2000 Chevrolet 1500 LT / V8 - 4.8 liter, 220K miles and I am the original owner.

I am trying to figure out a hard start / rough idle on cold mornings. It goes through ~30 seconds of rough idle / rpm surge when starting, then acts just fine once warmed up and out of the driveway. This does not occur during the warmer times of the year (anything above 60 degrees F). I am able to monitor fuel trims, oxygen sensors and other OBD2 data and have enjoyed this learning process for troubleshooting.

I am checking to ensure that the IAT sensor (connected within the MAF sensor) is working as expected. There are ~5V getting to the sensor with the ignition on (checked with multimeter) and my OBD2 scanner is reading the correct air temperature entering the air plenum (for example, about 60 degrees in the morning). My Haynes manual says I should be able to read the resistance values (Ohms) when my multimeter is placed across the IAT's terminals. However, my multimeter is not showing any resistance values (Ohms) when placed across the leads. I was expecting to see ~4000 ohms at our current temp (60 degrees F) and then decrease in ohms as the engine temperature increased.

Question: How is it possible for the OBD2 scanner to report an IAT temperature, but I am not reading a resistance value on my multimeter? The replacement OEM (MAF/IAT combo sensor) is about $170 and I won't replace it until I rule out a few other things such as intake manifold air leak,

Thanks in advance,

Garon

 

I have ordered another multimeter just to ensure my resistance (Ohms) readings are correct. I will let you know what I find.

 

I was able to read the resistance (Ohms) with my new multimeter. The resistance values were as expected and I could read the correct temperature on the OBD2 scanner. ~3200 Ohms = 69 degrees F.

I believe I am running lean (on cold mornings) for the first 30 seconds until more fuel is sent to the injectors. I am just not sure why this occurs when it is colder (or cool and humid).

 

If you’re still needing an answer….

The IAT sensor doesn’t influence air-fuel mixture as heavily as the ECT sensor. The PCM uses the IAT as a secondary reference because air temp is an indicator of air density, but its primary input is the ECT sensor.

Check the ECT the same way as the IAT by reading the temp in the morning. Use the IAT and Ambient air temp as comparison. Verify ECT circuit integrity by unplugging the sensor - the reading should change to -40*F - then jump the ECT circuits together - the reading should max out (can’t recall exact number).

What you’re describing sounds like a lack of fuel/fuel pressure due to system bleed down. Try cycling the key to on 2-3 times before cranking and see if starting improves. If it does, the fuel pump is the likely fault.