Out of the blue the car started running with a rough idle (revs a few 100 rpm on own). I have done the following troubleshooting and parts swap:

New Fuel filter and PVC. Cleaned K&N air filter.
EGR valve replaced (would not hold vacuum) - it was bad.
New IAC - could not really test, and it was old and had been cleaned a few times already. Set min idle.
TPS - tested okay and reset after IAC install.
Installed new O2 sensor (thought it may be reading slow based on datalogger data)
Installed new coolant and air temp sensor
Replaced cap/rotor with a known good used one (the last one I used).
Replaced distributor with known good. Replaced Prom with known good stock.

Confirmed timing - also ran timing light on all plug wires to confirm spark.
Tested new EGR valve. Tested EGR solenoid actuator.
Tested fuel pressure.
Tested cannister and valve.
Sprayed "carb cleaner" (fuel injection type) around arr vacuum connections and the intake manifold.
Filled up the gas tank (93 octane as usual).

Datalogger value for MAP was in spec. TPS readings also looked within spec.
I noticed swings in the O2 readings with the injector PW and DC. I thought the Injectors were chasing the O2 readings as it went up and down as if O2 was late. That's why I replaced the O2 sensor, but no real change in idle.

It does not idle rough when first started. But while driving it will surge.

The only thing I can think of now (outside of an engine issue) is the distributor or computer. These were both replaced in the past few years at the stealership.

Can anyone provide ideas or point me in the right direction for reading/understanding the data from the datalogger? I have done all of the steps I can find in the GM Techman, except specialized tests like injectors.

 

I don't know what to tell you but if you figure it out let me know. I'm having the same problem with my 86 IROC. Mine idle if completly normal until I come to a full stop after the engine is warm. I see a lot of other people with the same problem, but no solution yet. I have also tried about everything you have with no luck.

 

I ran a meter across the injectors. All but one was 16.7 - 16.8. One came up at 1.9. Ran the engine with it disconnected and saw no difference.

I am going to replace the set (all are 16 years old and have 140K on them).

I don't know if it will totally solve the problem, as it was the #4 (passenger side) and the O2 sensor is on the odd side (driver's side).
Datalogger did not have any excessive knock readings (I think there were 4 or so over a good amount of time) since the knock sensor is on that side.

The rough idle I have is visible at start-up, but once warmed up, I also get the revving. Obviously it is a cause for rough idle, but I don't know if this would also cause the revving.

Can anyone speak to this from experience?

 

Hi Guys,
I have the same problem "surging idel" when engine is warm.
Please advise if you have fixed your problem and what it was.
Also, at what temp. did you check the resistance on the injectors?

 

Anybody else figure this out yet? I too have a terrible "surging idle" on my '91 3.1 V6. Doesn't seem to have any power loss or surging throughout the rest of the rpm range, just at idle. I've replaced the fuel filter, dist cap, rotor, plug wires, and checked the plugs that all checked out fine. I'm at a halt for now until I find something here that points me in what direction to go next.

If someone else has solved their problem, please let us know what was wrong with yours.

 

I have the same problem with a couple of my cars, idle nice and smooth in open loop, then is kinda lumpy in closed loop. Will kinda surge in lower gears too. I have replaced alot of stuff trying to figure this one out, one has a MAF 1989 IROC 5L TPI, the other is speed density 1992 Z28 5L TPI. Both are 5-speed

I'd like to know what it is ! !

 

We "had" this problem too. A new set of injectors, problem gone.