I have (actually, my wife has) a nice 1978 Pre HE XJ-S with a strange problem. It will run great for about 30 minutes, then start loosing power at higher RPM, and eventually stall. After sitting on the side of the road for about an hour, it will stat up and run great again. I have verified this is not an ignition problem, it is not a fuel pump or delivery problem, and it is not a trigger board problem. I am reasonably sure it is either a MAP sensor or ECU problem. It is difficult to test, because it runs great at home, and when it does not run, it is at the side of the road 30 minutes from home. I am interested in buying a matched pre HE ECU and MAP sensor, if someone is parting out a similar car. Parts from a good running car would be great, but if you have a parts car that does not run I may be interested anyway. I an easily verify if the ECU or MAP sensor operate at home in my garage. Send me a PM or email if you think we an do business. I have great hopes of making thiis a reliable driver.

 

Just interested as to why you do not think it is an ignition problem?

 

Could you tell us what you have checked and why you are ruling out the fuel, ignition etc. We can help a bit more then.

Blocked surge tank, fuel tank not venting correctly, coil failing.

 

Jaguar XJS 1977 V12 5.3l Pulley Crank Shaft Damper / Drucksensor 73152A

Good guys

 

It has the crane ignition system, with new coil and rotor, and sparks vigorously when it is dead on the side of the road, both out the coil and at the plugs.
It has 30+ psi fuel pressure, new pump, clean surge tank with new filter screen, and has clear fuel out of the rails when it will not run. What it does not have is injector pulse, as verified by testing with a noid light. The trigger board is new. have tried to test the signal at the trigger board with a volt meter but am not happy with the result, but i can see it is different when it does not run. If I disconnect the trigger board and trigger with a test light to ground it will fire the injectors, but they do not fire from the trigger board. the gap from the trigger board to the rotor is about 0.030", if I pull the rotor uward it increases to 0.060", either reading is in spec. I would love to be able to scope the trigger board circuits, but I do not have a scope. Coolant temperature sensor has been replaced. no noticeable change with intake air sensor plugged in or disconnected. Throttle switch seems to operate properly, tested with volt meter. By process of elimination, and the change in readings at the trigger board circuits, I believe the ECM fails when hot. When it starts to fail, it begins as cutting out, on all cylinders under load. It will be OK at light throttle, low load conditions, but cut out at high throttle openings. Eventually it will not run at all. after sitting at the side of the road about an hour, it will start and run as if there never was a problem.

 

Read this many times to get my head tuned in.

The rotor has a magnet in the heel.

They can lose their magnetism with age and heat.

There are 2 shaped magnets, NO IDEA WHY, one is round and one is rectangular.

One shape is for the 3 wire trigger board, and the other is for the 4 wire trigger board, and I forget which is what as its been a looooong time.

One of mine had a weak magnet, eventually found that, and a new rotor was a breath of fresh air.

 

I was just going to say rotor arm ! 😉. It's obviously working to a degree as the car would never start. But might be failing with heat.

OP, I dont doubt your feeling toward ECU problem, and it looks like you have tested alot of things. Please don't be offended by our suggestions. New parts don't always meet original parts standards.

Test coolant thermistor with ohmmeter when hot, make sure it's not open circuit, someone else posted readings in another thread. As a note if IAT sensor is removed engine will run approx 15% richer, if coolant sensor is removed it won't start, ecu goes full rich. Check wiring from this back to ECU.

Pre He injectors flow a massive amount more than HE, something like 100ml more per minute. Maybe run some cleaner through them or check spray patterns.

Just out of interest check the dizzy mechanical advance. Rotor should move about 15° . Common on v12s for the mechanism to seize.

Make sure the fuel tank breather is working, cap should have a vent in it.

IF you think the ECU is over heating, get it out in the open, get some air or even a fan on it.
Jaguar did this with XKR's, he'll series 2 they even put a fan in the battery box.

I will ask my dad if we have any pre he ECU's. I know we broke our old race car but not sure what happened to it.

Have you tested the manifold pressure sensor on the inner wing, see if it hold a vacuum?
Not sure what the symptoms of this failing are but worth testing.
Also see how hot the injector amp is getting. Bonnet slam panel.

Hope some of these tests might help you.

Do you have a picture of your ignition set up?

 

Spray freeze is a mandatory accessory in my tool kit, it's useful for finding this type of fault. wait till the car starts to fail then get going with the freezer in a can and spray ONE part at a time I would start with the dissy.


From what you described below the ECU is working fine or the injectors would not fire when tested as you did.

The output of a Hall sensor is a pretty good squarewave. Does your meter have a Hz function, this will verify the trigger is working, also to measure the amplitude use the DVM on AC volts.

 

I have also thought the ECU was OK because I could trigger it with a test light, but that does not explain why the voltage on the trigger board circuits is different when the no start is present. This has been the case with 2 different trigger boards. I do not believe any of my meters measure HZ, I will look into that. BTW, the trigger boaed and rotor are a matched set and distributor mechancal and vacuum advances are working. I an pretty certain the injector amplifier is not at faule, that should only drop 3 injectors at a time. I do need to do testing on the manifold pressure sensor.