Bizarre engine lean code situation

The car runs great. If you start it and immediately drive there is no problem but if you start it, let it warm up, and then drive on the interstate it throws a lean code for both the right and left bank.

I've been struggling with lean codes since I got it. Resealing the intake manifold made it run much better and cleared the codes for a short time. They returned and a leak in an old vacuum line was found. They soon came back and a smoke test revealed four leaks around the MAF sensor. Those were repaired and now the present problem came up.

A professional did the smoke test and subsequent repairs but now he is stumped. Any ideas?

Thanks,

I can think of a couple things, based on my experience.
I put a fuel pump in mine last week. It had failed in what I consider a very unusual way, since any “in tank” pump failures that I had seen previously, were complete failures, or nearly so. After 10 to 90 minutes of normal running, the engine would go very lean, losing power badly, causing me to head for the side of the road. The pump did not, however completely shut-off, as the engine would idle, although very poorly. Turning it off for ten minutes, allowed it to resume completely normal operation---for awhile.
I’d check the fuel pressure at the port on the back of the engine, just in case something similar is happening to yours. Check it cold and after running for awhile. A normal reading will rule out the pump, pressure regulator, and filter (unlikely).

When you had your intake off, did you replace the PCV valve? It and the grommet and hose are likely vacuum leak points. If the rubber grommet holding the PCV into the manifold is hard, it can leak and should be replaced. If you replace it, buy a genuine GM; the Dorman replacement doesn’t fit!

Another possibility is the hoses at the vapor canister. I replaced all of the hoses to the canister, a couple years ago, as they were all in very poor condition.

When you first start the engine, the ECM keeps the mixture “rich” for some period of time, depending on temperature. After that, it tries for a normal mixture except for WOT conditions. This is when your issue “shows up”, so I’d guess that there’s a vacuum leak somewhere.

O2 sensors are a possibility, but both at the same time, seems unlikely---and they usually go to the rich side as they age, reducing gas mileage, rather than causing drivability issues.

Thank you very much paart. I will pass your ideas on to my mechanic he is very open to good info like yours.

Also check the engine coolant temp sensor that goes to the PCM (located in the water pump). If this sensor is reading high for some reason, it'll tell the PCM that and the PCM *may* lean out the air fuel mixture a bit assuming better fuel evaporation/atomization, which might be causing the lean issue. Have him scan the PCM and see if there's a difference in the sensor behavior between the two operating modes you described.

Well, I got the car back. The check engine light stayed off for a while but after a gas stop came back on showing lean fuel trim banks 1 and 2. I could reset it and drive for hundreds of miles without it coming back on but once I shut the engine off it comes back on upon startup.

I filled up with ethanol free gas and the light went off and has stayed off. Any explanation?

1/ What was done to the car? Has the fuel pressure been checked?

2/ I can think of two possibilities: There could be a clogged tank vent, that's restricting fuel flow, but I'd think that would cause at least some driveability symptoms. Does filling up with "normal" (with 10% ethanol) gas at least temporarily resolve the issue?

 Gas with 10% ethanol requires a richer mixture than "straight" gas. With "straight" gas a new O2 sensor will try and maintain a 14.7:1 ratio; that's pounds of fuel to pounds of air. With 10% ethanol added, that ratio will drop down to somewhere around 14.2:1 to 14.4:1. That's another way ethanol impacts your wallet, of which most are unaware.  

If it's not a vacuum leak, then it does seem to be some sort of fuel delivery problem, although probably right on the "edge' of being adequate.

Ooops--backwards!
 ---that's 14.7 pounds of air to one pound of fuel.
Otherwise, we would all be bankrupt!

If you reset the code, fill up with any fuel, and drive you can go without issue until you shut off the engine. Then when you restart the codes show. However, if you drive with the check engine light on, then fill up with ethanol free, the check engine light does not come on when you start the car and it stays off.

Filling up with 10% ethanol gas does not resolve the issue.

Using ethanol free does.

I don't understand it. The car drives ok either way but I never reach high rpm...I drive like the old man that I am.

I will check the vent though, good idea.

Thanks