Zenon

Hello all,
A month ago I installed a 7psi pulley on my P1SC upgrading it from the 5psi that it had before. I then went to my dyno shop and we tuned it for the additional HP. The car drove excellent for a week, but then one morning when I drove her around she felt really low on power. I took her to my dyno shop to see what was going on, since the car felt as she was running out of fuel. Sure enough she was really lean on top.

My fuel pressure has been going down for the months leading up to the problem, so naturally I thought it was my fuel pump. I bought the LPE fuel pump, also changed the fuel filter, with no prevail. I since then have checked compression, spark plugs and wires, even swapped out the injectors just to make sure.

The problem has been progressively getting worse, now the car won’t even idle right, but one day I decided to unplug the MAF sensor and the car started to idle perfect. The fuel mixture returned to stock. The car runs somewhat Ok, if I am not under WOT, but without the proper fuel metering I would not expect her to. I then figured it must be a bad MAF sensor, so I swapped it out with a friend of mine, but the car ran just as it did with my sensor.

The question is: What could be the reason the car runs so crappy with the MAF plugged in?

I know when the sensor is unplugged the car refers back to the VE tables, and that is why the car starts to idle great. We have tried to reinstall the tune, thinking something may have happened to it, but nothing we have tried so far has yield to any improvement!

P.S. There are no Codes.
Car is 1999 6 speed coupe


Thank you for all your replies in advance,
Vince

Todd157k

Did you check for low voltage / bad grounds? Low voltage will reduce the fuel pressure, make the MAF read incorrectly, and therefore, crap the mixture.

5 Liter Eater

It doesn't sound like you have a speed density tune so if you have a MAF problem you are bound to blow up your motor since you can't meter anything past atmospheric without it. Do you see any codes when you unplug the MAF? You may not be throwing codes because they may have been edited out with your tune.

I'd have to see some logs to see why you idle better without the MAF.

5 Liter Eater

Weird that swapping MAFs did not change the behavior though.

Zenon

Battery voltage seems to stay around 13.3-13.9 , all the connections on the maf are good.

5 Liter Eater
I do see a low voltage maf code when I unplug the MAF sensor, but when it's plugged in, there are no codes. However, sometimes the car will throw a bank 1 and 2 too lean, but again that is due to the car running really lean with the sensor plugged in. When I unplug it, the car goes to running normal again. Also, because I know the car is not metering the air when the sensor is unpluged, I drive it very easy, because I don't want to blow anything up.

AVB

I would confirm the issue isn't in the tune. The car can run w/ the MAF disconnected, this is normal and you don't have to have an SD tune to do it. It won't be optimal but it'll run.

Arnel

Zenon

Today I installed my stock fuel injectors and the stock tune, I also disconnected the piping from the supercharger to the intake and I put an air filter right before the MAF. In another word, the car is back to stock except that the supercharger is the idler pulley.
Well, the car runs the same!!! With the MAF unplugged it runs great, but when I plug it in, it runs like crap. Therefore, it leads me to believe that is not tune related. Instead it must be something mechanical…. I wonder if the MAF sensor that I borrowed could have been in the same condition as mine, where it’s bad just enough that it does not throw any codes, but would make the car run like crap.

Tony @ MPH

Hmmm, I'm curious. This is a long shot, but what happens if you unplug your O2 sensors instead of the MAF -- does it still run like crap?

Zenon

I have not tried to unplug my O2 sensors just yet, but both sensors seem to read in the 600-700mv range when at idle, which I think it’s the normal range.

5 Liter Eater

O2's would come into play w/ or w/out the MAF plugged in. It would still be operating in closed loop. Could be something with the wiring harness to the MAF.

AVB

Get it logged and see if the air/frequencies being metered are out of line.

sacsrpc

CHeck for any airleaks after the MAF. Cracks in the airbridge or loose hose where it connects to the tb.

Hool97

One thing to note is that the MAF is scaled. Therefore if the scale is off it will have to try to adjust with STFT and LTFT. This can cause a rough idle. If the car runs fine without the MAF, i would look there first. Also is your temp sensor in the MAF or out of the MAF that can also through things off if not reading correctly.

gottiman

I am having the same problem on my car, I guess the voltage going to the Iat sensor is not right, so my maf sensor code is going off. I replaced the maf but found out its the iat sensor causing the Misfire, maf performance code, and Iat sensor high voltage code. I have a wiring problem somewhere, a wire got pinched or burned, or a rat chewed on it i think.
did you check your codes? are you getting po101, po300, or po113?

Zenon

I am not getting any codes. Also, when I check my IAT reading it is usually around 98-120 degrees, which is where it should be, I think...

leojnknsC5

Does your MAF have a screen and did you remove it?

Zenon

Yes it still has the screen on it, the MAF has never been tampered with.
I will take it to the dealership tomorrow, and have them try to diagnose the problem. They have no problem with looking at it, since the car is back to stock mode now, except that the supercharger is acting as the idler pulley.

I will update with further news.....

gottiman

let mwe know if its a wiring problem. -tom

Zenon

Well took it to the dealership today and I hate to say it, but it was something stupid.....A vacuum leak.
The vacuum leak was located on the bottom of the hose where it connects to the PCV valve and the throttle body. I could not see it, but you can sure feel it. DANG… I feel like an idiot.

Thank you guys for all your inputs. At least I know that my engine is good, since I did compression checks and all that crap…..he he he

Todd157k

Glad they found it but how did a vacuum leak bring your fuel pressure down? Seems a little odd. Let's hope it is just black magic and gone.