Slight idle misfire

[captain150]

06 with the 4.2L, engine has about 130,000 miles. The engine is a 2004.
I just read the misfire diagnosis thread but I don't have the same symptoms.
Truck starts and runs fine when cold. When hot, it has a slight misfire at idle, but it's weird. I'll slow down to a stop light and it will idle fine for 30-60 seconds before the misfire starts. Or it will be misfiring in park, I'll hit the throttle and it will run smooth for 30-60 seconds again.

What I've done so far;

-Replaced the plugs (worn autolite platinums were replaced with the OEM AC Delco 41-103s)
-Cleaned throttle body, which was gunked up
-Cleaned MAF sensor
-Short and long term fuel trim are within 5% of zero, so it doesn't seem like a vacuum leak

The only code I have consistently is a P0418 which is for the SAIS. Every so often I get P0300 as well.
It occurred to me today since the truck and ECU are 2006, and the engine is a 2004, if that could play into this as well. I know GM made a lot of changes to the 4.2L for 2006, but I'm not sure if that could cause this issue. Whoever replaced the engine used the 06 exhaust manifold, the SAIS solenoid and hoses are attached to it as they should be.

With the new plugs and cleaned TB the thing runs way better at all other times and seems to start better.

 

[Capote]

Have you checked the intake manifold? It's possible some bolts are loose and there isn't a proper seal. I know mine have been loose before.

 

[Mooseman]

The SAIS is actually attached to the head, not the manifold. It could be defective. There was another recent thread that talked about it with similar issues. Even if your trims are good, you should still check for vacuum leaks using carb cleaner just to eliminate that.

I don't think the differences between 04 and 06 are significant enough to affect operation. The PCM and the coils changed. The 04 coils would work, just need to change the connectors. Make sure the air intake is on tight as this would affect the MAF. All engines back to 02 had provisions for the SAIS but wasn't implemented until 04 or 05. Older ones had a block off plate there.

The SAIS is a failure prone system that should just be removed using an earlier block off plate and have the codes tuned out.

 

[Capote]

The SAIS is actually attached to the head, not the manifold.

I know; was just stating that a vacuum leak at the manifold can also be a culprit in a slight misfire, unrelated to his SAIS code.

 

[BRomanJr]

Could be your front O2 Sensor? if it is the original, in the original manifold, they tend to get "lazy" and as they age are prone to not staying hot enough to work properly during idle. It can develop deposits on it and lie to the PCM after it has a chance to cool down during idle and the built-in heater is just not enough to keep it hot.

If you want more evidence before replacing, watch the sensor voltage during the misfire episodes. Should just replace if it has over 100,000 miles on it. If it has been replaced, was it a Delco or other OEM supplier? I recommend NEVER use a "universal

Remember, only the front one affects running, the rear is only for catalytic converter efficiency testing. The rear sensor stays much cleaner for a longer time.

 

[captain150]

Thanks guys. I'll check those intake manifold bolts, as well as the snorkel hose clamps.

The O2 sensor is certainly plausible. It would make sense that I don't get the misfire with a cold engine, since it runs in open loop at that point, and why I get smooth running for 30-60 seconds after returning to idle (takes that long for the sensor to cool down and start misbehaving).

I'll take a closer look at the O2 voltages...even if they look normal I'll probably just replace the thing, it's not all that expensive.