Marelli V12 idle stumble

[Brakebuster]

cool

EAV are still available new from heritage for not much money , the wiring to them can get a bit crusty as well

BB

[davidr]

Took my V12 in for MOT today (and it passed!!)
Anyway, the emissions readout showed the CO indicating that it's running lean. The idle hasn't improved and its misfiring and generally 'orrible.
As I've said before, I've checked for air leaks etc and drawn a blank.
So, calling all you EFi gurus out there - what could cause lean running. I'm thinking perhaps a naff sensor or something?
Any ideas?

[Brakebuster]

fueling is only determined by the TPS in relation to the TB position, other than that the car doesn't know how much air/fuel mix there is , ( i think )

so , extra air must be entering the system to lean out the mixture


is the gearbox modulator pipe installed on the back of the right bank ( as in no open vac holes ? )

there is also a vacuum pickup in the middle of the rear crossover pipe ( from the AAV ) , this may have dropped off , and i think this is where the pipe to the boot mounter ECU runs to ?

or could even be the inlet manifold to head seals perished ?

you taking the V12 or the show and shine car to Gaydon m8

BB

[davidr]

Hi BB. I have checked through for vacuum leaks and can't find any.
I was wondering about the various temperature sensors etc and if any of those can lean out the mixture.
I'm taking the Celebration to Gaydon. Even though the HE passed its MOT today the diff is pretty noisy and I don't want to ruin it!

[Brakebuster]

hmm

i will do some research on that


BB

[davidr]

I'm thinking of a couple of possibilities. First is that the fuel pressure is too low - maybe the pump is past its best?
Second is that I could adjust the idle screw on the injection ecu to make it richer.
Any thoughts?

[Brakebuster]

does your ECU in the boot have the vacuum reservoir in the vac pipe just by the ECU ?

if not, then this is a must do job , and they are still available new for about a tenner I think , it stops the ECU from ' fluttering ' with modulating vacuum by smoothing out the signal

there is also the engine bay mounted fuel mixture ' plug ' , this is a two pin plug by the ABS pump that tells the ECU which ' RON ' fuel is being used, its a case of plug in or plug out , I think I have ' plug in ' version telling the ECU I am using a higher octane fuel , but would have to confirm this ,

I would also be checking the TPS under the capstan , if this is not reading right ( a well known problem ) or the wiring is broken in some way then this will miss fuel the car as well

still a few things to check and eliminate

BB

[davidr]

HI BB - to answer your points I turn

I do have the vacuum reservoir fitted - see picture on page 2 of this thread!

The plug you refer to in the engine bay only changes the ignition timing. This link permits the selection of either of 2 ignition maps contained in memory. If the link is in place the ignition map suitable for 95 Octane fuel is selected and the ignition is advanced and if the link is removed the ignition map suitable for 91 Octane fuel is selected.

As for the throttle position sensor, I think I'll have to get my multi-meter out!

[Brakebuster]

cool

i'm out of ideas then


BB

[davidr]

OK - I checked my TPS this evening. At idle the voltage between the red and yellow wires should be between 0.32v and 0.36v. Mine was 0.21v so I adjusted it.
Now it is spot on at 0.34v.
Whilst I had the multimeter out I checked the resistance of the temperature sensors and all were OK.
Started her up and let her warm through - no improvement I'm afraid.
What I did notice was a couple of things.
The TPS appears to be working fine - as I moved it through its range it went smoothly from 0.34v to 4.74v so that's good news.
If I turn the throttle capstan sharply I can hear the injectors click - this is an enrichment strategy to add extra fuel as the throttles are opened quickly so response feels sharper and the engine isn't leaned out too far by the sudden increase in air flow. It's good that works as it demonstrates that the ecu is recognising the TPS moving.
When I take the vacuum pipe off the fuel pressure regulator (so that fuel pressure increases in the rail) it does smooth out a bit - not perfect but better.
So I'm thinking that fuel pressure is the issue. What I can't see is where I can get a fuel pressure gauge onto the fuel rail to check it. Has anyone done this before?

[davidr]

I think I've got a cunning plan for measuring the fuel pressure in the rail.
a while ago I got a new hose that goes from the rail to the outlet pressure regulator (left side). Noticed it on ebay and fitted it because the hose would be new and supple and the metal parts where shiny and new. EAC9986 is NLA from Jaguar but they do pop up now and again. As I'm a magpie, I kept the old one and I'm now going to adapt this so I can tee a connector in for a pressure gauge.
I've ordered a pressure gauge via ebay so another instalment when it arrives!

[davidr]

I've connected a pressure gauge into the return pipe from the fuel rail using the old pipe that I adapted earlier.
Results were :

When I turned the ignition to position 2 the pump ran for a few seconds as it should. As the pump ran the rail pressure was 2.25bar. When the pump stopped pressure dropped to 2bar and held.

When I started the engine rail pressure was about 2.25bar and held, fluttering a bit. Pulling the vacuum hose off the regulator made no difference. I let the car warm up and it idled rough with the characteristic stumble.

Next step was to put a hose clamp on the return pipe to the fuel cooler so I could adjust back pressure and increase the pressure in the rail. As it increased to about 2.5 to 3 bar the revs increased, the idle smoothed out and the exhaust smelled much cleaner.

So, fuel pressure is the issue. Both fuel pressure regulators have been changed so could the issue be the pump?

[davidr]

So I put the new pipe back on the car and moved to the boot.

What I decided to do was to put the gauge in where the filter would be. This showed the pump delivering a fairly steady 3 bar so it appears that my pump is healthy.
So now I'm proper confused!
Just to be clear, these are the steps that have been taken so far
The injectors have been cleaned using the Terraclean service.
Both fuel pressure regulators have been changed
The swirl pot and intake filter have been removed and cleaned
The fuel filter was renewed at last service
The throttle position switch has been checked and reset
The throttles have been checked and reset at 2 thou

My thoughts are, if the pump is delivering 3 bar to the filter then it is working fine. If there was any restriction in the fuel lines then the back pressure would be higher?
Maybe the filter is partially blocked and restricting flow? I think I'll order another one.
Or maybe the intake pressure regulator is restricting flow too much?
So I think I'll try bypassing the intake pressure regulator?
Any thoughts?

[Brakebuster]

good work

from memory i thought the pump ran at 4 bar , so maybe yours is a bit ' weak ' , i fitted a new lucas pump during my fuel tank refurb ( just before actually ) , before which i got the loss of power issues constant with the blocked pickup from the swirl pot , this may have also made the pump work too hard and wore it out somewhat......

so i think you may well be onto something , i would recumbent the use of a genuine lucas pump , as these aftermarket ones don't have the correct electrical connection terminals, and maybe arnt made to the same specification

nice bit of adaption

BB

[davidr]

Hi BB - my ROM says that the pump is rated at 2.5bar but that doesn't appear to be enough to give me smooth running.
Where did you get your Lucas pump?