Low and searching/bouncing idle - '91 Full-Size Blazer

[clay1937]

I have a full-size 1991 GMC Jimmy (same as Blazer) with a 5.7 liter (350 CID) V8, that has developed a low idle and searching / bouncing idle problem.

The SES light has never come on, and the dealer, who has looked at this several times now, has never found any codes in the computer.

When the engine is cold and is started, it runs fine, with the cold idle around 1,100 RPM. As the engine warms up (I typically let the engine warm up to about 100 degrees before driving), the idle drops down to about 800 RPM, then while driving, as the engine continues to warm up to operating temperature, the idle continues to drop to where it finally reaches it’s normal running 650 RPM, as it has done for the last 14 years I’ve been driving the truck.

When the engine finally reaches it’s operating temperature, (i.e., around 180 degrees), which typically occurs 2-4 miles into driving, the problem begins, where the idle will drop too low, to around 500 to 550 RPM, and begin to "search" for the right idle, bouncing up 50-100 RPM, then down 50-100 RPM over and over, acting as if the engine is going to stall and die...which actually has occurred a few times when the weather is really hot.

When pressing or holding the accelerator, which includes taking off from a stop, and just normal driving, it runs beautifully. Under a load, there doesn’t seem to be any ignition/timing problems at all

The problem returns every time deceleration from about 30 MPH occurs, or when sitting stopped at a light.

The idle searching / bouncing up and down is worse when the transmission is in Park or Neutral, such as putting the transmission in neutral at a red light, with bigger RPM bounces...around 100 RPM jumps and drops, than when the transmission is in gear, where 50 RPM bounces are seen.

Because of the low RPM (i.e. 500 - 550 RPM), and the 50 - 100 RPM idle bounces, the oil pressure gauge also bounces all over the place, because it seems the engine is struggling not to stall and die.

The carburetor has been cleaned many times, and the engine had a Motorvac service.

So far I have already replaced the following parts with no improvement:
GM Tune-Up Parts (cap, rotor, AC Delco plugs, wires)
PCV Valve
GM ECM (computer chip)
GM Distributor Module
IAC and gasket (aftermarket part...no longer available from GM)
GM TPS (Throttle Position Sensor)
GM EGR Valve and gasket
GM MAP Sensor
GM Knock Sensor
GM TBI Injectors (new...not rebuilt)

I recently read in a repair manual that if the temperature sensor is bad, that nothing in the injection system will work properly, so the next thing I'm going to try is replacing the Temperature Sensor, once it comes in (the 4x4 version of the GM part is only available on special order). I also have a GM O2 sensor to try after that.

I've had trouble finding the correct EGR solenoid, but have thought that may be a possibility too.

Other thoughts I've had include:

1. Replacing the distributor, which seems unlikely to be a problem, since the engine runs fine under a load during acceleration and driving, it's only a problem when decelerating to a stop, and when sitting idle

2. Replacing the thermostat (I'm running a 180 rather than the factory 195, but I've been running a 180 for the last 14 years I've owned the truck)

3. I have even considered that replacing the A/C/Heater temperature controls might help, (although this was replaced about 5 years ago when it quit working).

I also can't see that there would be a problem with the coil, or timing chain (i.e., play in chain), because the truck runs perfectly until the engine is hot, and under a load when driving...again, it's only a problem when decelerating to a stop, and when sitting idle

There don't seem to be any vacuum leaks anywhere, and the intake manifold gasket was replaced about 5 years ago, and doesn't appear to be leaking again.

Even though the truck has over 200,000 miles on it, I can't imagine that there's a problem with the fuel pump (i.e., low pressure) which was replaced about 8-10 years ago with an OEM GM pump that has maybe 20,000 miles on it since.

And since it’s a 1991 model year, there's no MAF sensor, Crank/Cam Position sensor, or even an air pump. It’s just TBI not SFI.

Someone made a posting of the exact same problem at http://www.automotiveforums.com/t183013.html but that was 4 years ago, and the solution was never posted. User GMMerlin posted a good article that will be helpful to me, but any other thoughts will be greatly appreciated.

Thanks!

Clay

[Schrade]

Loping idle is vacuum leak. 30 year old lawnmower, or a new car on the lot. If it sucks air where it ain't supposed to, it lopes.

Brick on pedal, block TB arm, anything to get a STEADY idle. 1200rpms or so... Feed starting fluid spray, A LITTLE AT A TIME, EVERYWHERE. Rev increase is the leak.

In a tough spot to pinpoint? Light a book of matches, blow it out while the heads are burnin', and feed the smoke to the leak. Best idea ever (MINE too )

[doop300]

Im having the same kind of problem, been thinking the tbi gasket where it mates to the manifold or a possible regulator problem, although I dont know where the regulator is on mine. Its my first tbi system, gotta start somewhere.