Tech question- running rich

[josh71485]

Hello all,

Ok, for some time now my car has been throwing the p0172 and p0175 codes. (System too rich bank 1, system too rich bank 2). It has also had a slight intermittent misfire at idle, and even sometimes a tiny stumble upon initial acceleration from a stop. Excessive popping coming from the exhaust on deceleration, and now and then I can catch a pretty good smell coming from the exhaust. This started about a year after I installed my blower and had it tuned professionally. Initially it ran great and had none of these problems, and then one day on my way to work, the MIL came on and these are things I've noticed since then.

2002 GT 5-speed

Mods are:
Procharger p-1sc, intercooled
Accufab throttle body and plenum
42lb FRPP injectors
Stock MAF with MAFia interface adapter (setting 4)
Focus pump
Kenne bell boost a pump with a 4psi hobbes switch(50% volume increase)
Mac off road prochamber
Stock cat-back with welded in flowmasters.

This is a regular procharger setup (blow through)

What would cause these problems?

I have a scan tool, and the freeze frame data always shows the malfunction happening at idle, when warm, 0 mph. Most recent shows 799 RPM, closed loop, ~25% calculated load (which seems pretty high), ~155 degrees coolant temp, 0 MPH. The code always gets thrown in very similar conditions though.

I can get other PIDs if needed. If anyone has any ideas, please let me know.....I've hunted and searched on the forums and talked to everyone I know and been living with this for a year now.

Any help would be greatly appreciated.

 

[wmburns]

symptom of a contaminated MAF

My normal expertise is in fixing/trouble shooting stock set ups. Your set up is far from stock.

I am going to take one swing at this and then recommend that a professional tuner be brought in.

Have you cleaned your MAF? A contaminated MAF will overstate the air flow at idle and under state under load. This causes the motor to run rich at idle and lean under part throttle.

Evidence of this can be seen by looking at Long Term Fuel Trims (LTFT) at idle and part throttle. If it is constantly going from subtracting fuel at idle to adding fuel under load, then likely the MAF is dirty.

Never use a product to clean the MAF that will leave a residue! Use only products designed for the job.

Be careful running oiled air filters as this will contaminated the MAF as well.

 

[josh71485]

The MAF was the first thing I looked at since the code showed up in both banks. Sometimes bank 1 will trigger the MIL first with the bank 2 code stored as 'pending'. If I don't check the codes right away, they will both be there.

Anyway, i cleaned the MAF with electrical contact cleaner and reinstalled. No difference. I also cleaned out my IAC valve at that same time. Also, no change.

My LTFT's for both banks are pretty stable at -25
STFT's start close to 0. as the engine warms up, they slowly work their way to -20.3. This is all at idle. Stepping on the gas pedal richens the STFT values. Interestingly enough, the calculated load decreased as engine RPM increased.

I did notice some ridiculous variations in STFT's

Some data I recorded....
Frame 8- 1448 RPM, 16.9 % load, ST FTRM11 32.0%, ST FTRM21 29.7%
Frame 9- 1406 RPM, 17.3 % load, ST FTRM11 21.1%, ST FTRM21 -91.4%

Quite a swing considering a difference of maybe 2 seconds, same MAF flow, ignition advance, IAT, coolant temp, etc

Also,
oxygen sensor 1, bank one shows voltage switching between .925v and .009v
oxygen sensor 2, bank one shows voltage of steady 0.00
oxygen sensor 1, bank two shows irregular switching between .940v and .024v
Oxygen sensor 2, bank two shows irregular switching between .870v and 0.00v

My tuner said he turned off the two rear oxygen sensors....

As for bringing in the professional tuner, my guy went out of business about the same time this happened. Otherwise he would have it. There are no other tuners I would trust that are even remotely close to me.

 

[wmburns]

STFT's will jump all over the place. In fact, be concerned if they didn't.

The LTFT are at the fuel trim limit which is why the DTC was thrown.

I still suspect the problem is with the MAF. Perhaps the "basis" settings of the MAF has moved, the switch failed, or the MAF is just plain bad, or something along those lines.

Try doing some data logs graphing the MAF, TPS, RPM, and load while doing some pulls. Suggest NOT doing WOT pull as this will switch the PCM into "open-loop" mode.

Someone else will have to help from here on. Any boost experts out there?

 

[GoBabyVroommm]

I'll jump on this as well some ideas: Ill list them anyway if someone has already.

1-O2's
2-Maf
3-If your running a check engine code, depending on the code it starts messing with timing. You have a blower so it might be cause problems when it does that.
4-Cracked vacuum line maybe.... I wish i could see the car it would help so much more.

I believe its a stupid sensor.

 

[josh71485]

I too suspect that the air entering the engine is not being measured accurately. I can't help but wonder if the problem lies with the MAFia though. Is there a way to test either component? I was always told that you can test a MAF by unplugging it at idle and seeing if the engine runs better when forced to run on pre-programmed fuel/timing tables. I have done this and it has not really changed much. I wonder if the result would be similar if I was to bypass the MAFia altogether and just hook the MAF directly to the wiring harness like from the factory. Obviously, this would just be a test at idle, as I do not want to risk pegging the meter and leaning out my AFR's significantly under any kind of load.

Is MAFia failure unheard-of?

Humid days seem to make the engine exhibit it's worst characteristics- I was even wondering at one time if the barometric pressure sensor was sending erroneous signals or just not functioning.

 

[wmburns]

Another thought of things that could cause a chronic rich condition is related to the fuel pressure sensor. Either it is bad or the vacuum reference line is disconnected, blocked, or leaking.

Consider datalogging the Fuel pressure. Or just replace it as there are numerious post regarding apparently good fuel pressure sensors that are in fact, bad. The part is not $$.

The report of good running after the tune would indicate the tune is OK. It is reasonable to look at a failed component as the cause for the change (assuming no other changes made).

OBTW, your idea to test the MAFia will not work because the voltage output of the MAFia has been indexed and the tune adjusted to accept the modified MAF. The stock MAF would have a stock voltage output which the PCM is not tuned for. In fact, I suspect that the motor would run even richer.

For your idea to work, you must also adjust the tune to match the MAF.

 

[josh71485]

The FRPS has been another suspect. When the signal is viewed through my scan tool, it shows a pretty standard 39-41 PSI at idle. It then increases from there as load is increased. At the same time however, my electronic sending unit auto-meter gauge shows 26-28 PSI at idle. Not sure which would be more accurate. I need to get a mechanical gauge and hook it up just to get a reading that I know is right.

I will have to verify that there is vacuum at the sensor. I do believe I tapped into my vacuum right there to run to my vac/boost gauge though. I get about 20 inches at idle


You make a good point about my theory of testing the MAFia

 

[wmburns]

I don't think you understand the difference between an atomspheric pressure and a delta pressure.

An external pressure guage will measure pressure relative to the atomsphere. The FPS measures pressure relative to the intake manifold. This is called DELTA pressure.

As boost pressure goes up, the absolute pressure has to go up to maintain the correct delta pressure.

So, for a delta reference, the pressure should remain in the 38-41 PSI range.

For an atomspheric reading, it is is normal for the reading to be low at idle. Remember that the intake is under VACUUM at idle. The pressure should rise as boost rises. It should always remain boost + 40 PSI.

Try this test. At idle, disconnect the FPS reference line. The external gauge and the FPS now have a common reference and should read the same (about 40 PSI).

Remember, this is for a test. NEVER drive the car with the vacuum reference line disconnected. If done, the A/F ratio will go lean under load.

If the FPS reported pressure is increasing under load, this does not sound correct.