Originally Posted by RIWWP

Chances are the leak is in one of your hoses, and as the hose warms up and gets flexible it bends more under the vacuum in the line vs the higher ambient pressure, opening up the hole more and making the leak larger

Originally Posted by Phil Bate

First let me apologise for what will inevitably be a long post. I feel I should give all the details in the hope someone that likes to help does not have to berate me for a short and non descriptive cry for help!


I'm trying to chase a positive fuel trim at idle on my own car (231 S1), and am running out of things to check. I would be greatful for any suggestions.


On starting from cold and leaving to idle, as soon as the O2 sensor kicks in and closed loop begins the total of the trims equals bang on zero, up until the engine has run for a few minutes and ECT starts nudging 65 degrees C. From here, the trim starts building and I end up with around 3 to 4% positive trim total. Not bad, but I am used to seeing 1 to 2% on my engine.


If I leave the car idle longer however, as everything heats up the total trim goes higher, and after the fan has cycled once or twice it will hit up to +11%.
If I bring the revs up and let the trim stabilise after enrichment, anywhere above 1800RPM the trims are near zero, same when driving. The cut-off is very distinct.

Listening to the car, watching live data and inspecting the logs I found that it is not constant. Occasionally while the trim is high at idle, the revs will blip up to ~890 then settle, and the trim will come down for a while (as if something has gotten easier for the engine), then RPM will start dipping slowly as if struggling and the PCM will open the throttle momentarily and up the trim again.

If I stop the car and let it heat soak for a while, it will keep the positive trim on restart, unless it cools right down (overnight) and then it will be back to zero.
Something is causing the trim when it gets hot, but it is not directly related to ECT, more like something attached to the engine or in the bay soaking in the heat.

The engine:
I rebuilt the engine at 54K miles with a street port and D585 coils. It now has about 67K on it, and I compression tested it about two months ago with 7.1 across the board - very even results.

Fuel:
I recently fitted a low mileage R3 pump assembly, the high pressure filter is clean as a whistle and it looked to flow just fine on the bench. Having been through a few failed fuel pump issues before I know this one is good, and STFT when varying throttle while driving is nice and stable. I flow tested all my injectors after rebuild and they were very evenly matched and at the top end of the tolerance.

Air:
The MAF rate at idle seems happy at 5-5.2 until I get the high trim when it can drop to 4.9, but looking at the logs it seems the flow is being affected by the problem rather than causing it. Filter is clean, I have tried two different MAF sensors. I have had the UIM apart a few times, gassed all pipes/joints, drenched everything with spray lube and even blew the lot with smoke this weekend but can't find any leaks.
I did find the evap purge solenoid can be fun if there is pressure in the tank, but disconnected it and capped the intake port for testing.

I can see 229g/s at WOT on a cool day.


Spark:
I run the well used dwell table for the D585s. This weekend I caught one of the Magnecor leads arcing to a bolt on the coil bracket. None have any insulation damage, so I spiral wrapped them and rerouted to avoid any metal contact. No visible arcing now (but could they be internally damaged?). Tried different spark plugs including the brand new set I have on now. All connections seem snug and fully seated. The coils have some discolouration where the metal brackets contact the plastic casing.
The spark plugs are getting coked, and there is visible carbon on the rotors.


Exhaust:
Standard manifold (header?), free flow cat back, no leaks. The CAT is OEM and visibly fine, no rattles and running temp. is good. I have tried swapping the front O2 sensor.


As the plugs are getting coked I guess that the PCM is simply 'seeing' a lean condition under this RPM and adding fuel unnecessarily. Also as it only occurs under 1800RPM (doesn't the firing order swap somewhere around that point, ie. trailing first at idle?) and can cut in and out I am leaning towards ignition.
I am picking up a set of standard coils and leads tomorrow to attempt to rule out the D585s and Magnecors. If anyone has any further suggestions or observations regarding what I have done so far please let me know, as if the coils/leads aren't at fault I'm not sure what to try next.

Many thanks
Phil

Originally Posted by TeamRX8

sorry, it was supposed to say "not a BFD"

BFD is slang for "big f***ing deal" i.e. American sarcasm

under 5% can be normal ...


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