I set the minimum air and TPS but the problem still persists. On a cold engine idle is fine at 600rpm in drive. Once it reaches operating temperature, the idle speed goes down to 450.

What could it be? I suspect my CTS is bad. That's because the car runs like crap when its cold. I get all sorts of hesitation and stuttering. Now since it got cold here, the slighest pedal input on a cold morning causes rpm to drop suddenly like it's gonna die. What's going on here? I had this problem last winter too but I dealt with it the lazy way - just let the motor reach operating temp before driving away. I have noticed the all the hesitation and stuttering disappear suddenly, when the temp gauge shows about 75*. It feels like the computer suddenly goes into another mode. The idle slows down a little bit and throttle response becomes crisp and very quick. Is it switching from open loop to closed loop? Don't take the reading of the temp gauge as gospel. It isn't very accurate, especially at low temps - something to do with the limited range of the sending unit. I swear I wish had a scan tool, but I can't afford one right now.

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'88 IROC 305 TPI
Gutted airboxes
160 degree T-stat
Advanced base TPS voltage
Relocated IAT sensor
Momo steering wheel
Ram-air setup coming soon
Flowmaster muffler

 

88,

I'm sorry to hear it's getting so cold in Arkansas. It was in the 20s here last night. A few miles up the road, 10" of snow fell during the day (Hey, thanks Canada!). Better get your woolies on.

You don't need to have a scanner to diagnose the problem, but it would make reading sensor inputs a little faster. Since you set the TPS, I presume you have access to a DMM. You can check the temperature sensor using the DMM on a resistance scale. I've included a cross-reference table with the appropriate data.

The CTS may be a problem, but I would be more inclined to suspect the MAT sensor. Even though the air temperature may be cooler, the coolant may have some latent heat. The MAT is used to indicate actual air temperature and the ECM offsets fuel accordingly. As long as you're checking the CTS, you might as well compare both. Since you've relocated it, checking it should be easy.

As far as the sudden change in operation, the ECM enters closed loop mode as low as 105°F as long as the oxygen sensor has a varying input. Cold spark advance and cold power enrichment ends as soon as the coolant reaches 133°F, and highway mode spark advance is enabled above 140°F.

And you're right about the dash gauge. The indication is probably not that accurate. Considering that, I trust you used a separate handheld tachometer to set the throttle minimum air position instead of the dash tach. The minimum air position idle speed should be 450-500 RPM. Since your hot idle is evidently in the same range, you might want to set the minimum air again, making absolutely certain that the IAC is clean and fully closed. If you have even a little IAC leakage when setting the minimum air position speed, the hot idle will tend to be lower than the target speed of 650 in DRIVE. As always, perform adjustments only when the engine is a full operating temperature.

Another thing to consider is the fuel pressure. If the pressure is a few PSI low, you could be a little on the lean side. A weak oxygen sensor can have a similar effect, but shouldn't be a consideration until you reach closed loop mode.



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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

I changed the IAC motor and cleaned out the whole throttle body at the beginning of this year. Could it get dirty so quickly?

I don't have a handheld tach or a reliable thermometer, so those are somethings I'll have to look into.

------------------
'88 IROC 305 TPI
Gutted airboxes
160 degree T-stat
Advanced base TPS voltage
Relocated IAT sensor
Momo steering wheel
Ram-air setup coming soon
Flowmaster muffler