FAQ: Mass Air Flow (MAF) Sensor Issues?

hmmm. fuel pressure regulator? I see fuel pressure on your list already, but if your FP regulator is stuck or something, and FP was very low, this could definitely result. It would seem to run okay but be way off all the time.

Wow, haven't thought of that, Derek. I do have an extra fuel pressure regulator I could swap in. Adding to the list...

1) MAF sensor
2) O2 sensor
3) LIM gasket (check spark plugs)
4) vacuum line leak
5) fuel pressure
6) fuel filter
7) fuel pressure regulator

have you unplugged the maf to see if the LTFT changes in VE mode?

Good idea, Matthew. I will try it.

Filled with a fresh tank of gas, half & half from two stations. Same result, just making sure.

I then ran a bottle of Techron through for good measure.

Unplugged the MAF sensor on the way home. This changed everything. LTFT actually returned to near zero (-2.1) with the MAF unplugged, but under heavy throttle, it went right back up to being lean, and when stopped idle, LTFT dropped to super rich (-40!), almost stalling the engine. Threw a P0102 code for MAFF out of range, no surprise. Plugged it back in, code was reset, now back to where we were. Based on the result, I believe the MAF is functioning correctly. I will clean it anyway, but now I'm suspecting a vacuum leak more and more. Just need to look for it.

I am thinking if I can't find a vacuum leak, I am due for a new O2 sensor anyway. A couple questions:

1) Is LTFT determined by feedback from the O2 during idle and cruise? Would a bad sensor cause a lean condition?

2) Can coolant seeping in to the cylinders from the LIM gasket cause the O2 sensor to fail?

Thx for help.

I can't answer question #1 but I can Definitively say YES to question #2.
I've had it happen and have seen it in the shop before.
After replacing the LIM and solving the coolant leak and replacing the O2 sensor's we would do a Motorvac as it would help clean out the CAT.
But we were a dealership spending other peoples money. You can probably get away with some Seafoam after fixing the leak and poor combustion.
But I would continue looking for any other vacuum leak as well.

After thinking about question #1, I would think that it would have an effect during Open Loop operation. It should.

Could a Motorvac or Seafoam actually clean off a dirty sensor? No need to worry about the cat.

1) MAF sensor
2) O2 sensor
3) LIM gasket (check spark plugs)
4) vacuum line leak
5) fuel pressure
6) fuel filter
7) fuel pressure regulator

Sensors are very easily damaged by foreign substances.
Especially when hot.

This honestly doesn't jump out to me as a vacuum leak based off the symptoms and code. I had a vac leak for a long time before actually finding the cause of it. I had my LTFT pegged at 16.4 at idle and if the tune allowed it to correct more than that, It would have, the STFT was also pegged at 20.4 or whatever the max is. It went away very quickly with throttle and the car basically drove fine. There were no detectable symptoms and I didn't have any major KR problems from it. If the only time you see relief from the LTFT being pegged is when the PCM commands deceleration enleanment, I lean towards fuel supply problems.
What really makes me doubt a vacuum leak in your case is that it's still badly lean with throttle which doesn't happen with a typical vacuum leak. In fact, with boost, I would think the effect would reverse if the leak is bad enough.
I don't think a fuel pressure regulator could cause a problem this severe. I would check out the fuel system. I would change the fuel filter for good measure and hook up a guage and check fuel pressure since you have one. Clean the maf sensor too. After that, I would suspect the O2 sensor.

Thanks very much. I originally didn't suspect vacuum either, and after reading your post I am leaning away from it. A vacuum leak shouldn't cause a lean condition, right? I'm going to get under the hood and inspect anyway, clean MAF sensor, and replace spark plugs today.

AA wrote:

Could a Motorvac or Seafoam actually clean off a dirty sensor? No need to worry about the cat.

1) MAF sensor
2) O2 sensor
3) LIM gasket (check spark plugs)
4) vacuum line leak
5) fuel pressure
6) fuel filter
7) fuel pressure regulator
8) fuel pump

Fix.t

The problem has been solved; turned out to be one of the simplest fixes, although a little tricky to identify. It was the MAF sensor - needed a cleaning.

After unplugging the MAF a few days ago, the result led me to believe the sensor was working, but I didn't know for sure if it was working correctly. I tentatively ruled out the MAF sensor, but I hadn't actually looked at it yet.

When I opened the sensor to inspect, I noticed some buildup on the wires. A quick session with CRC electric parts cleaner removed the build-up. Reinstalled, took the car for a long drive, and the scanner eventually showed near LTFT between -1.6 and .8 at cruise, and a lock-in at 0 for WOT. There is no more audible knocking, and KR levels are back down to 1.5º max. I managed a brief blip of 3º, but not sustained. Before I was seeing 5-6º just barely trying to accelerate on the freeway.

