From Andy Levy:
It's possible that the block heater is sending some heat up (either via
convection or conduction) near the IAT and other sensors, making the
computer think the ambient temperature is higher than it is. This would
lean out the mixture and cause stumbling.
From Kevin Hoegen:
It could simply be a small amount of
gaseous build-up or condensation in vapor form (caused by heat build
within the block) that is released quickly when the engine
turns-over.
From Will Coughlin:
I think that what you might have happening is that the block and and internals are getting warm, while the air outside is still cold. As you know, a cold engine requires more fuel to start and run than does a warm engine. If you have a warm engine, and the PCM sees a cold air temp (IAT sensor) and a cool/cold coolant temp (Coolant Temp sensor), the PCM won't know the engine internals are warm. Therefore, when you crank it up, the PCM immediately throws more fuel into the mix (kind of like a "choke" function) because of the "cold start" condition it's seeing from the sensor input it has. The engine internals are warm and do not require as much fuel to run properly and are being fed too much fuel. When the engine stumbles or misses because of an overfuel condition, the PCM picks up on it and backs the fuel off accordingly. What you could do to correct or help this condition is to acquire on of Bernd's IAT adjusters. You could then: turn the key on and let the PCM take a second or two to analyze the sensors and you could use the IAT adjuster to make the PCM see an IAT input that is higher than the corresponding outside ambient temp. So say it is 10° outside when this conditon occurs one day. The next day, try adjusting your IAT adjuster up to 30° or so and see how it affects it. If it makes the condition better, but doesn't cure it, continue moving up the temp scale on the adjuster the next morning. If you get to a point where the truck takes longer to start or won't start, you've gone too far and you need to come back some.It wouldn't take you long to figure out how to adjust it according to the outside temp
From Jeff Trithart:
I am in Alberta, using or not using my block heater makes no difference in the way it starts. We have been experiencing -30 and colder temps at night and the truck just starts. Most of the time, I forget to plug it in. I run full sythetic oils thru out the truck, not sure if it makes big difference, but the truck starts fine every morning
From Bob Tom:
I used my block heater for the first time last winter ... unusually early, cold spell (below 0F) and I had not changed back to the 195 tstat from the 180 nor brought my coolant mix back up to the 50-50 mix (was starting to get ice crystal formation!).
Anyways, I had the same experience. I believe that the pcm was getting conflicting sensor signals and it took a bit of time before it figured it really should be in cold-start mode (i.e. very rich).
BTW, I read that about a 1 hour period with the block heater should be suffice. Anything longer may not be beneficial. This is a rough guide dependent on conditions of course. I did notice that I was getting some "sizzling" sounds when it was plugged in for more than an hour.