Surging Idle on my 3.1L

[Dean]

Okay here is the story. My '96 GP overheated, very hot. Did a flush to it and did not solve the overheating problem, it got worse. The thermostat was good, but wasn't cooling, notice gasy smell in the coolant. Nasty smell. So, I tore the engine down discovered the rear head gasket had a hole right through it, between 1 & 3 cyliders, water jacket area. Fortunately, nothing went inside the cylinders. Had the heads milled, installed new gaskets and all. Now I get this surging idle and will not stabilize. You have to keep it rev'd at 800 rpm to prevent it from dying off. Do not under stand what the issue would be. I checked all the vacuum leads to the intake manifold and everything is connected and cleaned. I even took the throttle step motor out and cleaned it and replaced the O-ring on it, thinking this maybe an issue, but the idle is stll erratic.

Anyone have clues or suggestions?

Thanks!

[geoffinbc]

Hmmm 96 GP has the 3100 and I don't think it needs to do an idle relearn if memory serves me correctly.

But it sounds an awful lot like your problem. If you are certian everything is back together right then you will have to drive the car around for a while and it will re learn its own idle. While doing so you will have to prevent it from stalling by 2 foot driving.

The quick fix to this is to have a programmer do the relearn.

Do a search on idle relearn because there is a driving procedure to go through to quickly make the car do its relearn.

[Dean]

Surging idle issue

Okay, I replaced the IAC valve on the TB and also replaced the Throttle Position Sensor thinking these may have gone bad when I was cleaning the TB. The Idle still won't stabilize, even at the car fully warmed up (waited till the cooling fans kicked on). I checked all vacuum ports three times and everything is connected.

When you drive it, it drives fine. But when you come to a hard stop, the idle drops below normal and car stalls out. Then if you come to a gradual stop it maintains the unstable idle and idles for a few seconds then car stalls.

I tried the idle relearn three times, didn't work for me.

I was now suspecting the device that is located and mounted on the lower set of studs that hold the coil pack, so I looked at the GM shop manual and Chiltons, and they do not explain what this device is. It has to two power leads going to it and has two hoses connected to it as well (I assume one comes from the fuel pressure regulator; and the other ties to a fuel line, a 1/4-inch rubber hose which routes to a hardline going to the back of the car). It has a third port that doesn't look like it was connected to anything before my engine teardown, it has a 90-degree rubber connector. Do not know what this may have connected to!?! I checked for any lose hoses in the back of the engine and saw nothing. Do not know if this device went bad when my car over heated!?!

I also looked at the EGR. Unbolted it from the manifold and inspected the little plugger at the bottom. It moves freely and does not stick. I put it back on and unplugged it started the car with it off, and the car still exhibited the same surging/erratic idle. So I plugged the EGR back in.

Confused!?!

Anyone have any suggestions or another method of process of elimination??

Thanks!

[Juglenaut]

RE: Surging idle issue

Did you reprogram the Mem-cal??
Did you test for fuel pressure across the RPM range??

Did you check the Evap purge solenoid??
Is the map functioning normally?
Are you getting any black smoke on start up, cold??

[Dean]

Surging idle issue

I have figured out the problem. The primary O2 sensor went bad. When I hooked up a OBII scanner and read the data coming through. I noticed that the Voltage between primary and secondary O2s were very different. The rear secondary read high amount of voltage and the front was really low, close to nothing. I replaced it the primary O2 and it is now gone. The car runs normal. Surging idle is gone.