P0171 and P0174

[geekman]

I've been getting persistent P0171 and P0174 "too lean" codes on my Jag XJ with the 4.2 engine. It runs fine, but the EML is illuminated.

I initially went around the engine bay, spraying all the pipework and joints with carb cleaner and monitored the short term fuel trims. I noticed that the values changed significantly when I sprayed around the PCV valve and pipework, so I ordered new ones. I fitted those parts, but although the fuel trims now don't change when I spray around that area, I've still got the same codes coming up.

I tried using a cigar to smoke out the leak, but I couldn't see anything obvious. I noticed that the lid of the air box was slightly loose, so I reseated it, but again I can't imagine that would cause it, as it's before the MAF sensor. These are the values at idle with the engine fully warmed up. Is there anything that looks suspect here? I forgot to screenshot the long term fuel trims, but they're around 10-15% at warm idle.

















Thanks

 

[skaif]

I can't add anything that will help but this made me smile and sounds pretty badass.

I tried using a cigar to smoke out the leak

I hope you are constantly chewing a toothpick when not with cigar and you've used the cigar cutter to definger many criminals.

 

[Malt_Vinegar]

I am chasing a similar error on my Wife's A4 tfsi. You have my sympathy...
I have replaced the MAF, O2 sensors, PCV, various coils (had misfire codes also, which are now gone), various seals and other misc related parts, and it keeps coming back.
The car drives fine. I can reset the code, and it stays away for a little before triggering again sometime days, other times minutes later.
It occurs at really low revs, mostly when you are moving off from a standstill as you bring up the clutch.
Usually around 850ish revs.
If you avoid the revs dropping too far, or just drive hard, there are no issues..
I now suspect some small but annoying little air leak somewhere, otherwise it could be the fuel pump playing up perhaps?
We are at the point now where sending it to a garage for a diagnosis just seems silly for what it is worth, so will likely just change it if the code/EML ever stops the MOT

 

[Deleted member 66701]

FYI re the Lambda. It's a narrow band sensor, so it can only tell if the exhaust gas is rich or lean but not by how much, therefore that Lambda trace is entirely normal as it transitions the switch point multiple times.

 

[geekman]

I can't add anything that will help but this made me smile and sounds pretty badass.

I had to put my mouth on the intake pipe and was worried that people walking past would think I was one of those men who *really* love their cars....

I am chasing a similar error on my Wife's A4 tfsi. You have my sympathy...
I have replaced the MAF, O2 sensors, PCV, various coils (had misfire codes also, which are now gone), various seals and other misc related parts, and it keeps coming back.
The car drives fine. I can reset the code, and it stays away for a little before triggering again sometime days, other times minutes later.
It occurs at really low revs, mostly when you are moving off from a standstill as you bring up the clutch.
Usually around 850ish revs.
If you avoid the revs dropping too far, or just drive hard, there are no issues..
I now suspect some small but annoying little air leak somewhere, otherwise it could be the fuel pump playing up perhaps?
We are at the point now where sending it to a garage for a diagnosis just seems silly for what it is worth, so will likely just change it if the code/EML ever stops the MOT

Yes, issues like this can be difficult to track down. I really want to avoid taking it to a garage as I know they're just going to throw parts at it, and I stand a decent chance of ending up in the same situation as you. In my case, the fuel pressure looks fine but then I don't 100% know what I'm looking at tbh.

FYI re the Lambda. It's a narrow band sensor, so it can only tell if the exhaust gas is rich or lean but not by how much, therefore that Lambda trace is entirely normal as it transitions the switch point multiple times.

Thanks, I was actually wondering if it was a lambda sensor so that's helpful.

I took it for a short drive between traffic lights yesterday and recorded the LTFT, STFT is now pretty much perfect at idle which makes me think there might not be an air leak?

https://youtu.be/2om-4eXXe00

 

[skaif]

For what it's worth I had something similar on the micra and despite changing mafs, lambdas and throttle bodies it still hasn't gone away. We just leave it and drive.

 

[geekman]

For what it's worth I had something similar on the micra and despite changing mafs, lambdas and throttle bodies it still hasn't gone away. We just leave it and drive.

It doesn't have to pass any kind of MOT so I could do that, I'm just concerned about any damage it could do in the long term. I'm planning on taking it on a 3000 mile drive in a few months so I'd like it to be as close to perfect as possible.

 

[Firestar_3x]

Lack of fuel? What is the pressure at the fuel rail and it is within spec?

Can you easily take the lift pump out of the tank and see if the sock is dirty?

 

[geekman]

Lack of fuel? What is the pressure at the fuel rail and it is within spec?

Can you easily take the lift pump out of the tank and see if the sock is dirty?

From my scanner I can see that the fuel pressure alternates between 309 and 333 kPa at idle, no idea if that’s good or bad or where it’s measured from. If I rev it to about 3k rpm in park it goes to about 360 kPa.

I’d put this on the back burner a bit as I lent it to a friend for a couple of weeks and after all, it does drive fine, but I’d like to get it sorted if at all possible. I’ve no idea about accessing the tank but I could definitely look into it.

 

[Dogbreath]

Check the charcoal canister purge valve, if this is stuck it will cause your symptoms.

 

[geekman]

Check the charcoal canister purge valve, if this is stuck it will cause your symptoms.

Interesting, thanks. I'd have thought that would give some sort of evap code though?

 

[geekman]

Finally got this sorted. Turns out the new PCV valve they sent me was faulty Got a new one on now, and no more check engine light.

 

[Nasher]

I had errors like this on a car and it was a bad ground wire to the MAF sensor plug.