FAQ: Mass Air Flow (MAF) Sensor Issues?

Abaddon wrote:

Eldo wrote:

As far as the pressure-regulator goes, how many PSI are there in a 'ton' of fuel pressure?

I believe a "ton" of psi's are nothing but a plethora of single psi's, therefore, enough to run the car

Hmmm - I was thinking that, depending whether it's English or Metric, a 'ton' of fuel pressure would be between 2,000 and 2200 PSI...

Eldo wrote:

I see that you've covered all the usual suspects but one - have you checked for a 'classic' vacuum line leak? On an open-loop cold start, there's enough enrichment to handle a small vacuum leak; a warm start is leaner... Have you checked all the vacuum hoses with eyeballs and spritzes of carb-cleaner?

As far as the pressure-regulator goes, how many PSI are there in a 'ton' of fuel pressure?

Not really basing the "ton" on scientific data, but rather a strong burst of fuel from an ill timed depression of the the fuel rail valve ( don't try that at home). As far as vacuum line leaks, we have replaced a bunch of lines, not each and every one, might try the carb cleaner spray trick. But from what I am reading online, a poorly operating Fuel Pressure Regulator can cause this symptom.

EDIT: Sprayed everything down around the intake with carb cleaner, but no change in the idle. Where would one insert a fuel pressure gauge on our cars to check the fuel pressure?

BMD wrote:

EDIT: Sprayed everything down around the intake with carb cleaner, but no change in the idle. Where would one insert a fuel pressure gauge on our cars to check the fuel pressure?

On that schrader valve that you depressed.

L67 wrote:

Opened up the pins in the connector to get better contact and its working again...cross your fingers boys. P|

I don't know if your around Matt, but can someone tell me how to open up the connector at the MAF sensor to check the contacts?

BMD wrote:

L67 wrote:

Opened up the pins in the connector to get better contact and its working again...cross your fingers boys. P|

I don't know if your around Matt, but can someone tell me how to open up the connector at the MAF sensor to check the contacts?

Disconnect the plug from the MAF sensor and then use a mini-flathead (or other small poking device) to bend the pins inside the holes of plug upward so that when you plug it back in the pins have more of a contact with the pins on the sensor.

Okay, thanks Matt. I am still curious if anyone has ever opened up a connector? Are they ruined if they are opened? If you have a bad connector, do you need to splice in a new one?

BMD wrote:

Okay, thanks Matt. I am still curious if anyone has ever opened up a connector? Are they ruined if they are opened? If you have a bad connector, do you need to splice in a new one?

If you have the connector kit (I think it's Kent-Moore) it's no problem at all - the factory connectors are intended to be field-repairable. You can find the kits on eBay. They are not cheap BUT if you find one of the sites that is still selling stuff from dealer closings I bet you can find a kit for way cheap. Most people who need such kits (dealers/indie mechanics) have them.

Alternatively you can get most connectors pre-wired in correct wire colors, to splice into the harness, from RockAuto.com or in my case my local NAPA.

Great info, thanks albert!

My Dads 99 PA just got this code, here is the freeze frame data I retrieved from the car...

Fuel Sys 1            CL
Fuel Sys 2            NA
Calc Load %        2.75
ECT (celsius)       27
STFT B1 %           0.00
LTFT B1 %           16.41
MAP (kpa)             33.00
Eng RPM              1043
Speed (km)          0
MAF ( g/s)             7.67
TPS %                   0.39

Can someone translate this? We are on our second MAF by he way.

- in closed loop
- Not applicable
- engine load 2.75% (scanned at idle?)
- coolant temp 27*C
- short term fuel trim 0%
- long term fuel trim 16.41% <--------------buttload off
- manifold pressure (baro sensor) 33kpa
- 1043 RPM
- not moving
- mass air flow 7.67g/sec
- throttle position at .39V (idle)

See if you can scan while driving and see what the short term fuel trim and the MAF is reading.....

Doesnt the ltft peg at 16.4?

