www.jec.org.uk

I have a 1985 V12 Cabriolet which developed the following problem a while back and I can't so far diagnose and clear it -
Car starts fine and drives/accelerates normally for about 20 mins., then it
starts to missfire (seems to missfire on a couple of cylinders to start
with) which gets worse if I load the engine. The engine almost totaly cuts
out unless I lift my foot when it fires normally again. If I try to
accelerate the same happens. The engine will stay running if unloaded and
light throttle but if required to pull a little it starts coughing again. It
got down to 20mph going up a slight incline and me playing tunes on the
throttle! When left for +30 mins normality returns until I have driven it for
another 20 mins-ish (got hot again?) when the above symptoms start again. If I idle the engine to temperature for 10-15 mins and try to rev up it falters with muffled coughs/bangs on pick up. When this happens (stationary or driving)the fuel pump is distinctly noisy.
So far I have replaced the fuel pump with a known good one (twice in case!) - no change and have opened the swill pot/sump tank and it looks like new - no crud whatever - and the filter was in place and perfectly clean.
Any thoughts, please - could it be the fuel pressure regulator, say?

A couple of things occurr. The timing of the problem is suspiciously close to the cut out of the '15 minute timer'. Are all the vac hoses connected correctly and in good order and is the ignition timing correctly set?

Second thought, is the aux air valve functioning correctly? (If it's the original and it is, it's pretty unusual.)

Which ignition system do you have on your '85 XJS? The Lucas AB14? If so, when was the amp module last changed? They do go south eventually and the symptoms are not unlike those you describe: as the module warms up it fails to trigger reliably.

When the symptoms appear what fuel pressure do you have?

Hi, I was reading the thread and was intrigued to see the comment about the 15 minute timer, thing. What is that you're referring to? I have never come across anything like that????
Thanks

I have had problems in the past with the amplifier, in my case the car would cut out and then restart when cooled down.

I changed the module inside it with an Intermotor replacement, and bought a secondhand AB 14 off Ebay to carry as a spare.

Pity you are not close as a quick substitution would prove it one way or the other.

The fifteen minute timer is an interesting suggestion, shorting out the wires at the temperature sensor bypasses it.
This switches out the solenoids operated by the timer.

What revs is the engine idling at?

I suppose another thing to check would be the Throttle position sensor. I suspect if it is the culprit it will read ok with a meter when cold.

The cts sometimes benefits by removing the connector and cleaning the contacts.

Regards kels.

Apologies for the thread creep but the 15 minute timer retards the ignition for the first 15 minutes of operation, if the engine starts with a temperature below 45°C. This was to promote a faster warm up, allegedly for emission reasons, although it was only fitted to B emission vehicles (UK/Europe) with the HE engine.

The system (and how to defeat it) are well described in Kirby Palm's book and the mod to reduce it to a 7.5 minute delay does work, I tried it.

It is part number DAC3797, and there's a good illustration of it on the Jaguar Classic Parts website.

Ah, I see, that's probably why I hadn't heard of it.
I live in Canada, and although I have owned, and still do own, several XJ-S cars over the years, both US & Canadian spec cars, I had never come across it. I've had a few XJ-S cars in the UK too, but I didn't ever have any problems with those cars that would have led me to it.

I am having the same problem. Also XJS-C located in Canada. I am presently checking exhaust but I noticed on a test drive yesterday that my fuel warning light was flashing as it was happening. I will be doing fuel pressure test and fuel delivery checks. She is an 88 and failed emission test because of problem. Passed at idle but had high unburned fuel reading while at rpm test. Now after reading about the amp I will change that out tonight.

In my experience the amp failure causes the engine to cut out completely. Normal service resumes as it cools down. I changed the HEI unit for one purchased from intermotor and it is currently running well. The complete amp is very expensive and the part required is a GM part.

Reading these threads makes me pleased I have an old S2 v12 running on carbs, all these clever bits leave me cold! Keep at it - Oldtimer

I have a S1 XJ12 swb also running on carbs, but I've rebuilt those bl**dy Strombergs 4 times!
Now the Opus is acting up, so it's not just the FI cars with problems

Hi

Reading your notes it seems remarkably similar to the problem I was having with my 3.6 XJ-S (see AJ6 misfire thread). Granted the engines are different, but check the screws holding the amplifier are tight and if it's on a mounting plate (aluminium?) that the securing bolts are making a good earth. It surprised me how much difference this made to the running even at tickover. Finally, I changed the crank position sensor which transformed the engine and cured all the faults.

I'm not very familiar with the V12 fuel injection electrics but hopefully this may help you or at least eliminate some of the potential issues.

Good luck,

Forgot to add, you say the fuel pump is noisy when it happens - possibly down to fuel starvation? You've checked for sediment so this could be down to fuel lines collapsing internally when trying to draw fuel through in larger volumes say when going up hills! This happened on my old Scimitar SE5A and the only cure is to replace the rubber lines. The good news is that you can buy the pipe from the main tank to sump tank from Jaguar very cheaply (I replaced mine) and the rest are just standard fuel pipe available from any good auto spares, just remember to fit new clips too.

Cheers

A noisy fuel pump can be caused by a blocked fuel filter.
Or more difficult the small filter within the fuel surge tank. To clean this you have to clamp off the fuel lines take the surge tank out and tap the retainin ring back on the side of the tank.
This will allow the filter to be removed. Thoroughly clean the tank out. Disconnect the battery first. Sparks and fuel = danger.

Regards Kels.