2.8l GMC S15, rough idle when cold

[grassulo]

I have a 1987 GMC S15 pickup, which when cold idles very rough, the whole truck will vibrate and the motor will sound like it's going to die. When it's started up, the idle is fine for about 15 seconds and then it drops down to sputtering, so I'm pretty sure the IAC is moving, but possibly not enough? we're these IAC's known to have problems, or could this be somthing else?
Once the truck warms up, the idle is fine, no sputtering or hesitation at all.

-Garret

[geoffinbc]

Yes on an old truck the IAC may need replacement. You should test the Engine Coolant temp sensor. It needs to read 3400 ohms at 21*C (70*F) so test at room temperature. This sensor instructs the ECM on how much fuel to deliver for a given temperature. If it is wrong it can really screw things up.

[grassulo]

I looked at it today and realized I was trying to test the wrong sensor. Turns out my truck has TWO coolant temp sensors in it, one for the gauge on the dash and one for the ECM, and I'd been trying to hook a ohmmeter to the one for the gauge. The one for the ecm is the one with the oval shaped weatherproof plug near the thermostat right? NOT the one with the square-ish non weatherproof two prong plug sticking out at the corner of the valve cover.

-Garret

[geoffinbc]

Right the one in the intake is what you want. Yellow and Black wire.

[grassulo]

I tested that sensor and it tested fine, about 3000ohms at 50 farenheit (really cold day for june!) the one for the gauge however is dead, no ohm readings at all, at any temperature, so that one will need to be replaced. The truck STILL idles rough when cold, I suspected the throttle position sensor, so I backprobed it and adjusted it, .92 volts with throttle closed, same issues, still idles rough, what else could it be?

I'm going to replace my Idle air control valve next, but what irks me about that, is that the idle rpm surges up at start, then gradually drops and gets rough, so I know the iac is moving, it's almost like the computer is pushing it out too far when cold, once warmed up and when in closed loop mode, the truck idles fine, it's only when it's in open loop mode that the problem occurs.

The rocker arms badly need to be adjusted, they click a lot, I have new roller tipped replacements waiting to be installed. Could this possibly be whats causing the rough idle when cold? But if so, why does it get better in closed loop mode and warm?

-Garret

[geoffinbc]

Your temp sensor is fucked. The reading should be HIGHER as temperature gets LOWER. Replace it.

[grassulo]

I replaced that sensor with a new one and my cold idle has gotten better, but still not great, I'm beginning to think its due to carbon deposits in the IAC passages and in the top end of the motor, have any of you ever tried running Seafoam cleaner through your brake booster hose to clean the top end? Thats what I'm going to try next.

-Garret