As a new member this is my first post.

I have the dreaded P0171 fuel trim system too lean code on my wifes 2008 1.2 9N4 polo which is running fine without any issues of roughness, staling or poor starting.

I have run the VCDS test for MAP and Lambda sensors which check out fine, I have checked for any vacuum leaks both visually and with a smoke test all of which are Ok with no leaks.

One thing I have noticed in VCDS 01 engine, measuring blocks, block 32 field trim at idle is high at 14% to 18% and understand that this should be below 10%.

The check engine light comes on anywhere between 1 and 5 days after a code has been cleared with the same P0171 fuel trim system too lean code.

Any advice would be welcome as I’m running out of ideas as to where to look now.

 

Hi

If the mileage is high , I think it is worth cleaning the throttle body  and recalibrating the throttle.

First Welcome to the forum.

 

Have a look at this http://wiki.ross-tech.com/wiki/index.php/16555/P0171/000369. & this http://wiki.ross-tech.com/wiki/index.php/Fuel_Trim_Info 

44 minutes ago, vwfan3 said:

Hi

If the mileage is high , I think it is worth cleaning the throttle body  and recalibrating the throttle.

Nothing to do with the cleanliness of the throttle body 

Many thanks for the reply's,

 

The throttle body is clean and I have replaced the seal and recalibrated and all is OK.

 

I have followed the ross tech guidance and have eliminated most of the areas on the list, apart from the fuel areas as if there was an issue with fuel delivery there would be issues with rough idling and performance, both of which are fine.

Any more thoughts out there?

 

On 3/29/2018 at 12:36, gerards19 said:

Many thanks for the reply's,

Any more thoughts out there? 

I wonder if the pre-cat lambda sensor is worth a go. I wish they were easier to get at and unscrew.

Both pre and post Lambda probes check out Ok in VCDS, and have already replaced the pre CAT one alreday with no effect.

 

 

 

 

On 3/28/2018 at 22:53, gerards19 said:

The check engine light comes on anywhere between 1 and 5 days after a code has been cleared with the same P0171 fuel trim system too lean code.

Any advice would be welcome as I’m running out of ideas as to where to look now.

We need to catch the EML light red-handed as it were.

Clear the code, and then as soon as the EML comes ON, make a note of the parameters e.g. temperature, revs count, etc.

It appears to be a long term fuel trim with at idle is at 15%, but with higher revs this drops to 0-1% which I understand is what it should be. The car runs fine with no rough idle or hesitation and assume that as the long term fuel trim passes a threshold it turns on the EML light.

I have read that this all points to a vacuum leak but can’t find one anywhere using a smoke test or spray test with either water or carburettor cleaner.

 

 

The vacuum pipe to the servo is known weak spot for cracking and leaking but not enough too effect the brakes 

Thanks for your reply

 

No signs of any cracking on the servo vacuum pipe and Its has been replaced without any change.

No harm in cleaning the MAP sensor.

It turned out to be a tiny leak on the EVAP purge valve and investing in a vacuum gauge found the fault, replaced the valve and now all good.