idle speed. 

The TBI and multiport injection systems use an IAC (idle air control) valve that is mounted in the side of the throttle body.  The throttle blades basically close completely and this control valve is a controlled air bypass around the now closed blades.   Similar to the carb version when it senses you are on the gas it opens to high idle and then catches it when you come off so it can smoothly ramp down. 

You could have mechanical issues not letting the throttle close.   You could have a sticky valve or plunger dodad on the carb.   You could have a miss adjusted TPS (throttle position sensor) so the computer doesn't know where the throttle actually is.  Someone could have messed with the physical throttle stops so there is too much air getting past the throttle blades for the computer to have any control.  If it only happens when its warmed up it could be a vacuum leak causing the computer to add fuel to maintain a proper mix. 

[Dave Shepherd]

Excellent info!!

[WTL]

I've had similar problems, albeit in a 90 Deville with the 4.5 and fwd.  It's called sail on. 

It seems I have mostly cured it on mine by replacing a nonfunctioning isc motor (plunger) and I recalibrated the tps.

For what it's worth, I think it's a pretty common problem on all these cars.  I've read lots of complaints all over the net regarding it.   

[Maynard Krebs]

Thank you, gentlemen.

The same behavior takes place in both of my '89 Caprices, EFI, 305 V-8.   Though one has far more miles than the other, they both are in fine mechanical condition (w/ great gas mileage), and are "pure stock", as delivered when new.

My thinking is that this behavior is deliberately designed into the car's emission controls (via ECM), partially via the theory that most emissions are caused when under high-vacuum conditions; i.e., decelerating.

Yes, it does feel like there's a hydraulic door-closing mechanism on the throttle... or like an old-fashioned dashpot.

What I need to figure out... is how to make them 'more livable'.

[cadillacmike68]

I'm pretty sure there is something mechanical here or a sensor malfunction. I've had a few mid 90s GMs all to way over 100,00 miles and have not had this happen. The actual ECP / PCM/ whatever its called, almost never fails, it's the inputs connected to it.

The old garbage in = garbage out rule of computers is in full force with the car PCMs.

[TJ Hopland]

Strange they still used the external thing on the cars that late.   Trucks got an internal IAC when they got EFI in 87.

On the EFI systems there is a very specific procedure for setting the idle speed and timing.   It varies slightly model to model but it involves jumper wires and disconnecting wires at the correct times to put the computer into the proper modes at the right times and setting things to a specific setting.

If someone tried to adjust things without putting it in the correct modes the computer will be trying to compensate while you are adjusting so you won't have any idea where you are adjusting too.   That then confuses the computer because its assuming it knows its starting points.  You do that to more than one part of the system they can all compound and make a big mess.

[Maynard Krebs]

Yes, TJ; the old-design RWDs were among the last to receive EFI because the thinking (?) had been within the corporation.. was that they would be phased out, sooner rather than later.   But the sales level of these 'traditional' GM cars throughout the 1980s made it almost impossible for GM to turn down that good cash flow.   To be sure, it happened eventually . . .

Yes, the trucks got it in '87; but not 'til '89 in the Caprice.   The RWD wagons still used 'electronic' carburetors for a long time, too.