This has been going on for over a year, but I want to fix it.

(P0101 - MAF sensor performance, P0171 - Fuel trim system lean)

148k miles, new plugs and ign. wires did not change anything.

It was throwing those codes fairly often a year ago, not as often now, but the rough running under light to moderate acceleration at lower rpm’* is consistent. Once it gets to rpm under acceleration, it smooths out, maybe with slightly less power than I remember it having before this started.

I read thru diagnostics for the 2 codes from my AllData account, but most of them require a DRB scantool to get anywhere in the diagnosis flow chart.

But, taking a cue from where the diagnostics appeared to be going, I put T-taps on the TPS and MAF sensor wires and took some voltage readings.

Results:
TPS - reference voltage was exactly 5 volts. TPS wiper voltage reading was 0.65 at idle, and increased proportionally with opening of the throttle. Change in the voltage was smooth and very repeatable with throttle movement.

MAF sensor – power supply was close to system voltage (right at 14 volts). Output was 4.32 at idle, approx. 4.0 volts at higher rpm’* with no load, dropped to about 3.4 momentarily with a sudden WOT application. Again, seemed smooth and repeatable. I don’t know what the normal voltage readings are supposed to be - the almost worthless Alldata account doesn’t go into any detail like that – everything relies on the DRB tool to tell you if something is good or not without giving you the specs. Or maybe that’* not Alldata'* fault – maybe that’* how GM does things?

So – bottom line, unless someone here tells me otherwise, I don’t think the problem is the TPS or the MAF sensor. Anything else that would normally cause P0101 and P0171 and result in rough running under light to moderate throttle at lower rom'*?

Now – while I was rapidly blipping the throttle open with the engine running, I did hear what at first sounded like a small valve or something rattling in time with engine speed – at first it sounded like what I would imagine a PCV valve would sound like if it was slamming open and shut in time with the engine. I ruled that out by temporarily pulling the PCV valve out of the valve cover and closing it off – noise still there when under sudden WOT.

I now think it is an exhaust manifold gasket leak. I only hear it when rapidly accelerating the engine with the hood up. Appears to be coming from the front bank – which is good since that would be the easier one to work on. I also stopped the engine and pulled one front bank plug wire at a time and restarted the engine after pulling each wire and blipped the throttle. The noise was still there *EXCEPT* when I had the no. 1 plug wire disconnected.

A question: Would it make sense that a leaking manifold gasket (hopefully it'* not a crack in the manifold) would cause those codes? Like maybe from a small extra amount of air getting introduced upstream of the upstream O2 sensor? Or would an exhaust manifold gasket leak be a separate problem from whatever is causing those two codes?

TIA for ideas.

Check out these write ups.
https://www.gmforum.com/showthread.p...78#post1390964
https://www.gmforum.com/showthread.p...36#post1390823

Go threw those and see if that helps.

If the maf is setting a code, it'* likely a maf issue. Maf issues can cause lean bank one issues (171).

Since you've been going for over a year like this, I'm sure your car will be happier now that youve decided to fix it.

That was a big help, Dan. It did mention that a leak in the exhaust could cause the lean reading. I think with the fact that there does appear to be a leak (from the sound and the spark plug wire test) and that a leaky exaust *can* cause the codes and symptoms, I'm going to replace the manifold and pipe gaskets - that should fix the leak, and likely the codes and rough running.

I want to explore that more, Bill. Do the voltage readings I got seem reasonable? The MAF operation descriptions talk about the reading going higher at increased air flow. My voltage *dipped* with more air flow. So if they meant the *voltage* increases with increased air flow, then maybe there is a problem with the MAF. But if what the voltage represents (air flow as read by the scanner) is an inverse relationship (i.e, air flow = -K voltage), then that would be evidence that mine is working correctly, though I don't have any numbers to compare to.

Does anybody have real numbers or specs. on this particular MAF design that I can compare to my readings so that I can either determine that the MAF is either good or bad? At $100± for a new MAF, I don't want to replace it just to see if that'* the problem.

Ahh - there is no edit function. Does that capability come with more posts or becoming a paid member? I needed to fix some minor typos.

Anyway - are there any detailed sketches of the complete exhaust system posted here, or any exhaust manufacturer sites that show accurate exhaust system layouts specific to the vehicle?

Any partucular details or gotcha'* on replacing the left bank manifold and pipe gaskets? Is it as simple as removing the top engine mount and a few incidentals, or does it get more complicated to get to things?

you can only edit for a certain period of time or before someone posts.

did you ever try looking at the sensor and see if its carboned up? they make a maf cleaner you could spray on sometimes that will get you by.

Thanks for the explanation.

I did clean it with the proper cleaner a couple of years ago. But I will do a visual on it. I have my doubts that the MAF is the problem since it is responding to air flow changes.

It still would be good to have some information regarding the normal voltages one should see from it. Do the GM FSM or diagnostic manuals not give expected voltage ranges under certain conditions, like idle, and which direction the voltage goes in when air flow increases (i.e., open throttle)?

If you are getting the code, the maf is setting off something based on what the PCM sees from it. Try this. Unplug your maf and get the car to idle onit'* own. May take a few minutes. Clear the codes, drive that way and know you'll have a maf code. See if the lean code returns. If it doesn't, then it'* highly likely your maf is bad.

The voltage and readings don't mean the maf is 100%. Say it'* 90% and that'* why you are getting issues. Another thing to check is the wiring going to it. Being over the exhaust sometimes that wiring can get brittle and have issues.

Bill - I had hernia surgery this morning, so it will be probably 4 or so days before I can get out there and do that test (Doc says no driving until Tuesday) - sounds like a good idea to at least rule it in or out. Will definitely do that and report back.

On the MAF wiring - the voltage measurements prove that the V+ and ground wires are good (and all indications are that they are solidly so - i.e., not intermittent). So, if the wires are a problem, it would have to be the signal wire going to the PCM. If things behave differently on your test of unplugging he MAF, then that would be pretty strong evidence that that wire is good (i.e., unplugging would be the same as an open signal wire, and if that wire were shorted to ground from bad insulation, then I would be getting 0 volts out and not the changing volts in the range of 3.4 to 4.32 that I'm getting.

I meant to comment about that in my last post - Before my original post, I did unplug the MAF sensor connector and started it up. It had no trouble running and idling right away. Not sure if that means anything.

Anyway - When I'm back up and about, I will follow your instructions about doing that, resetting the codes, and seeing what code or codes show up.