V12 fuel trim

[M100_]

Hi,

I have a 2011 Rapide and have been trying to troubleshoot an irregular idle and have tried everything I can think of. I have abnormally negative long term fuel trims ~ - 13 to -15% on all "4" banks... ie 2 sets of 2 two banks. I have tried all the obvious checks and am now stuck so I am hoping for some help / advice.

Currently performed the following checks/work

Symptoms:
Rough (not surging) idle - like an intermittent misfire - although feeling a rough idle event doesn't always increase the misfire count.
Some low level misfires - seems to be random(ish) in cylinder selection)

Parts:
New plugs
New coils
New Manifold gasket (trims would be positive if vacuum leak anyway)
New PCV valves
02 sensors replaced by previous owner (2019)
Original fuel injectors, cleaned, flow-tested and matched
MAF sensors cleaned.

Checks:
Fuel pressure is 55psi above manifold pressure
Manifold pressure sensors work
Disconnected PCV system - no change to trims
Disconnected fuel EVAP system - thinking it could be the source of excess fuel - no change to trims
02 sensors - response to vacuum leak or artificially increasing fuel pressure (by refencing the fuel pressure to atmosphere and not manifold vacuum) works as expected.
Primary catalysts visually inspected and are intact.
Compression test is fine for all cylinders.

Only non standard parts are the secondary cats which look to be high flowing sports cats.

What is really confusing in that all 4 "banks" are running similar negative long term fuel trims. Any suggestions would be very welcome as I do not know what to try next.

Edited by M100_ on Sunday 17th October 07:38

[amv12]

Hi,
It’s my understanding that the fuel pressure should be between 39-41 psi above manifold pressure.
Although this can be driven higher if the fuel temperature rises.
Can you read the fuel temperature?
Although there’s a pressure/temperature sensor in each rail, only the primary is wired to give out the temperature.
If the temperature reading does appear high, possibly swap sensors and see if anything changes.

[M100_]

Thanks for the reply, and the suggestion, would definitely fit with the global "phenomenon" that I'm seeing. I gave it ago:

Yes I was also expecting around 40psi... from but the AM material I have found says to check fuel pressure is at least 40Psi above manifold. I would expect the the injector timing related to fuel temperature/density would probably be adjusted outside of the fuel trims but I shouldn't assume anything!

I can only read fuel temperature information on the primary bank (secondary bank reads 0). All the Foxwell scanner gives me is a "temperature sensor supply voltage" of 2.7V however I'm not sure what temperature this corresponds to. Sensor is Bosch 0261230093/ Ford MoCo 3F2E-9G756-AD. Only data sheet I can find is here

After swapping the pressure/temp sensors (ie Bank 1 with Bank 2) my observations were the following:

• No change in temperature voltage reading
• When the sensor (its a combined temp and pressure sensor) is at atmosphere (engine off) I noticed I get a pressure reading of about 4psi ... not zero as expected...could be normal with 0psi outside of its operational range.
• Swapping the sensors ultimately ended in the same result however for the first few minutes (cold start) it did appear as if short term trims were trying to get the overall trim back to zero... eg they were fluctuating around +6 to +10 however after a few minutes the swapped back to +- 4, keeping long term trims between -14 and -16. Will try and repeat a cold start and see if this repeats
• Coolant sensor looks OK
• Air temperature sensors looks OK

Does anyone have the voltage to temperature relationship for the sensors (Bosch / Ford MoCo 3F2E-9G756-AD) so we can conclusively rule this out?

Any ideas as to why we are initially OK (ie short terms terms of +10 look to be correcting the long term trims) but then we revert back to "rich"?

Thanks in advance, this one has me scratching my head.

[Fullmel]

Probably already done but has the engine oil been changed?

[M100_]

Fullmel said:

Probably already done but has the engine oil been changed?

Yes, oil has been changed.

[amv12]

Hi M100, I would like to send you some information via a PM, so would it be possible to check the, allow members emails, box in your profile.

[M100_]

amv12 said:

Hi M100, I would like to send you some information via a PM, so would it be possible to check the, allow members emails, box in your profile.

Thanks, I've made the changes.

[laracer]

Did this problem get resolved?
What if anything fixed it?