Yes, but I've noticed my scan tool likes certain decimal values. Like it will say 1.8 but never 1.9. Maybe BMD's scanner maxes at 16.41 instead of 16.4? Whatever, it means it's extremely lean - dangerously lean. This happened to me a long while ago. I found two problems: my MAFF translator (AFC) was malfunctioning, and I had a vac leak at the blower. Fixed both and the problem never returned. I'm not sure which one caused the lean condition, or if it was a combination of the two.

I think a failing or dirty MAFF sensor could cause this. If it failed entirely, or were disconnected, should throw a DTC.

Thank you guys soooooooo much for responding to this. I don't have an actual scanner, its a code reader that provides freeze frame data as well and yes, we acquired that info at idle in park.

Scott - What should that LTFT reading be at idle?

Aaron - What is a MAFF translator and where is it located? And the blower you are referring to is for the HVAC system, correct? How is that related to engine vacuum?

I wanna help my Dad out here, he has already spent a pretty penny on a new MAF sensor ( returned for a second...) and a new IGN module for his no start condition, now this. Any info will be greatly appreciated.

LTFT should be 0 all the time. plus or minus 5 is pretty normal though. this just means the car is trying to pull fuel or add fuel.
When it starts to get outside the +-5 range it may be you need a tune up or to tune for the mods you have added.

The AFC is something aftermarket AA added to help adjust the A/F for his car

the term "blower" usually refers to the supercharger.

Thanks Mathew,

Mr.Riviera wrote:

the term "blower" usually refers to the supercharger.

Shoulda realized that one.

Well hes got new.... IGN module, plugs, MAFF, IAC, TPS, oxygen sensor, fuel filter, coolant sensor, we cleaned the throttle body, checked and replaced vacuum lines... the only thing left that I can think of aside from getting the injectors checked is the Fuel Pressure Regulator. In my FSM it says that "If the fuel pressure is too low, poor performance and a DTC P0171, could result" But his car drives really smooth and strong????

The 96 rivs have a lot of vac lines. Sure all of them were replaced? Some of them are not even avail from the dealer anymore.

Edit: sorry i went back to your original post that this is for a park ave not your riv.

What is the mileage on the PA, is it the L67 ultra or something else?

Yeah his car is a Supercharged L67 with 130000 miles . Not every vac line has been replaced. The vac lines we did replace we did with vac hoses, very tight fit.

BMD wrote:

Thank you guys soooooooo much for responding to this. I don't have an actual scanner, its a code reader that provides freeze frame data as well and yes, we acquired that info at idle in park.

Scott - What should that LTFT reading be at idle?

Aaron - What is a MAFF translator and where is it located? And the blower you are referring to is for the HVAC system, correct? How is that related to engine vacuum?

I wanna help my Dad out here, he has already spent a pretty penny on a new MAF sensor ( returned for a second...) and a new IGN module for his no start condition, now this. Any info will be greatly appreciated.

The LTFT basically tells you how the car has been running, and the STFT tells you how it's running right now. Based on what the front 02 is saying, the PCM will add/subtract fuel accordingly. This is reflected in the STFT. If the STFT remains high (+), the LTFT will go up and up until it can't go any higher. P0171 is the result. Basically, LTFT makes coarse adjustments in order to get STFT as close to zero as possible.....got me?

If you've replaced everything that you have, and you're convinced that you have no vac leaks and what-not, I'd be looking at the 02 sensor for a skewed reading. The fuel trims are directly affected by the 02 reading......

I haven't really been looking closely at this thread, but..
have a look and make sure one of you rear plug wires is not touching the 02 sensor wire. Its been known to cause all sorts of funny things if it is...

Abaddon wrote:

The LTFT basically tells you how the car has been running, and the STFT tells you how it's running right now. Based on what the front 02 is saying, the PCM will add/subtract fuel accordingly. This is reflected in the STFT. If the STFT remains high (+), the LTFT will go up and up until it can't go any higher. P0171 is the result. Basically, LTFT makes coarse adjustments in order to get STFT as close to zero as possible.....got me?

