Just had our frist really cold spell here in Mo. (9 degrees) Car had been doing fine til yesterday morning. Started it up and it didnt want to idle. It had been going up to about 1000-1200 rpms til it warmed up and then back down to 550. Now I have to hold the gas pedel down to get it to start, if not it trys to stay down at 550 rpms when you frist start it. And when its as cold as it has been it just dies. Once it warms up its just fine. How does the tune port on my 91 L98 regulate the fast idle? Computer controled, im sure. But is there a sensor or something that tells it "Hey Its Cold, Idle faster" or is my computer going bad? Not sure where to start checking. Thanks for any help you can give me. Oh yeah and my gas mileage has went in the crapper. The average indicator on the dash is saying 18.9 mpg it was around 30mpg. Put a new battery in and it reset everything on the dash and noticed thats where it reads now. Not showing any codes, runs great when warm. O2 sensor? Guess I need to put it in the garage and throw some money at this winter. Cant complain, 150,000 miles and hardly a lick of problems, til the last few months.

 

Do you have a scan tool? It could be the engine coolant temperature sensor. It is the only sensor aside from the intake air charge temp sensor that tells the ECM how hot or cold it is. If you have a scan tool you can check it easily. On a cold day I scan it, compare the the ECT and the IAT sensor readings. Both should be close before the engine starts. Also you should check the ECT connector, sometimes the sensor goes bad and starts leaking coolant then the readings get skewed and then the ECM don't have a clue as to how cold it is. Or it could go the other way and it may think its -40 out which could be bad because then it will over fuel the engine. Or even flood it out.

 

Thank you very much for the info. Will check those out tomorrow..

 

What "scan tool" are you reffering to and where can i get it? I am having similar problems on my 85.

Thanks

 

It is almost certainly the ECT sensor or the wiring between it and the ECM. If you are familiar with carbed cars, consider the sensor as the choke stove and the ECM as the choke. When the choke closes, because the choke stove is cold, the engine gets a rich mixture to keep it from stalling while it's warming up, and the choke engages the fast idle cam to kick up the idle speed.

Your car dies when it is cold and it doesn't have a fast, cold idle. Check to be sure the sensor hasn't become unplugged or that the wires haven't chaffed (shorted out) or broken (open circuit). You can test your ECT sensor with a regular DVOM. My FSM lists the different voltages (or maybe it is resistances) for different ambiant temperatures, but if you notice the volts or the Ohms changing as it warms up, that is proof enough that it's working.

RACE ON!!!

 

Actually... I've seen these early MAF cars do that (mine itself has done it) as a result of MAF sensor issues - apparently faulty temperature compensation circuitry in the MAF itself. It may be just me, or coincidence, but I've had a Couple of "performance" aftermarket MAF sensors that just went all sorts of crazy when the ambient air temperature fell into the single digits.

Think about how a MAF sensor works - air moving through it cools off a heated wire (if it's an OEM Bosch type) or heated wafer. The colder the air the more cooling it does, obviously, so there has to be compensation for the ambient air temperature. If that compensation isn't correct, the output signal from the MAF won't be accurate.

The adjustable MAF unit I am using right now needs to be fiddled slightly this time of year (to fix idle problems) also. I expect that once spring comes I'll be turning the screw back to where it Was before it got cold...

Just a possibility... a scan tool reading when you start it up on a cold morning, compared to one idling during Warmer conditions, will show what the MAF output does. Air temperature Shouldn't change it - the compensation Should be happening within the MAF itself.