Hmm very interesting info. Well i can say i did notice a difference from cleaning my MAF sensor from the other thread, but realized that wasnt the problem. I just changed the spark plugs yestrerday and its a night/day difference, wow i cant believe it. No more losing boost and hesitations/leaning out under full throttle, so smooth and even more powerfull..

But i do notice one thing that has not gone away, going into first, if im going slow then start accelerating it feels like a dead spot, like turbos kicking in or something, maybe that's what it could be. But is that what you have ruiner? I wonder if i should just buy a new MAF sensor to be fully "fresh" in this car, im starting to love it after yesterday haha

 

Keep in mind, when I thoroughly cleaned my MAF for the first time in September with the electronic cleaner (the ONLY variable in the equation), my STFTs returned back to normal.

An important note for reference (assume a warmed up car):

My STFTs with what I thought to be a "contaminated" MAF at idle were -15 to -22%.

*I then thoroughly cleaned my MAF with electrical cleaner*

After cleaning, my STFTs at idle were typically -5 to +2%.

After 2+ months of running, my STFTs have returned back to -20 to -22% yet again.

*I then half-assed cleaned my MAF - not as thorough as the first time*

After reinstalling my MAF, my STFTs were back in the -12 to -15% area at idle.

 

I changed my plugs @ 27k and have 33k on my car now. I highly doubt that it is my plugs given the fact that the "hesitation" IMMEDIATELY cleared up after I cleaned my MAF for the first time. I do not recall getting hesitation after changing the plugs.

The ONLY variable that I changed was the MAF cleaning. As soon as I did that, my low-speed hesitation was eliminated.

I think that my MAF was (is) on its way out before I cleaned it. I probably extended its life so to speak by doing so, but who knows.

Perhaps the electrical cleaner, being cold, had some effect? Is the temperature sensor inside of the MAF or is it in another place near the throttle body? Did cleaning it with the super-cooled cleaner somehow affect the results and mess with the atomic lineup in the metal (such is how you can cryo-treat rotors and make them more solid). Did that skew my data? I really dunno. That would be a question that I need to ask the Porsche techs.

 

It looks like you are not into open loop which would ZERO out your Fuel Trim readings at WOT at any engine speed. Your air flow at idle of .76 lbs/minute is right, but the air flow at WOT (is all over the place. It's peak of 31 seems low. This is assumiing you kept the pedal to the floor for the logging. You should have a smooth iincrease in air flow readings and I suspect you should have been around 36 - 38 lbs of air.

I would check the wiring in in the MAF cable. For some reason, you are not going out of closed loop at WOT. Is a pin broken in the connector? My guess is that your MAF is fried.


After looking at the graph again, it looks like you punched it for a second to go open loop rather than log a WOT pull. Is that correct?

 

Yes, I was WOT in most cases. You can see on the graph where both the right and left bank on the STFTs zero out after I let off of the throttle for a few seconds (the RPMs should be falling at that point). It is strange, to say the least.

 

A log of a steady WOT pull in 3rd gear would show more accurately what the MAF is doing. Take a reading of your calculated load at idle along with intake temperature. If those show right and the MAF increases smoothly up the full range, you may have a fueling problem unrelated to the MAF.

You graphs do not have any range indictaed, only a number. What is the low and the high of the air flow at idle.

 

Scale ranges:

STFT = -20 to 20
Airflow = 0 to 35lbs/min
RPM = 0 to 6750

I might try and do a 3rd gear pull tomorrow. It seems as if my fuel trims are learning positive (both long and short) during accelerated pulls and negative during idle.

I will say this, though, my symptoms are EXACTLY as described below:

When a MAF sensor gets contaminated, it skews the transfer function such that the sensor over-estimates air flow at idle (causes the fuel system to go rich) and under-estimates air flow at high air flows (causes fuel system to go lean). This means Long Term Fuel Trims will learn lean (negative) corrections at idle and learn rich (positive) corrections at higher air flows. Short term fuel trims will run even more lean (negative) at idle as well as they are the quick-acting fuel trims so to speak.

 

When you at WOT, the STFT (and LTFT) should be ZERO at all times. There should be no movement of them at all.

 

I'll log throttle position for you as well. I know what they are supposed to be, but it doesn't appear as though they are. I am leaning towards my MAF being on the way out...

 

I would think that at WOT, you should be seeing around 38 lbs or more of air as measured by the MAF. That's what I get in my 996 and you should be pumping in more air than I am. I understand what you are saying as to what you saw by cleaning the MAF (going from a rich mixture to a lean) but that is the complete oposite of what will happen if the system is running properly. A MAF will read less air and cause the engine to run lean. That is why I suspect that it was just a coincidance that it happened the way it did and there is something wrong with either the MAF itself or it's related wireing. I think you also said there was no change in idle when you disconnected the MAF. Make sure your vacuum line is connected properly to the fuel pressure regulator too.

You know it's going to turn out to be something simple and obvious in the end, so don't lose any sleep over it.

 

You might be right. I agree that I should be pulling in more air as well. I seem to recall it being in the mid 40s when my system was running well, but don't quote me on that. Maybe the MAF is all screwed up.

However, there is an obvious correlation between me cleaning my MAF and system returning to a more "normal" state for 2+ months. There is no denying that as it was the ONLY thing that I changed and thus, the only variable. I did it that way on purpose so that I could check my progress. I noticed a physical improvement and the datalogs back that up.

Whatever my issue, it has to affect both banks. Fuel pressure? That would affect both. A loose vacuum line would affect both as well. However, a loose vacuum line typically would not result in a smooth idle. I do know that when the MAF is gone/busted, many systems will resort back to a stock table and estimate what it "thinks" is the airflow at a certain RPM (the best that it can given throttle position, rpm, etc). The airflow at idle will be virtually the same even if the MAF is defective. I would assume (and this is ONLY an assumption) that Porsche programmed in a stock air flow rate table just in case the MAF dies. I am sure that the ECU will err on the side of caution and pump in more fuel just to make sure that the engine can function. I DID smell more fuel when the MAF was not connected...

 

I don't think the 996tt long fuel trims go to zero in open loop mode. They are still affecting the full power open loop mixture.

 

Wow this is so damm confusing

 

A bad MAF causes over rich at low speed. The ECU adjusts itself leaner (neg fuel trim). The neg fuel trim runs overly lean at higher loads.
The excessive neg fuel trim can throw various codes.
The lean misfire may throw misfire codes.

 

Yes, that is exactly what I was experiencing as I only get the codes at idle (very negative STFTs) and my LTFTS are positive after a drive under throttle:

When a MAF sensor gets contaminated, it skews the transfer function such that the sensor over-estimates air flow at idle (causes the fuel system to go rich) and under-estimates air flow at high air flows (causes fuel system to go lean). This means Long Term Fuel Trims will learn lean (negative) corrections at idle and learn rich (positive) corrections at higher air flows. Short term fuel trims will run even more lean (negative) at idle as well as they are the quick-acting fuel trims so to speak.