Thanks to everyone for your help. My car runs great now, and if you've ever wondered how important it is to clean your MAF sensor, I'll say this experience proves it can be EXTREMELY IMPORTANT. From now on I'm going to clean mine every 10k miles.

10k might be overkill, but i do agree that a clean MAF can make a world of difference.

I clean mine with MAF Sensor Cleaner (available at auto parts stores) every time I remove the air filter for cleaning.

Today i leted my car bescanned..and got a code P0171. what coudl it be?
car idels bad.

Vacuum or exhaust leaks before the CAT.
With all the recent work you've done, try spraying carb cleaner in and around the intake and vacuum hoses and see if you get any change in RPM's
Just DO NOT use carb cleaner on the MAF. Use MAF cleaner.
Next I would look at what the FSM has to say about the code.
P0171 = Lean Mixture Bank #1

i hear some vacum leak sound beheind the engine and between the brake booster. but cant find anythgin when lookign there.

sorry rick i dont know what in english mean FSM.
i checked on rockauto wor maf and when saw the picture knowed what the shortname meaned.
i took the MAF off. it had a bit dirt on it so i used compressed air to blow it off. how do i know if its not broken?
on doing works on the engine it may have got damaged?

jusp pissed off. have done so much and now weard things start to happen.

defendly need to get better vacum lines. those there are too soft when it gets hot under hood.
also the TC light keep comming sometimes on and then goes off again. but abs dos not come on at all. so abs sensors should be fine..

also...there was one bad thign when i buted the engine together. the gasket between SC and throttlebody wasent good enaf. and it started to leak. it should have had one extra hole in it bit it dident. so it made water pressure in it and broke it on bottom. so some of the water was sucked into the intake and made the car make white smoke from exhaust. could have been that the o2 sensor or how they are called in the exhaust got damaged?

If the MAF sensor had any visible dirt on it, that was likely part of your problem. Mine had just a small piece of build-up on the front wire. I would use a spray cleaner designed for electronics, or MAF sensor cleaner instead of compressed air.

So, maybe the MAF was the reason for the P0171 code, and your rough idle could be from a vacuum leak. Probably was a combination of problems causing multiple side effects.

Automotive Acronyms: https://rivperformance.editboard.com/info-reference-f50/automotive-acronyms-t480.htm#4394

Timo,
FSM = Factory Service Manuals
MAF = Mass Airflow Sensor......It is very delicate and susceptible to damage by using the wrong type cleaning chemicals on it. (As Aaron mentioned)
You can use Electrical Contact Cleaner or a product made specifically for it....Mass Airflow Cleaner.
If there is an extreme amount of dirt or grease on it you can use one of these cleaners on the wires in the airstream and use a very soft toothbrush if needed.
You NEED to FIND and FIX the large vacuum leak near your break booster for sure.

My gf's regal has been running with a P0171 code for about a year. It idles smoothly and mileage has not suffered. In fact, it's been increasing and has stabilized at an avg of about 24 mpg. Try clearing the P0171 code and see how long it takes to come back, if it does at all. Like Aaron said, I suspect your idle is unrelated to that code.

your gasket story could have set off the code with the 02 sensor -by water coming into contact with a hot 02 sensor, or a dirty (or damaged, after blowing compressed air onto it) maf COULD trigger a lean condition. Did you have the P0171 code BEFORE "cleaning" your maf ?

I would clean the maf with contact cleaner, and unplug the battery for 15 minutes to reset the pcm.

Like in Jason's case, my lean condition didn't hurt fuel economy, either. I remember filling a tank and getting 22.2 MPG (likely this was better than normal because I was being very gentle on the gas until I sorted the problem out). My overall average for a year is 20.9 MPH.

i havent had a MAF plugged in for 2 weeks and on my last trip home i got 23.5mpg doing 75 the whole way. that's about what i normally get. i have a VE tune, but it's not finished or near enough to run with no maf all year.

Matthew, what does your engine do when you stop for a red light? Anything different than normal? Have you thrown any codes related to MAFF?

i readed it right now and truyed...i had some minutes ago a test drive again...and the ses cone back on ..
i truyed to unplug the maf but omg...it cone even worche!...the engine turned on SC and started to shake even more...just crazy..

What did you have to do to correct the coolant leaking in from the intake /supercharger area that you mentioned previously.?
Do you think you still have a leak internally.??

Snowdog wrote:

i hear some vacum leak sound beheind the engine and between the brake booster. but cant find anythgin when lookign there.

also...there was one bad thign when i buted the engine together. the gasket between SC and throttlebody wasent good enaf. and it started to leak. it should have had one extra hole in it bit it dident. so it made water pressure in it and broke it on bottom. so some of the water was sucked into the intake and made the car make white smoke from exhaust. could have been that the o2 sensor or how they are called in the exhaust got damaged?

Did you use the O-rings in the kit that seal up the coolant passages.?

Have you been able to solve these problems yet??
And if so, what did you have to do to fix it. ?