E85 Code P0171 & P0174 Help!

S

snoopaloopy

Member
I've got a 2004 Z4 3.0i with the M54 engine with only 49,000km but I've been dealing with a consistent P0171 & P0174 codes here and there that kept coming back.

I believe I already addressed the vacuum leak with a Amazon smoke machine. Plugged it in the main air intake hose. I already replaced the lower intake hose, vaccumm caps behind the intake manifold, as well as changed DISA o-ring to proper old ring. Smoke test didn't show anything else.

What i noticed is Long term fuel trims at idle and acceleration are around 15-20+% consistently while short term trims fluctuate than -10%-10%.

Acceleration are smooth and powerful with no issue during the ride. I tried both regular and sport mode. Revs went to 3-5000 rpm fine. Last night, at the end when I cruised at 50kph back home last 3-4 km the engine light came up with P0171 code (confirmed) and P0174 (pending). I cleared the code today, drove just fine on the high way, only for the CEL to come on again when I pulled over to drop off my friend.

I did notice the gasket on my fuel cap is cracking and I already ordered new one but I doubt this would lead to lean codes. Could this be bad pre-cat O2 sensors?

Is there a way I can record the OBD data to confirm? It's getting pretty frustrating.
 
A decent code reader should be able to display live data from the exhaust sensors to see what they are doing, pre cat when viewed in graph form on a good scanner should be producing a regular wave form as they rise and fall through their voltage range, the post cat sensors usually display as a pretty static line unless they are picking up high level of unburnt fuel. Since you are sure you have cured air leaks then sensors the next logical check to be made.
 
Can you humour me and unplug your maf sensor. Car will then resort to a default setting. If you then find when you touch the brake peddle engine revs increase , let me know as I might have an idea.
 
colb said:
A decent code reader should be able to display live data from the exhaust sensors to see what they are doing, pre cat when viewed in graph form on a good scanner should be producing a regular wave form as they rise and fall through their voltage range, the post cat sensors usually display as a pretty static line unless they are picking up high level of unburnt fuel. Since you are sure you have cured air leaks then sensors the next logical check to be made.
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Thanks. I did check the log from the scanner from the last drive and I did notice something odd. This is for pre cat sensors:

IMG_3721.png

the sensor for Bank 2 seems to be completely missing for first part of the drive.

While the post cat sensors look consistent but I don’t see the data recorded for the portion of the the drive where the Bank 2 pre cat sensor data is missing.

IMG_3722.png

It’s even more odd because P0171 indicates Bank 1 not 2
 
Your post cat sensors should not be oscillating. This indicates unburned fuel getting past the cats. Bad cats? DME commanding too much fuel?
 
BumpyZ4 said:
Your post cat sensors should not be oscillating. This indicates unburned fuel getting past the cats. Bad cats? DME commanding too much fuel?
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Does that align with the trim codes then given LTFT around 20%?

Bad pre cat sensors asking for too much fuel than needed?

But I think this was also during the time I pulled over to drop off a friend.
 
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snoopaloopy said:
Bad pre cat sensors asking for too much fuel than needed?
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Your Bank 1 Sensor 1 looks good. Bank 2 Sensor 1 has some issues since it was not showing a signal in the first part of the graph. As Mike6 suggested unplug your MAF and check the fuel trims again, with the MAF disconnected the DME will switch to a default fuel curve. If your fuel trims correct themselves your MAF is bad.
 
Mike6 said:
Can you humour me and unplug your maf sensor. Car will then resort to a default setting. If you then find when you touch the brake peddle engine revs increase , let me know as I might have an idea.
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Sorry just saw this. Should I unplug MAF while engine running or while it’s cold?
 
Mike6 said:
Can you humour me and unplug your maf sensor. Car will then resort to a default setting. If you then find when you touch the brake peddle engine revs increase , let me know as I might have an idea.
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I unplugged the MAF and turned engine on. Revs did go up initially at start up when I pressed brake peddle but after that it stabilized and didn’t happen anymore.

Fuel trims still high

1754000210147.png
 
This will sound strange, but I had high fuel trims like you. I found a few minor vacuum leaks, did the DISA valve, unplugged the MAF and still no change. What it ended up being was a tank of bad gas. Drove it down to about 1/2 a tank and filled it up with fresh gas and the fuel trims dropped in half. I then drove it until it was almost empty and filled up again and the fuel trims were dead on and I've never had any issues since.

Did the high fuel trims start after fueling the car??
 
BumpyZ4 said:
This will sound strange, but I had high fuel trims like you. I found a few minor vacuum leaks, did the DISA valve, unplugged the MAF and still no change. What it ended up being was a tank of bad gas. Drove it down to about 1/2 a tank and filled it up with fresh gas and the fuel trims dropped in half. I then drove it until it was almost empty and filled up again and the fuel trims were dead on and I've never had any issues since.

Did the high fuel trims start after fueling the car??
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It might. What sort of gas do you put in? I’ve been putting in 94 but only so not sure if it’s station specificz
 
The zed is not supposed to like continuous use of supermarket fuel, if you do use it try a premium fuel of super quality. Some forum members have reported issues with supermarket fuel as it doesn't contain additives like shell or Esso. No idea if it's true but whenever I put in a tank of supermarket stuff I always make sure my next fill is top quality.
 
I had fault codes for the post-cat 02 sensors on my 2nd 3.0Si.

My Indy told me to only run it on Shell Super 99 RON and put some Cataclean in the tank. The codes came back once but another dose of Cataclean and sticking to 99 RON cured the problem. Then I read the manual and it recommended using at least 98 RON whenever possible. :rolleyes:

I believe the M54 engines are also designed for 98+ RON so that might be worth a try.
 
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