Please Help! P0171 & P0174

Please Help! I have been chasing this problem for 15 months now.

I bought my 1997 Mustang GT, 5 spd in May 2003. About a month later (no mods yet), the Check Engine Light (CEL) came on. I went to Autozone and had the codes read. It cam up with P0171 and P0174.

These codes mean: P0171 (O2 Bank 1, Sensor 1 Lean) and P0174 (O2 Bank 2, Sensor 1 Lean). I have done alot of research on these codes, and have found that what this means is: The O2 sensors are reading a lean condition and are adjusting the fuel mixture richer and richer until it exceeds a predetermined value, then sets the CEL.

The car had 65,000 on it and I figured that replacing the O2 sensors was not a bad idea. So I did that and reset the light, drove for a few days and the light comes back on. While working on the O2's I noticed that the Passenger side exhaust flange to pipe gasket was leaking and I decided to replace that as well, reset codes, light back on. I took off the IAC valve and looked into port, the intake was clean! There was a light coating of soot, but no carbon chunks like I expected. Cleaned IAC and re-installed. Took off the EGR valve, also clean and functioning properly, installed a TOMCO carbon screen kit, and reinstalled, as you can imagine, the CEL came on again. I also looked (listened) for vacuum leaks, and used carb cleaner to help, to no avail. I also replaced the Mass Air Flow sensor, spark plugs, and cleaned my air filter with no result.

Next I started checking all of the sensor votages. The Coolant temp Sensor, and Intake Air Temp Sensor measured out OK. I adjusted the TPS voltage to .92V, and checked the voltage at the DPFE or Differential Pressure Feedback EGR sensor. The voltage here showed high at idle, but the EGR was sealing fully, and when i applied high vacuum to the EGR valve, the sensor changed. I removed and inspected the DPFE sensor, and noticed a lot of buildup (corrosion) inside one of the ports. So I replaced it.

This time, When I replaced theDPFE sensor, I reset the computer again. The CEL did not come on for almost two weeks. I thought I had it fixed, then one day BAM it comes back on. Most recently, I replaced Both tube kits for the PCV system, and also installed an adjustable fuel pressure regulator, as I had noticed that the pressure at idle was 31psi (vac hose on) and 41psi (hose off). I adjusted these up to 41psi (hose on) and 51psi(hose off).

Here are some observations by me.

The CEL ONLY comes on at idle and after the car is warm. I.E. driving to work, almost there, sitting at a stop light.

It never comes on on the expressway, or in town above idle driving.

The fuel pump is loud (whining), and I can hear it from the back of the car. When turn signals flash, the pitch changes, but I have measured voltage there and it seems ok.

The only thing I was suggested to do that I have not had a chance to yet, is to hook up a scanner that shows real time data stream, and watch the O2 sensor voltages while using an unlit propane torch to search around the intake for vacuum leaks. I am told that the O2's will read really rich if a problem is found.

I am stumped. I am having a hard time with this because I am an ASE certified mechanic, (right now I work for the Service Parts Organization of a major motorhome chassis manufacturer), and I should be able to do this!

Please, anyone that can help I will greatly appreciate anything you can tell me!

Sorry for the long post, I just want to make sure I mention everything.

Thanks!

I have.

I cleaned the MAF, then I replaced it.

After I cleaned my air filter I cleaned the new one....

Yep, forgot to mention it but I did.

Arrrggghhh!

Might it be a grounding problem? Also, what about the fuel pump noise?

That's one thing I did not do. I only checked pressure. Do you think it may be a volume issue?

You know, the first time it came on, I did not, but I have since installed one. The kit came with an upper (after MAF) and a lower (before MAF). I thought that may be the issue, so I reinstalled the stock upper. I have also installed a 1/2" aluminum plenum spacer, and a diablosport delta "flip-chip".

Thanks for everyones replies, I really appreciate them. My good friend in MIchigan (I'm in Illinois) just bought a scanner that does real-time data display, so I'm going to do the propane enrichment trick as soon as I get there.

Any other ideas?

sx199 said:

check the evap system for stuck valves or a brocken/loose line. also i believe that you mention a chip on the pcm. i work at a ford dealer and see problems with aftermarket chips ocassionally. take the chip off see what happens. good luck

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You know, I had an exhaust leak between the RH exhaust manifold and the h-pipe, which I fixed. But the evap purge hose was routed very close to this area and may have seen heat. I visually inspected the hose, and there were some tiny cracks in it, but I sprayed it with carb cleaner and it didn't seem to make a difference. Maybe I'll just replace those hoses anyway.

BTW the chip was installed after the problem persisted for a while. I put it in hoping it would richen the mixture enough to fix it.

This winter I am going to be installing a PI intake and cams, so if there is a vacuum leak at the intake, hopefully it will be fixed. I am also replacing the fuel pump, as this was another possible cause.