Somehow you got into another post, instead of replying to this one until now, and that is how we lost communication. I assume the three codes set at the same time. P0171 and 174 set when the fuel trim values reach a maximum of 25%. The fact that both of the banks are showing lean, is more than likely due to a central induction type fault. Did the plenum boot get replaced, from what you say it appears as though it did. There is a flat plastic piece clamped(or supposed to be) to your air induction pipe. The reason I say supposed to be, is sometimes this clamp is removed for one reason or another, and the piece can be dislodged and causing a central unmetered air leak. Check that this piece is fully connected to your induction pipe. A simple backfire through the intake can unseat it if not clamped. You can use a cable tie to replace the clamp, it should function. There is also a pressure blowoff on the intake below that plenum, it would have been visable with the intake removed. Sometimes, a backfire can deposit a chunk of carbon deposit onto the seat of this valve, and it can cause a vacuum leak. As always, someones work quality can also introduce a concern, I am not doubting the credibility of your mechanic, it is just food for thought. The reason that it is intermittent, is that it could be present at one time, say at idle when manifold vacuum is high, and at cruise it(fuel trim) can be brought back to acceptable limits, or vice versa. Of course fuel pressure is a possibility, lo pressure may cause lean conditions, but I am not sure of the P0101.
As stated, a scan tool that can view data can be helpful. If the fuel trim can be observed going lean, spraying(nothing flammable if at all possible) around the induction area and monitoring for a change, can sometimes be more conclusive than say, a vacuum leak that can't be seen or heard. I am not sure, but there may be a programming calibration upgrade on your vehicle that will address P0101, if I remember it was for 101 only though. It is just something to consider, and maybe coincidence. If you post the full VIN, I can validate that. The maf sensors seldom fail, and it probably didn't need replaced. The quote given that it needs replaced when presented to an oiled air filter is false, it can be cleaned with contact cleaner. I don't suspect you have injectors clogging, that would be more cylinder or bank specific, as opposed to both banks. Have you possibly filled with a high ethanol fuel(higher than 10%), I have not had experience with that scenario, but have heard that it can cause problems. Unfortunately, there are not many vacuum sources out of that manifold. If you can view fuel tim, you can also disconnect and cap the sources, and see if any make a difference. You have a PCV source, possibly a line behind the steering pump, the fuel pressure regulator line, and the power brake booster. The booster hose can be pinched closed for test purposes, no need to disconnect. Hope this helps some, and keep replying to this post.
Saturday, September 18th, 2010 AT 12:37 AM