Marelli V12 idle stumble

[davidr]

Hi BB. As you say, the micro switch is to tell the ECU that the throttle is completely closed so fuel supply is shut off on the over run. It's also why there is a brief dip in the revs as the car comes into idle as the revs drop and the idle setting on the ECU takes over.
One way to check it's working is with the fuel computer. Switch it to show instantaneous consumption whilst driving and it should show 99mpg when you lift your foot right off the pedal and coast. That's the only time consumption will be that low!!
It's my understanding that it should click off just as the throttle capstan starts to move. That's why I was interested (and perhaps confused ) by SteveM's description.

[poprivet]

Hi David, I can't help personally but there may be some info in the various documents at

http://www.jagrepair.com/JaguarXJSElect ... Icodes.htm

in the technical section.

Even if it doesn't provide the answer, there is a lot of other interesting stuff there including A/C info.

[davidr]

Had an interesting evening in the workshop last night. To start with, listened to England winning a cricket game which was a pleasant surprise.
Secondly, I had another go at checking and adjusting my throttle settings with my now well used 2 thou feeler gauge. When I was happy I started her up with the air filters off and outer half of the air box removed. She ran up well and appeared to be much smoother. I then put the filters and boxes back as she ran and all was still well. Finally I plugged the air temperature sensor back in and the stumbling idle returned. Took the sensor out and put the multimeter on it and it's resistance was far too high. This would lead the ECU to think that the ambient temperature was very low and throw more fuel in. ( I think it's that way round!).
I've ordered another one so we'll see if that fixes it!
I also have had some loss of power issues and misfiring on the motorway which goes away when i start throwing it around bumpy roads or stop and start again. In other words, classic symptoms of the fuel swirl pot and filter needing a clean. Hopefully I'll get round to that job one evening next week.
I'm hoping that these two things will improve matters!

[Brakebuster]

cool

BB

[davidr]

I got the new air temperature sensor today and fitted it. The sequence of events went like this
First the engine was running on the old air temp sensor. Idle rough and stumbling
Second I unplugged the sensor. The revs increased and the engine smoothed out - not perfect but better.
Last I plugged in the new sensor and the revs dropped and the idle was better than the old sensor but still not as good as I want. It's not bad, but it's not good either.
Still have to clean out the swirl pot as I'm running down the fuel level.
Wondering why the car runs better with the air temperature sensor unplugged? And why does it idle faster? I guess it runs richer with the sensor unplugged? Points to the mixture being too lean at idle? maybe there's an air leak that I still haven't found?

[Brakebuster]

have you proved the vac pipe to the boot ecu is good ?

i have also found benefits to fit the vac reservoir next to the ecu , this holds the vac and prevents fluttering of the pressure

this is available from heritage, and is a 30 second fit , and costs about a tenner CAC4968 ( ebay )

i think Steve fitted one of these to his V12 ?


BB

[steve_m]

Not me but I have ordered one following your post (£9.62 from blackcountryjaguar).

[davidr]

Actually, it was me! Fitted one a while ago. Didn't feel it made any difference.
I cleaned out my swirl pot earlier - plenty of crud in there. Will post some pictures when I can be bothered to go through all the fuss of resizing them etc!

[Brakebuster]

lol

at least you felt good by fitting it......


swirl pot sounds like the culprit


BB

[davidr]

A few pictures of what came out of my swirl pot and fuel pickup. I do wonder why the filter mesh is so fine and quite how much work the inline fuel filter actually does.
Haven't had a chance to run the car properly yet - mostly because it doesn't have much fuel in it!!

[Brakebuster]

that filter is 100% the cause of the poor idle and lack of power , and if that much debris is on the pickup filter I would say the tank needs pulling and sealing on the inside ,

I too have wondered what the main inline fuel filter actually does, as this pickup filter protects the pump , the main filter must only be there to stop pump parts going to the engine when it implodes ?

BB

[davidr]

The lack of power issue only happened once on a long steady drive and this clogging of the intake filter is undoubtedly why. As you say BB, pulling the fuel tank and flushing, sealing and possibly replacing it is now on the winter jobs list.
I haven't driven the car since flushing the swirl pot. I had intended to give it a blast this evening but the weather prevented play. I did start it up and let it warm up in the garage though. I can't say that the idle was any better.
I did check out the vacuum pipe by pulling it off near the ECU and the suction was strong so I guess the pipe is OK.
By the way, i took a picture of the vacuum reservoir we were talking about earlier just for reference for other people reading this.

[oldtimer]

Hi DavidR,
Three questions :-
1. Why do you wonder why the pick filter is so fine ? Surely it is doing it's intended job.
2. Have you any idea when this filter was last cleaned/ inspected ? Is it referred to in any servicing documents.
3. Tell me more about the process of sealing the inside of the tank, like ,what is it and how is it done ?

Oldtimer

[davidr]

Hi Oldtimer - answers to your questions
I'm surprised that the pick up filter is so fine because there is another filter after the pump which I would have thought would catch the finer particles. This pick up filter is not part of the service schedule but the fuel filter is. But as I've shown, the vast majority of the muck gets caught in the pickup filter. Maybe the pump is damaged by finer particles?
The pickup filter has never been cleaned as long as I've had the car (12 years).
As for sealing the tank please refer to BB as he's changed his!

[dhdove]

Swirl Pot and pick-up Filter
Guys......
......it's an XJS "thing", much discussed and detailed here for many years
The tanks corrode badly, the swirl pots corrode badly, you break down. QED.
I would link you to my own woes with photos ten years ago but they seem to have dropped off the Bulletin Board......
TonyS