Manager Forum

I have a customer with a 2004 Impala 3.8 with a P0171 lean condition bank 1. The fuel trims seem to fluctuate a lot. Most the time both will be in the + double digits.

Looking on Identifix shows the MAF sensor or naturally a vacuum leak being one of the more common problems with this code and vehicle.

When I unplug the MAF the fuel trims still are double digits. Checking the MAF with lab scope shows 2.5 - 3 grams/sec at idle.

The fuel pressure & volume is good

When I smoke the vacuum system no external smoke is visible. When I take the oil filler cap off it's full of smoke. If I spray carb cleaner in the valve cover the short term fuel trim goes -20 and the engine will speed up slightly.

Should their be smoke in the oiling side of the engine and should carb cleaner effect the idle and fuel trims? I wasn't sure if the smoke could be getting through the PCV valve (it's inside the plenum below the MAP sensor) or am I seeing a leaking intake gasket?

Well, I'd be thinking that smoke should not be getting past the PCV valve. If it is, then I'd have to fault the valve. I'm thinking you may have leaking intake manifold gaskets in this situation.

Am agreeing with Tim with a slight curve. When you take the oil fill cap off does the engine start to stumble and run worse or stay the same? Was this vehicle in other shops before and what was done? If it was tuned somewhere else the improper PCV may be installed. If the intake is internally blown usually the vac through the oil fill will peel the chrome off of a trailer ball or in some cases i've seen whistling noises coming from the air being sucked through a rear main seal. Another issue may be incorrect fuel or a weak injector. The pressure from a good smoke machine may push pass a pcv. I'm seeing incorrect fuel more and more. I would play detective and really grill the customer about what was done just before all this happened. As techs we know just happened means in the past two or three weeks and customers think 20 minutes.

Steve is correct. Smoke will go right through a pcv valve and fill the crankcase. To prove an intake leak that is creating a vacuum on the crankcase, pull the pcv out of the valve cover and block the valve cover hole. Pull the fresh air hose off and place a compound gauge in the hole. Run the engine. You should build a slight pressure from the blowby. A leak is indicated when vacuum is present.

If you have just one bank skewed, you can usually rule out fuel pressure and MAF right away since these would act on both banks.

ricmorin wrote:Steve is correct. Smoke will go right through a pcv valve and fill the crankcase. To prove an intake leak that is creating a vacuum on the crankcase, pull the pcv out of the valve cover and block the valve cover hole. Pull the fresh air hose off and place a compound gauge in the hole. Run the engine. You should build a slight pressure from the blowby. A leak is indicated when vacuum is present.

This is a 3.8 and does not have the PCV valve in the valve cover with a hose going to it. It's inside the plenum with the MAP sensor on top of it.

ricmorin wrote:If you have just one bank skewed, you can usually rule out fuel pressure and MAF right away since these would act on both banks.

I agree with this statement but doing some research on iATN and Identifix it seems this isn't true. I have found many fixes with the MAF only kicking on this code.

When pulling the oil filler cap of it does not change the idle of the engine. I have seen them kill the engine when pulling the cap off, this one shows no difference. I just wasn't sure about the smoke being in there.

I guess I'm back at square one on this one.

Ah yes. Been a while since I've seen one of those. Have you had the pcv out? I've seen the little o-rings in there fail and cause lots of vacuum to pass.

I have looked at the PCV valve and O'rings. It all seems in good shape. The car runs fine other than the light is on and looking at the fuel trims shows it's got something going on.