Fuel trim diagnostic puzzle for you
 

I greatly enjoyed the recently posted Schrodingers Box youtube tutorials on fuel trim diagnosis. My current situation is baffling me however so I'll throw it out to all of you.

I have a CEL with a single code P1130 (lean on bank 2). It first came on a month ago but cleared itself after a few days. It just came back on this week, same code.

I have done a bunch of data logging using a bluetooth OBDII and DashCommand. The only thing that stands out are consistently high LTFT values, 8-11% on bank 1 and 6-9% on bank 2. The STFT values look normal to me, fluctuating mostly in the +-5% range in response to what appear normal O2 sensor traces.

In my few hours of logging the LTFT values vary very little, but do seem to very slowly (over tens of minutes) decrease while driving at speed.

However every so often they spike to like 20%, but sharply and only for a few seconds, seemingly in response to opening the throttle and corresponding high MAF rates.

The image shows about 3 1/2 minutes of data. LTFT and STFT traces in the upper panel, Speed, RPM and MAF in the lower pane.

So what is your diagnosis? Are these spikes triggering the CEL?

The stock answer for this CEL code is intake leak and I do plan to bring it into the shop next week for a thorough diagnostic, smoke test etc. to resolve this. I didn't expect the data to look like this however, shouldn't an intake leak result in lower LTFTs when accelerating and cruising at speed? Why don't these LTFT numbers change much at all, is this a feature of Porsche adaptations unlike other cars like in that guys videos?

Cheers, Richard

http://986forum.com/forums/uploads01...1405050929.jpg

Richard,

You didn't mention your MY, so I'll assume that your DME is version 7.2.

For a P1130, there are several fault areas to look for (as you know, that code applies to idle range only):

1) Incorrect signal from MAF sensor
2) Intake air system leaking
3) Fuel pressure too low
4) Volume supply of fuel pump too low
5) Fuel injectors fouled
6) Exhaust system leaking

The proper way to troubleshoot this in in these steps:

1) Check the signal from the MAF sensor
2) Check the exhaust system for leaks (leaks ahead of the front O2 sensor can cause LTFT issues)
3) Check for leaks in the intake
4) Check fuel pressure/volume supply
5) A fuel injector on that bank could be fouled


I also sent you a PM.

Regards,

paul...

Your STFT are "normal" because they have adapted to the new "norm" of the LTFT values. Most likely you have a incorrect MAF signal, intake air leak, or improper fuel pressure. You can be fairly positive its not a fuel injector or exhaust leak as it appears both banks are seeing the increase in LTFT.

If you have a fuel pressure gauge and adapter for a schrader valve start with fuel pressure as its easy and cheap. Next do the normal testing for faulty AOS (which can be found all over this forum), and lastly the MAF testing.

You'll probably end up replacing the MAF and or the AOS and this problem will be resolved.

On a side note it seems like that is a high number for the MAF reading...I remember mine being ~11g/s but I might be thinking of something else.

The MAF readings I think are fine. My idle values are 3.5 - 4.5 g/s typically. (The value shown on the picture is not at idle).

I have a couple more naive questions.

Since the STFT values are fine, I assume that the engine is adapting just fine and not running lean or rich when in closed loop. If the MAF is reading low due to bad/dirty MAF or additional unmetered air, would the car then be running lean in open loop, or does it use the last known LTFT values and hence be close enough?

What defines the idle range for DME7.2? Nothing in the data indicates why the code would have been thrown as the idle data always seems to be less than 11%.

Good call- I didn't catch the RPM.

Both banks are adding fuel (+LTFT %) to compensate for a long standing lean condition. The MAF is either reading more air than is actually passing through the TB, or you have an unmetered air leak.

Generally speaking cars run more rich in open loop as they are not relying on secondary o2 sensor data until the cats are warmed up. Its using the computers "best guess" based on MAF, IAT, etc then it will switch into closed loop and monitor the cats.

I still think you have something going on with an air leak or MAF issue

I'm the one with the how-diagnose-resolve-engine-issues-using-durametric

What device are you taking the readings with?

The LTFT are a bit high meaning they are adding fuel to compensate for a lean condition. Perhaps the MAF is not right on spec, or a small air leak at the intake.

Mine are around +5 and I have read that up to 10 is not abnormally high. Over 25 with throw a fault code.

You MAF is reading fine.

The concern is the LTFT spikes on one bank. Either a loose connection to an O2 sensor, failing O2 sensor or interference in your signal reception for your reading device.

I also have signal spikes and figure its just data capture errors or interference.

Could be loose connections though but I replaced the front O2 sensors and a multitude of other things that had no impact on my signal spikes.

Since you are only getting these spikes on one bank by readings and the fault code confirms it on one side only, I would check the 2 bank2 O2 sensors.

Try swapping the O2 sensors from bank1 to bank2 to see if the problem then switches to bank1, which would confirm bad sensors.

If Bank2 still acts up then I would look for bank 2 header or intake leaks.

Fuel trim issue RESOLVED!
 

Follow up on this over a year later.

First the car threw another CEL later last summer. I asked a 'new to me' shop very close to my home to look into it while doing the yearly oil change. He supposedly did a smoke test and found no intake leaks, claimed the MAF was fine, and cleared the codes which did not return. The LTFT values were still running +8-9% when I got the car back however. So much for curious mechanics.

This week I got another CEL, hooked up the scan tool and found the LTFTs running over 20% on both sides!!

So I went down to the auto parts store, bought a can of CRC MAF cleaner and a set of security star bits for a total of $22. Gave my MAF a good thorough cleaning, tightened the clamps on all the intake manifold joints and now the fuel trims are all at 0%.

Should have done this last year.

Clean your MAF, easy and solves many issues.

Thanks for posting this follow up!