Jaguar Repair: xj6 1987, bright sun light, bright sunny day

Question
car starts fine with one try/engine idles up and down in park/when i'm driving at speed starts losing power if i give it gas too much it dies! will restart in a couple of tries/does the same thing eventually

Answer
Hi William,
You symptom is useless info as it does not tell you anything other than something is wrong.

You need to first run a compression test, 125 to 180 PSI on all cylinders with little difference between cylinders. ( be sure to hold the throttle open when testing and spin the engine over at least 4 or 5 revolutions on each test)

You must test fuel pressure at the time of failure. It must maintain from 32 to 42 PSI at all times. I place a "T" in the line just at the fuel rail and run the hose out from under the hood and place the gauge under a wiper arm so you can see the gauge as you drive. (Be sure to put clamps on all the joints you make.) Any industrial gauge that can read up to the 40s PSI will do.

You must be able to see spark at the time of failure. This is easy to do by just putting a timing light on any plug wire and tape the trigger down and put the timing light under the other wiper arm. (If it is a bright sunny day you may have to tape a piece of cardboard over the end of the light on the windshield so you can see the flash of the timing light in bright sun light)

With these three simple tests you can narrow the problem down to compression, fire or fuel then test the faulty item. Compression don't usually come and go so you will probably find the problem in fire or fuel.
Let me know
Howard