Need help with how to proceed when throwing only codes P0171 and P0174

[adream8]

Hi Guys.

2003 Boxster S. 78K. Check engine light came on for first time.

Code reader gives me P0171 and P0174. Those two codes only.

P0171 = fuel system too lean (bank 1); P0174 = fuel system too lean (bank 2).

Did a search here on the forum and found surprising little relating to these specific codes firing together only.

The best generic write-up I can find is a Repair Pal post, here:

http://repairpal.com/OBD-II-Code-P0171-and-P0174

Article suggests that it wouldn't be O2 sensor related, as it would be very unlikely for both sensor sides to foul at once. Makes sense to me.

It seems that the two most likely problems are:

1) the MAF sensor (dirty and/or faulty) (clean and do a barometric pressure scan test)

2) a vacuum leak somewhere (intake manifold gasket, vacuum hoses, PCV hoses, etc.) (visually inspect and possibly smoke test)

It seems that less likely culprits are:

3) out of date PCM software (update) (how do I tell?)

4) poor fuel delivery (failing fuel pump, clogged fuel filter, fuel injector failure) (test fuel pressure, check/change fuel filter, test injectors)

Can any of you Porsche experts out there tell me if I'm on the right track, or give me further valuable insight into this throwing this pair of codes only on a Boxster? ie. What's the common failure? What should I check first? What can I deduce? In what order should I proof things out? Etc..

Thank you!

*** POSSIBLY WORTH NOTING -- I AM NOT EXPERIENCING ANY NOTICEABLE CHANGE IN THE CAR'S PERFORMANCE. IT SEEMS TO BE IDLING AND RUNNING AS WELL AS IT ALWAYS DID. [That makes me lean toward MAF (sensor related) rather than vacuum leak (physical lack of fuel or too much air). Though I suppose it may be that the car's computer is compensating well enough for whatever physical air/fuel problem may be.. so perhaps my reasoning is faulty there..]

[JFP in PA]

Well, so much for "generic write-ups"; these two codes are indicative of the car running overly lean on both banks. It is not O2 sensor related, and it is rarely MAF related; and what connection it could possibly have to the PCM software totally eludes me, but more likely it is because of an air leak into the intake system or poor fuel delivery (volume and/or pressure).

Possible fault causes:- Incorrect signal from MAF sensor- Intake air system leaking- Fuel pressure too low- Volume supply of fuel pump too low- Fuel injectors fouled- PCV valve leaks- Cap of oil filler neck not closed correctly or seal is damagedLooks like you have an air leak...

[adream8]

Thanks for responding, JFP.

Other than PCM thing, which I don't understand (I'd like to), the write-up seems to be exactly in line with the possible fault causes you listed, so I'm not sure why you said "so much for"..

Why do you say it's an air leak or poor fuel delivery necessarily, as opposed to a bad MAF sensor? Do MAF sensors rarely fail on Boxsters? Or would it likely throw another code on top of the P0171/P0174 (too lean on both banks) codes?

In what order would you tackle testing out the possible fault causes?

Should I test the fuel pressure and clean the MAF sensor first, since those are relatively easy for me to do at home for low/no cost? Then if that doesn't work, take it to a mechanic to pay for a smoke test, since I don't know if I'd be able to find an air leak at home? And finally as a last resort, buy and replace the MAF sensor? I'm hoping it's not the injectors, but I guess if all the other tests/fixes failed at that point, I'd have to have them checked. I will of course check the oil filler cap seal first.

Any further elaboration is appreciated.

Thanks!

[JFP in PA]

Thanks for responding, JFP.

Other than PCM thing, which I don't understand (I'd like to), the write-up seems to be exactly in line with the possible fault causes you listed, so I'm not sure why you said "so much for"..

Why do you say it's an air leak or poor fuel delivery necessarily, as opposed to a bad MAF sensor? Do MAF sensors rarely fail on Boxsters? Or would it likely throw another code on top of the P0171/P0174 (too lean on both banks) codes?

In what order would you tackle testing out the possible fault causes?

Should I test the fuel pressure and clean the MAF sensor first, since those are relatively easy for me to do at home for low/no cost? Then if that doesn't work, take it to a mechanic to pay for a smoke test, since I don't know if I'd be able to find an air leak at home? And finally as a last resort, buy and replace the MAF sensor? I'm hoping it's not the injectors, but I guess if all the other tests/fixes failed at that point, I'd have to have them checked. I will of course check the oil filler cap seal first.

Any further elaboration is appreciated.

Thanks!

PCM is "Porsche Communications Managament" system, or the nav, stereo, and cell phone, hence why I don't think it is involved.

Your MAF really is not doing much until 3,500-4,000RPM, so it is rarely involved in these codes. While MAF's can and do fail, most people do not know how to correctly test them before doing so, and they are often changed without testing them first.

What you need to be doing is getting the car scanned with a Porsche specific system (PIWIS, PST II, Durametric) all of which can see the MAF and other sensor values in real time, quickly eliminating those functioning correctly from contention. You would need a proper fuel pressure test tool to check the volume delivery and pressure as well.

As for backyard checks, look over the connections, hoses, oil filler cap, AOS system, etc. looking for signs of a leak point. You would be surprised at how often these problems come down to a small hose that fell off its connection, or one that has rubbed a small hole in itself.

[adream8]

Good stuff, thanks!

I will focus on vacuum leaks first then.

Been searching the forum and finding some good posts related to air leaks, including yours about using a slow stream of propane to discover them.

Think it might be time to invest in a Bentley service manual, so I can learn the anatomy of this engine, and trace all the potential leak points.

Haven't really need one until now, but I'd like to start saving a few bucks and DIY where I can, and save the mechanics for the big jobs.

(I'll be changing the oil and the air filter this week, so I may as well at least spray clean the MAF sensor while I'm in there. Probably couldn't hurt.)

Thanks again!

Edited May 29, 2013 by adream8

[adream8]

Found the problem, JFP.

It was the AOS. It's diaphragm had begun to tear. Was sucking air into intake and creating a strong crankcase vacuum, substantially above what it should have been. Could barely budge the oil filler cap when the car was running.

Fortunately (and amazingly) there was no oil yet in the intake, so I was safe. And lucky.

Replaced the AOS, and now all the parameters (including fuel trim) are back within spec and the car is running perfectly. DO notice a difference in performance.

Figured I'd post, just in case it might help anyone else down the line.

Now I just have to change the oil and swap the engine mount out, and she'll be all fixed up.

Thanks again.

Edited June 5, 2013 by adream8

[JFP in PA]

Found the problem, JFP.

It was the AOS. It's diaphragm had begun to tear. Was sucking air into intake and creating a strong crankcase vacuum, substantially above what it should have been. Could barely budge the oil filler cap when the car was running.

Fortunately (and amazingly) there was no oil yet in the intake, so I was safe. And lucky.

Replaced the AOS, and now all the parameters (including fuel trim) are back within spec and the car is running perfectly. DO notice a difference in performance.

Figured I'd post, just in case it might help anyone else down the line.

Now I just have to change the oil and swap the engine mount out, and she'll be all fixed up.

Thanks again.

Glad you found it, I expected there was an air leak in one or more of the systems, and the AOS is always a favorite leak point.