Rough Idle When Cold?

[Brewtal66]

Hey Everyone,

Having a weird issue with my wife's 2001 Yukon XL that has the 5.3L in it. Just this week, when I start it up after sitting in the cold all night, it runs on 7 cylinders. Once it warms up a tiny bit, it runs absolutely perfect. No check engine lights or anything out of the ordinary.

The plugs and wires are new, maybe 10-20K on them and they are AC Delco iridium tip ones.

Any ideas where to point me? I was originally thinking maybe bad O2 sensors, but when it's that cold it's running open loop, so that wouldn't make sense.

Thanks for the help/input.

-Tim

[GregWeld]

sounds to me like a sticky injector ---- that warms up and begins to fire some fuel -- no fuel -- no lighty offy!!

[mitch_04]

Check for intake gasket leaks. We replace them very often on the Gen 3 pickups. We hook up a smoke machine to the intake and almost without fail, smoke will rise from below the intake. You could try spraying carb cleaner or starting fluid around the base of the intake and see if the idle comes up, but it doesn't seem to work as well as it did on old small block Chevys and such. Find a buddy who smokes, pop off the vacuum line to the fuel pressure regulator, and make him blow a cig or two into the line. Or find someone with a smoke machine, much easier!

It should shoot into closed loop very quickly with heated oxygen sensors, so you could also check your long term and short term fuel trims. The fuel trims might give you an insight into what is causing the problem, if it's adding fuel or taking it away.

I don't know if you have any scan tool or similar tool, but even the cheap ones that link with your phone will tell you a lot that can help. The intake leaks don't always set codes, when they do it's usually a lean code or even the MAF since it gets blamed for miscalculating air.

For checking the injectors, you can hold the tip of a screwdriver to the injector, then your ear on the handle. Should be a hard handle, not cushy. You will hear the injector tick and it's obvious if one of them isn't opening as well.

Lots of possibilities to cause a rough idle in cold weather, but I'd check the intakes first, we fix them constantly.

[minendrews68]

hmmmm, rough idle when it's cold....
sounds like a Chevrolet to me

[blitzer454]

x2 on the intake manifold gasket. My truck had a similar problem. Only ran rough when it was cold and only until the engine warmed up. I replaced the gasket and all was good.

[Brewtal66]

Originally Posted by mitch_04

Check for intake gasket leaks. We replace them very often on the Gen 3 pickups. We hook up a smoke machine to the intake and almost without fail, smoke will rise from below the intake. You could try spraying carb cleaner or starting fluid around the base of the intake and see if the idle comes up, but it doesn't seem to work as well as it did on old small block Chevys and such. Find a buddy who smokes, pop off the vacuum line to the fuel pressure regulator, and make him blow a cig or two into the line. Or find someone with a smoke machine, much easier!

It should shoot into closed loop very quickly with heated oxygen sensors, so you could also check your long term and short term fuel trims. The fuel trims might give you an insight into what is causing the problem, if it's adding fuel or taking it away.

I don't know if you have any scan tool or similar tool, but even the cheap ones that link with your phone will tell you a lot that can help. The intake leaks don't always set codes, when they do it's usually a lean code or even the MAF since it gets blamed for miscalculating air.

For checking the injectors, you can hold the tip of a screwdriver to the injector, then your ear on the handle. Should be a hard handle, not cushy. You will hear the injector tick and it's obvious if one of them isn't opening as well.

Lots of possibilities to cause a rough idle in cold weather, but I'd check the intakes first, we fix them constantly.

Originally Posted by blitzer454

x2 on the intake manifold gasket. My truck had a similar problem. Only ran rough when it was cold and only until the engine warmed up. I replaced the gasket and all was good.

Thanks for the help. I sort of forgot about the issue since the car stopped doing it, or at least as bad. But now it has done it bad again. I did buy a super cheap DTC reader. It has thrown P0300 code two times, which is just random cylinder misfire.

For $20 I can replace the intake manifold gaskets, I'll try that and see what happens. Unfortunately it only happens when it's really cold and only for maybe a minute, not really long enough for me to play with it and try to diagnose it.

I'll try that and report back if it worked or not. Thanks guys!

[Brewtal66]

Originally Posted by GregWeld

sounds to me like a sticky injector ---- that warms up and begins to fire some fuel -- no fuel -- no lighty offy!!

That's possible. I have run two fuel injector cleaner bottles through it. Not that it's guaranteed to fix it, but didn't seem to make even a little difference.

[randy]

Spray throttle body cleaner around the intake while running and see if the tpms jump, borrow a real scanner and read the injectors and history misfire codes

[Brewtal66]

Just to update this a bit. A little over a week ago I replaced the intake gaskets. Didn't visually see anything wrong with the old ones, but there are new ones in place.

It hasn't done the running on 7 cylinders thing since. It hasn't been as cold out either, but still hasn't done it. I'm still keeping an eye on it that first minute it runs when it starts up.

If anything changes, I'll chime back in. Thanks again for every ones help!

[mitch_04]

Hope it works for you. Visually, they aren't like the older gaskets which are usually an obvious blowout. The section will just be a little flatter and may have a slight discoloration. I've replaced oodles of them and rarely is it an obvious leak. Our smoke machine makes it an easy find, otherwise they can be very difficult to diagnose.