QuestionQUESTION: Hello, we are hoping that you can help as we are having huge problems with our 1990 XJS. The car failed to start it went to 5 other garages before coming to ourselves. We changes the dizzy cap, rotor arm, amplifiers, plugs and leads and the car started and ran for about 1 minute before dieing. We removed all the plugs as it smelt of fuel cleaned them then checked for spark and replaced. The car did the same again ran for 1 minute before dieing? We then replaced the fuel pressure regulators and this made no difference. Our car does not have the cold start injector which is often responsible for this problem? Can you advise? Any help will be greatly appreciated.
kind regards, mark
ANSWER: Hi Mzrk,
I have no experience nor info on any Jaguar after the 1987 XJ-6. However, on any kind of car that runs for a minute and cuts off, you should be able to isolate the problem to a small area so that you may have a chance to correct the problem. You need to go back to the basics. All engines require only 3 things to run, Compression, Fire and Fuel with conditions on each.
Compression will not usually come and go so you need to work on fire and Fuel.
The trouble here is that you are replacing parts without doing any testing and you may run out of money before you find the problem that way.
You do need a couple of tools but you will find that they are less expensive than what you have already spent.
Get a fuel pressure gauge (for injection systems) Before I bought a regular fuel injection gauge I used an industrial 0 to 100 PSI gauge and fittings. (not expensive) (Your system probably works in the 30+ PSI range)
Timing light is something you should have anyway as long as you are owning cars.
Here is how to run the tests, connect up the fuel pressure gauge so you can monitor fuel pressure and connect up the timing light so you can monitor the flash. Start the engine and set the speed up to about 2500 RPM and just wait and watch. When the engine starts to die look at the fuel pressure and the flash of the light. (I like to put the timing light on the coil wire so I can see a steady bright light) If the light and the fuel pressure stay good until the engine stops rotating then you have eliminated "Fuel supply" and "Fire from the Coil". If these are good go to the next test, get a can of either starter fluid or something like WD-40 and when the engine starts to die spray into the intake to see if you can alter the way it is faltering. (If this car has two intakes like the older ones have someone help and use two cans of spray. If you can't alter the cut off of the engine then you have eliminated fuel as a fault. Next test is to put a vacuum gauge on a vacuum port and note manifold vacuum at 2500 RPM and watch the gauge as it dies. (DO NOT MOVE THE THROTTLE AS IT DIES) If the reading goes to 0 just before it dies you may have a partially stopped up exhaust system which requires an exhaust back pressure test to be sure. (If you have never done this let me know and I will go over the procedure for you)
Do these tests and let me know the results of ALL the tests and I will tell you the next set of tests.
Howard
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QUESTION: we have already carried out most of these checks. The fuel pressure sits at 2.5 bar (35 ish)psi. The timing stays were it is, and is not adjustable, as it is ECU controlled, with no advance mechanisms or retard mechanisms. We have put injector pulse lights on, which flash. When the engine dies, pressing the accelerator, or leaving it still makes no difference? it still cuts out. The plugs come out of the engine wet, and black and do not spark at all. If you clean the plugs it will start straight away and then run for a minute. The exhaust isn't blocked. We have been told it could be the temperature control switch for the ECU? or the thermotime switch? but we don't know where they are. Thanks for helping
AnswerYour car is out of my range of cars I have info on or any experience on but I do know that thermo-time switches only control cold start injectors and only in the key "start" position. As it looks like excess fuel is your problem it would be a good idea to disable the cold start injectors (probably two)(I have had a Bosch/Lucas cold start injector stuck open once) Also look for raw fuel in the vacuum line to the fuel pressure regulator and as you say check the coolant temp sensor for the injection system (usually a two wire sensor close to one of the thermostats, the older cars had it on the left thermostat housing.) (most temp sensors for gauges are one wire). The coolant temp sensor is tied into the throttle potentiometer
All of this is from my experience with the older Bosch/Lucas systems. So it may or may not be of any use to you. I always looked at what had the most control of fuel mixture and checked to see if the problem was over all cylinders or just one bank. It was very rare to find a bad ECU. When mixture was determined to be the problem I always looked at what had most control. All injection systems have to have some kind of air flow/mass or vacuum control, coolant temp control and some kind of cold start control. Not all have a throttle control or air temp or feed back control (0-2).
Sorry that I can not be of more help as I quit working on cars after the 1987 cars were out.
Howard
