Rough Idle When Cold, Random Misfire, Lean Codes

[kleake]

I have a 2000 4x4 Suburban with the 5.3L engine. For the last several years it has had a rough idle and stumble when cold. The stumble is sometimes still there once warmed up, but the rough idle goes away.

 

On summer mornings, start it and go, with only a little rough idle, but not enough to really cause issues. On cold mornings below 50 degrees and very bad below 30 degrees, it will start and barely stay running. Many times it will set codes for random misfire, and several times has also set "Bank 0 lean, Bank 1 lean", although those only set once in a while.

 

These issues have been a pain, but when it gets serious is when I am towing my camper. Normal towing down the road you would never know, but pull into a park and try backing up a hill. Even though it doesn't miss during normal driving, it will miss badly even when hot while I am doing that. Even a couple of times I had to put it in 4-low in order to get the rpm above the miss and have enough power to back up the hill.

 

Over the last couple of years I have replaced Spark Plugs, MAF, MAP, TPS, Fuel pump, and Fuel pressure regulator. I have also cleaned the EGR valve, although it was very clean to begin with. I also plugged it, and it didn't make any difference. I have monitored fuel pressure, and even though it doesn't hold the pressure when I turn the key off due to the check valve in the pump, it does hold 43-45psi (if I remember right) at all times while running.

 

I am at a loss, but have recently heard about intake leaks. I am thinking of pulling it to find out, but does this sound like the intake or something else?

[Sjaimeeyenm]

It's your manifold gasket. When it's cold the gasket shrinks and you are getting a vacuum leak. As the engine warms up the gasket expands and no more vacuum leak, hence no more misfire. Change your manifold gaskets and it will fix your misfire

[Snow White]

my 09 is doimg the same minus the codes but at 85,000 i have to try and convince dealer manifold gaskets are the issue, any tips for proving that?

[Snow White]

turns out it was a throttle position sensor, no wonder i thought i had a vacuum issue lol

[efritz]

Listen to

 

 

It's your manifold gasket. When it's cold the gasket shrinks and you are getting a vacuum leak. As the engine warms up the gasket expands and no more vacuum leak, hence no more misfire. Change your manifold gaskets and it will fix your misfire

He's right, it's your manifold gaskets. Change them and it will go away 99% of the time.