Marelli V12 idle stumble

[Brakebuster]

interesting,,,,,,

i got mine a long time ago , but jaguar still sell new ones

CBC5657 @ £211.85 Lucas 4FP

used and new can be found on ebay , and Bosch also make a version that is dimensionally the same , but has threaded post terminals rather than the 'spade ' terminals on the Jaguar specific Lucas unit

BB
have you checked the ground of the injectors, if this is compromised then they might not be opening strongly enough to allow the right amount of fuel into the plenum ?

just a thought

[davidr]

Remind me where the ground is for the injectors?

[Brakebuster]

the injectors are always live, they go from there to the power resistor behind the right headlight , this then sends the signal to the injector ECU which switches them to ground as per its programme

injectors are arranged in four banks of three ( 1,3&5 - 2,4&6 ) per bank , so four switching ground circuits in total for the V12

BB

[davidr]

Just to bring this thread up to date - this is how the issue was finally resolved.
First thing I did was to try by-passing the inlet pressure valve. This made no difference so I felt that I had narrowed the issue down to the outlet pressure valve. I had changed this earlier but it was looking suspect.
Back then I was faced with a choice of a pattern part for £36 or the genuine one for £95. Guess what I did? Well of course I congratulated myself on being so financially astute and did the obvious thing. That was now starting to look like a bad decision so I bought a genuine one

Above shows the pattern part on the right and the genuine one on the left. Interestingly, the pattern part is a better physical fit as it has both of the locating lugs.
The genuine part is made by Bosch as you see below. There are no marks on the pattern part I could see to indicate who made it.

The Bosch part number and rating can be seen here

As you see, its rated at 2.5bar and I've already established that the pump is giving 3 bar so that appears to be correct.
It only took a few minutes to swap the pressure regulators from the pattern one to the Bosch one. The car hadn't been run for a while so I knew there wouldn't be any pressure in the rail but I was still careful not to spill any fuel as I was about to start the car. I also fitted my adapted return line with the pressure gauge in it.

I turned the key a few times without engaging the starter to let the system purge the air through. 4 or 5 times does it. By the time the gurgling of air in the system had stopped I could see that the pressure was 2.5 bar without the engine running. That's about 0.25 bar higher than with the pattern regulator.
On starting the idle quality was much better. It was smooth and there was no fluffing. When I felt the exhaust flow it was even with no obvious misses. Interestingly the smell of the exhaust was much cleaner.
With the engine running at tickover, the pressure in the rail was about 2.25bar as below

When the revs were raised the pressure in the rail decreased as it should. (the purpose of the fuel pressure regulator is to maintain a constant differential pressure between the fuel rail and the inlet manifold so that the injectors have a constant pressure gradient of 2.5 bar between inlet and outlet. It does this by means of a diaphragm and spring balanced against a vacuum pipe taken from the inlet manifold).
When the vacuum pipe is pulled off the pressure also rises as it loses its compensation.
So the moral of the story is that I fell for false economy!

[ptjs1]

David,

Well done! Perseverance and logical thinking solves the day!

Your point about pattern parts is interesting. I must admit to almost never fitting pattern parts to my XJS unless I have to. A recent case was the heater valve on my 4 litre which was leaking. Genuine parts weren't available and I had to fit a pattern valve. Time will tell if that was also false economy!

Cheers

Paul

[davidr]

Thanks Paul - a little off topic I know but when I fitted a pattern heater valve (again I felt the cost of a genuine one too high) I was disappointed at how quickly the finish deteriorated. Obviously the plating was wafer thin. I guess you do get what you pay for!

[Brakebuster]

good sleuthing there


BB

[pimgmx]

Am following this with interest: I'm building a preventative maintenance list for my recently acquired '89 5.3, and will be paying special attention to the fuel system.
The injectors have already been cleaned and I will be building a new injector loom this fall. Hoses & ferrules are also on the list. This topic makes me consider to also take care of the regulators then...

That Bosch part number in your pics is really helpful: this broadens the part search to a much wider network of parts suppliers, with the same regulator apparently used on Porsches and BMWs of the era. Best UK price I found is roughly 1/3 less than directly from Jaguar...