If you've replaced everything that you have, and you're convinced that you have no vac leaks and what-not, I'd be looking at the 02 sensor for a skewed reading. The fuel trims are directly affected by the 02 reading......

Let me see if I got this straight, so the PCM takes the O2 info and adjust the fuel mixture accordingly to keep the STFT as close to zero as possible by adjusting the LTFT, even if its to its detriment?

@ Karma, will do, thanks.

Well, yes and no. If the PCM reads lean, it will continue to add fuel until it's no longer lean (STFT as close to zero as it can). I believe P0171 sets when the LTFT reaches about +12 to +13%. It can only add so much fuel......so really, it's not to it's detriment, but merely trying not to blow holes in your pistons

EDIT: just add to Mr Riv.....yes, LTFT "pegs" at about 16%, and STFT "pegs" at about 20%.

I've never had a DTC for LTFT, reaching as high as +16.4 on my Riviera. It may be supposed to set the P0171, but it didn't actually happened in my experience.

Quote :

Let me see if I got this straight, so the PCM takes the O2 info and adjust the fuel mixture accordingly to keep the STFT as close to zero as possible by adjusting the LTFT, even if its to its detriment?

That shouldn'tt happen, because LTFT typically follows STFT's lead. If you are really maxed out lean at +16.4, your STFT should not be 0. Your scan might have been taken too soon, before the LTFT learned. If you can scan again, see if the situation has changed. Usually, if STFT is zero, LTFT will soon try to match it.

BMD wrote:

Abaddon wrote:

The LTFT basically tells you how the car has been running, and the STFT tells you how it's running right now. Based on what the front 02 is saying, the PCM will add/subtract fuel accordingly. This is reflected in the STFT. If the STFT remains high (+), the LTFT will go up and up until it can't go any higher. P0171 is the result. Basically, LTFT makes coarse adjustments in order to get STFT as close to zero as possible.....got me?

If you've replaced everything that you have, and you're convinced that you have no vac leaks and what-not, I'd be looking at the 02 sensor for a skewed reading. The fuel trims are directly affected by the 02 reading......

Let me see if I got this straight, so the PCM takes the O2 info and adjust the fuel mixture accordingly to keep the STFT as close to zero as possible by adjusting the LTFT, even if its to its detriment?

@ Karma, will do, thanks.

STFT= short term fuel trims
LTFT= long term fuel trims

how long is long, and how short is short i dont know.

the O2 sensor adjusts the fueling and the fuel trims change as a result. STFT changes fast and may not be at 0 all the time. LTFT is like an averaged value of the STFT that shows how the air/fuel has been over time.
For purposes of tuning i look at LTFT more than STFT

AA, i have never had the code set either and as much as i screw with my car i'm at 16.4 or -22 pretty often when i start changing stuff around.

I think I get it now. Learning new things is the one upside of having car trouble . In any case, as far as repairs go, I think I'll put my Dads, old O2 sensor back and see if that code re-appears, thanks guys, I'll update when I get more info. By the way Karma, O2 wires and spark plug wires were not touching one another.

AA wrote:

I've never had a DTC for LTFT, reaching as high as +16.4 on my Riviera. It may be supposed to set the P0171, but it didn't actually happened in my experience.

There are 15 conditions that need to be met in order for P0171 to actually set. Tuning and playing around for a day or two is more than likely not going to throw the code.



If any one of these don't get met, P0171 won't set.........

I drove around for a couple weeks with LTFT pegged at 16.4. It wasn't from experimenting, my AFC had gone haywire, and I had a major vac leak. No P0171 code, nor any code was set.

Most of those 'conditions' are irrelevant. They occur whenever the car takes a trip. The only ones that matter (assuming everything else is in order) are LTFT & STFT. I cannot remember if my STFT was maxed at +20, but I know LTFT was +16.4.

It would seem that if STFT is not maxed, the system is not actually lean by the PCM's definition. This could explain why BMD, Mr.Riviera and myself didn't get the code?