I am getting P0171 on a daily basis so I hooked up my OBD2 reader and watched the fuel trims, these are the results:


Short term and long term fuel trims within +/-5 at idle (this should discount vacuum leak)
STFT and LTFT within +/- 7 at 2500 rpm in neutral
STFT at +/-7 at 3000 rpm driving
LTFT at +19 (both banks) at 3000 rpm driving


Things I've done to fix this so far:


Recently changed fuel filter
Cleaned MAF sensor
Replaced IMT O-rings
Replaced spark plugs
Cured misfire on bank 1 with a new coil pack
Replaced valve cover gaskets on bank 1 because of oil in 1 plug well


Since the problem is on both banks and only while driving I am assuming that the next step is a weak petrol pump.
Do you have any other suggestions or diagnoses I can try?

Thanks

 

Have you checked Fuel pressure?
Wouldnt condemn pump until pressure shows drop either all time or above certain rpm.
Mine is around 58-60 psi?
Any other codes or pendings?
only other thing I can think of is exhaust leak but not sure how X type exhaust is configured, given both banks are showing lean?

 

Sounds like you have a vacuum leak. Try looking at the PCV breather hose. If yours is a smooth one instead of a ribbed one chances are it is split underneath. Also check the brake booster check valve. Also check the accordion hose between the MAF and the throttle body to make sure it isn't split and is attached well at both ends.

 

I'm getting 55 psi at idle and 58 with throttle so that seems to be okay. I will check while driving and when the lean condition is happening.
Thanks for the tip.

 

I have the proper PCV hose and a new brake booster vacuum pipe fitted.
I will check the accordion to see if I can find splits.
I cleaned the MAF sensor and I saw that the little rubber gasket (like a small IMT O-ring) was stretched but I can't find a part number for it. Do you know where I can get a part number and if that could cause such a big lean condition?

 

It's an odd one.

I think I'd try to get more data, in the hope of inspiration, by getting engine hot, park, then have someone watch LTFTs as you set off and drive around looking for how soon (with a hot engine) they go large. Then when they go small again. Repeat a couple of times to see if fairly consistent.

Try to do some uphill & downhill as well, steep as you can, as that'll cause high engine load and hopefully also downhill should be engine overrun so injectors switched off.

I'm not saying it's not an air leak, just after clues.

Avoid WOT as then it goes OL i.e. trims are garbage.

 

Have you tried a smoke test,many things leak on those lovely X types
areas to check are the brake booster check valve,the pcv hose the intake boot from throttle to airbox and the imt o rings
If all leaks are good then i would lean toward a maf if both fuel trims are what you posted
also sounds silly,but what shape are your battery terminals ?

 

That's not silly

 

So just for those car nerds bored on New Years I have an update.
Work done:

Replaced broken valve cover to concertina cable.
Replaced PCV connector with a proper jubilee clip so no leaks there.
Checked all inner manifold gaskets and connections, no faults.

Errors and ongoing issues:

Code P0171 still.
LTFT on driving (3000+ RPM) 19%+ on both banks still
Sounds of vacuum near serpentine belt (may just be my ears...)

Following are diags from my OBD2, first 3 are idle, second 3 are at 2600 revs:

 

This is a strange one. As I understand it, if you have a vacuum leak the long term fuel trims will be high at idle and decrease with the rpms as the amount of leaked air is basically constant and decreases as a percentage of total air through the system. You don't have that indication. The fact that both banks show high LFT yet no P0174 is curious.

Have you done a hard reset to clear the fuel trims and see how they change?

I've attached a chart of causes for P0171 DTC. It seems that you have eliminated the obvious causes and now the more obscure remain. An exhaust leak would affect only one side unless both leak. Have you had any other work done recently? How many miles? How is the car running?

 

Hi Clyons,

Yes I understand the same as you about trims and it seems to eliminate a vacuum leak. The car has 129000 miles and runs like a dream (except for the latest squeaking at the back).

These are the numbers when I am driving (the LTFT can spike up to 25% sometimes):

 

Yes, not any of the usual air leaks.

So, maybe when something flexes or a valve activates.

When the code sets it will also store freeze frame data. It's worth looking at in detail for more clues. Do it a few times to see how similar they are, hoping for hints. If they're very different it'll rule out some things that if they're similar will be possibles so look closely.