Vacuum leak - P0171 - causing high LT Fuel Trim - Mazda 6 Forums : Mazda 6 Forum / Mazda Atenza Forum
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post #1 of 3 (permalink) Old 08-02-2018, 09:31 AM Thread Starter
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Vacuum leak - P0171 - causing high LT Fuel Trim

2004 6S/3.0L

Got a P0171 CEL the other day - lean condition in bank 1. Car idled a little rough and surged a bit but was fine under load while driving. I was hoping it was a dirty MAF but discovered a vacuum leak this morning. A hole in the PCV hose. Ordered a new part but have patched the whole with duct tape. Seems to have fixed the idle surge for now.

Which leads me to my next question. I'm using a OBDII tool with DashCommand and was watching the fuel trims. My STFT is steady at 0% no matter what. (is that normal?) My LTFT is sitting at 20% at idle and goes to 10% under load. Is that normal? Any other read out I should be looking at? CAT temps are around 1200 and go a bit higher at WOT.

I plan to check the fuel trims again after I replace the PCV hose, but just curious of these readings are normal or something I should be concerned about.

Current: 2004 Mazda 6S 3.0L Wagon | 2015 Mazda CX-5 | previous: 1990 Miata, 1992 Mazda MPV V6
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post #2 of 3 (permalink) Old 08-03-2018, 01:26 PM Thread Starter
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Well after a day I ran another log.

Looks like the LTFT is no longer pinned at 20% at idle.

Under load the STFT sits around -3% and the LTFT is around 3%.

Will be replacing the PVC valve and hose tonight.

Have noticed a small cloud of white smoke at start up for the past few days.
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post #3 of 3 (permalink) Old 08-03-2018, 01:55 PM
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LTFT (long term fuel trim) generally stays somewhere between +- 10%
The STFT (short term fuel trim) will move according to moment to moment needs.


The fact that your LTFT changed after the leak repair shows that the ECU found a new base from which to operate.


There are quite a few threads here about fuel trims for the v6 but it sounds like where you are is now normal.
Make sure your plugs and coils are in good shape and that you don't have a misfire (even subtle)


A puff of white smoke at startup...hmmm. White "smoke" is usually coolant. A puff at startup is generally valve guids/seals (but that is blue smoke)


A you slowly loosing coolant? I suppose a crack or head gasket could cause this. Seems odd (unlikely) that it would start right after the vacuum fix.


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2004 6s Wagon 3.0, 5-speed ATX, Performance White (A2N)
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