2004 V8 runs rough when cold, P0171, P0174

[Sexsymbolx]

Hi everyone!

I really have to dig into my 2004 V8 LS and find the solution to this one. I've seen these codes listed on the V6 and people seem to suggest that the PCV is the culprit. Not sure about the V8.

The chec engine light is on and I'm pulling P0171 and P0174 codes. There are a couple of odd things that I should mention that might make sense to someone.

When it's cold, as in below 60 or so, it runs rough in the morning until it warms up. If I stop on a decline coming out of our driveway it starts running much rougher.

Also, if it's wet and rainy, often the check engine light will shut off.

Third, if I'm idling at a stop with the wheels straight ahead and I turn the steering wheel slightly to the left, the engine surges a little.

I occasionally have a surging issue with this car as well.

The spark plugs and coils are all about 5 years old. Yes, I'm sure that I should replace the plugs, and I have had a couple of occasions where the throttle stepper motor has gone into failsafe mode, which is probably an indication of a bad coil, from what I've read.

I know it's a lot, but does any of this make sense to anyone? Could be a combination of things.

Thanks!

Tim

 

[joegr]

I'd start with a hunt for intake/vacuum leaks. It could be the internal seals on the runners of the intake manifold, or it could be the PCV plumbing.

I wonder if excessive oil is pooling in the intake manifold? (If I stop on a decline)

There is a boost sensor on the power steering that the PCM reads and responds by increasing engine power slightly at idle if the pressure goes up. (I turn the steering wheel) This is correct operation.

If your surging is at very low speeds, it's the transmission shifting, and it's normal.

 

[Sexsymbolx]

Thanks Joe!

I'll start there.

The running rough on a decline thing is only when started from cold very first thing in the morning.

 

[Sexsymbolx]

I have an update-

I found that the short crankcase vent tube was somewhat askew where it connects to the air cleaner outlet pipe. The air cleaner outlet pipe has a small crack at that connector, so the crankcase vent tube isn't sealing solidly. (Also the O Ring on the vent tube was dried out and hard). I replaced the O Ring and used a little electrical tape around the connector.

The light went off and the car was running smoother for a couple days.

Then last night the light came back on. Car is running a little rougher again. I checked and the connection seems to be solid.

Any suggestions where else I might be finding a vacuum leak? Or is there a recommended way to figure out if this connection is still leaking?

Thanks!

 

[RigsLS]

Perhaps you should remove the plastic intake tube and inspect the bottom side for cracks. When putting it back on, verify it goes on properly at both ends with the clamps. Any unmetered air getting in past the Mass Airflow Sensor will throw off the fuel trim. At the MAF sensor housing check the torq bolts, I had to replace the whole thing on my 01 years back when I first got it because where one of the torq screws fastens was stripped inside the housing and wasn't allowing the top portion of the MAF to be tightened down tightly and evenly. It basically was sitting loose in one corner and when pushing on it I noted the possible air leak.

Might not be your immediate issue but the intake snorkel from Airbox to Throttle Body is often overlooked.

Further more, you could take an unlit propane torch and wave it around all lines while it's idling. Any slight immediate jump up in RPM would indicate a vacuum leak at such point.

GLWR

 

[Andrizzle]

I found my vacuum leaks by spraying carb cleaner around the intake seams (where plastic meets metal portion). All of my seals in this zone were worn down and letting air pass when it was cold. I ended up using gasket maker with the old seals and it worked great, no more issues.