Jaguar Repair: Jag xj6, jaguar xj6 series 3, bright sun light

Question
1985 jaguar xj6 (series 3)
Hello Howard,
Here's my problem, car runs great in the drive way, jack up the rear wheels in drive car runs great. Put in on the street in drive car takes off like cat problem (sluggish) removed exhaust same problem. What else could create this problem. ( Already rebuild head, new head gaskets, cams aligned properly.) Please help:(

Answer
Hi Maria,
If you have set ignition timing to specs it needs to be separated between a fuel problem or an ignition problem.
To do this you need to be able to see ignition at the time of failure. The ideal way would be to have a scope on the engine while the car is on a dyno. But not many people have that option. In the Jag dealerships I worked in we had a scope but to put the car under a load and watch ignition without that kind of equipment you can do the next best thing.

Put a timing light on the COIL wire (not a plug wire as you would setting timing)and run the wires out from under the hood and put the timing light under a wiper arm so you can see the flash of the light while driving. If it is a bright sunny day you have to tape a piece of cardboard over the light so it can be seen in bright sun light. Tape the trigger down and drive the car under the conditions that it is having the problem. I found that even a occasional misfire in the primary ignition will show as a flicker in the light that looks like a steady light. This is a rough test of ignition up to the coil. To test cap, rotor and wires you would need to put the light on each plug wire and make the six road tests.
Don't rule out spark plugs as I did recently on my own XJ-6. When I had a miss under load. I removed all the plugs and just looked at them. I have been looking at plugs since my first Jag dealership job where I predelivered two new MK II Jags for Eleanor Roosevelt in 1960. So I assumed I knew what a good used plug looked like, at least to a 98% accuracy. I have no scope in my shop at home now so I was forced to take it to a friends shop who does and there is was. A bad spark plug. Big embarrassment! So don't leave any stone unturned or it will come back and bite you.

The only thing in fuel you can easily test is fuel supply. to do that you need a pressure gauge and a long hose and clamps and a "T" fitting. A fuel injections testing gauge may be expensive but you have just buy an industrial gauge (0 to 100 PSI) is what I use at home. Injection hose and a metal "T" can be purchased at most any auto parts store. Put the "T" right at the fuel rail where the fuel line comes to it and run the hose out from under the hood and put the gauge under a wiper arm facing inside and be sure to put clamps on all hose connections because the pressure is 32 to 42 PSI. Driving the car the pressure should read about 32 PSI but when you step down on the throttle hard the pressure should go up to about 42 PSI. This only checks fuel supply to the injection rail under the condition of failure. It does not check fuel into the engine.

If you do all this and still have the problem you need to find a shop equipped to test Jags. That is about all you can do in a home shop.
Howard