I’ve been trying to trace a problem with acceleration on my 97 XK8 with 44k miles and hoping someone has had a similar problem and an answer.

on startup it runs normal for at least 20 minutes. Only after sitting for maybe 30 minutes or continued driving does the problem occur. Then, upon hard acceleration it stuffers, misfires, and hesitates. Resuming cruise it runs smoothly. If I force the rough running I can occasionally get a code- lean on one bank, otherwise no code is thrown.

i have swapped ECU and ignition modules, checked fuel pressure, cleaned MAF, changed spark plugs and fuel filter, and cleaned terminals. Wiring looks good and not brittle.

 

You’ve already done a lot and it seems to be an intermittent problem, which is really annoying.

I would do a smoke (big Churchill Monte Cristo cigar is best) check for air leaks into the intake between the MAF and throttle body. Maybe a break in the accordion on air tunnel or breather pipe to cam cover.

 

Could it be battery ?

 

I wouldn't expect a minor air leak to cause a misfire at full throttle, and a major air leak would show at idle.

You checked the fuel pressure, but was that under acceleration? It does sound like a possible fuel starvation issue, so monitoring the fuel pressure while driving may be necessary. I have done this by extending the line to the pressure gauge. The fuel pump may be failing or some of the injectors may need cleaning. Do you get a misfire code for individual cylinders?

 

Have you checked the engine mounts. It may be that the mount or mounts are bad and the engine is moving under the hard acceleration causing your problem.

 

I was getting misfire's on bank 2, So bad the engine light was flashing,was told it could be timing out on that bank,
Slipped tooth or chain stretch, the misfire's were intermittent, so i could not see how it could be slipped tooth or chain stretch (told this by jaguar inde)

I changed all plugs and coil pack's on bank 2, still getting intermittent misfire's but not as bad (no flashing light on the dash)
To cut a long story short, i changed my fuel pump's and to date no misfire's.

 

I also had replace the battery, checked for vacuum leaks, and did a running fuel pressure test. Fuel pressure remains steady under hard acceleration although I did not perform the test when the car was exhibiting misfire.

The fact that the problem occurs when the car is thoroughly warm, well past temperature gauge reaching operating temp, leads me to believe it is something that takes a while to heat and begin to malfunction. I’m guessing an electronic component but have eliminated the most probable already. I haven’t tested the O2 sensors which would explain one bank lean but my reading indicates that in general an O2 sensor would not cause drastic misfire, only reduced performance and increased fuel consumption. But then, I couldn’t find any specific on if a Jag XK8 O2 sensor could be more dramatic.

 

Flashing MIL = catalyst damage so maybe you've a damaged cat.

 

Yes that is correct,i no longer have misfires but im getting the cat code,putting some cat clean in the tank soon,hopefully it might help.

Could your misfire be injector related and rising fuel pressure as the fuel get's hot with the engine temp,
"Just throwing thing's out there,may have no relevance what so ever"

 

The ECUs in the early cars have been known to suffer from bad capacitors on the circuit board. Has your ECU ever been rebuilt? From memory, misfiring is a symptom of a failing ECU.

As usual, getting the actual codes (ideally from a Jaguar-aware code reader) would help.

Best of luck, keep us posted.

 

I think the capacitor problem was isolated to the 97 model year.