2002 GMC Sierra Cold Rough Idle

AYBAIR76

MEMBER
2002 GMC SIERRA
V8
2WD
AUTOMATIC
109,000 MILES

When the outside temperature is about 40F degrees or below, my engine idles really rough until it is warm. It recently started coding P0300(random misfire). This problem started last winter. Ran throughout the summer with no problems at all. Started up this morning at 25F degrees with a rough idle. I gave it some gas to increase the RPM for about a minute, then about one mile up the road at a stop light, it was still idling a little rough.

In the past, I seem to remember the idle staying above 1000 RPM for a good amount of time on cold mornings. However, now, it starts up at about 1100 and quickly, within 20 seconds, starts to ramp down to 500, where things get rough.

Here is what I have done so far.
During the summer, I replaced all spark plugs and I recenlty installed new plug wires. Made no difference.

I replaced the coolant temperature sensor. No improvement.

As suggested by a friend, I placed my old Coolant temp sensor in my freezer for about an hour to cool it down. I then plugged it into the connector while the engine was still warm. The engine did not want to start when I cranked it. I held the gas pedal down a bit and finally got it to start and the idle was fine. Not sure how to interpret those results. Friend thinks it must be the computer.

I wanted to replace the mass airflow sensor since I read that it has fixed the issue for others, but everyone keeps telling me not to unless there is a code for that.

Some think it could be a vaccuum leak. There are few vaccuum hoses on this engine and they all seem to be ok. Perhaps it could be the intake manifold gasket.

What do you think?

3 Replies

BRIAN 1

MECHANIC
1,030 POSTS

It is the intake leaking, very common on these.99 through 05 GEN III truck engines may experience any of the following:

A cold rough idle that diminishes as the engine warms up.
Possible DTCS P0171, P0174, or P0300.
Random misfires at idle. The intake gasket seals may be leaking around the intake ports, causing a lean condition. The intake gasket seals may leak around the bottom side of the port, making it a challenge to detect the vacuum leak. If leaking intake gasket seals are found, replace the intake gaskets. The old version of the intake gaskets were orange in color. Use only the teal green colored gaskets for a repair. The intake plenum should also be checked for excessive warpage that may cause an uneven clamping pressure of the seal. If the plenum is found to be warped it will need to be replaced. Take carb cleaner and spay it around the intake and look for the RPM's to increase and thats were the leak is, start there first before you throw more parts at this vehicle.