Hi all,

I have been experiencing what I think is a misfire at between 1300 to about 1800 RPMs when under load. So I decided to change the spark plugs as I have no history from the previous owner of a spark plug change. After the nightmare I had with changing them the car has felt much better since. But I can still feel what I think is a misfire in the same rev range.

If I push the car to about 2000 RPM and above then I can't really feel any engine vibration/misfire.

The car idles fine.

For instance, when I approach an incline I can feel the car feels a bit underpowered and I have to rev the car more than I used to in order to maintain the speed. I can also feel some hesitation when accelerating from a standing start and at certain speeds especially when in cruise control. The revs can drop and rise even at a fairly constant speed and even when not under any load.

There doesn't appear to be any difference in my MPG even though I feel I am driving the car harder than I used to since this misfire has been noticeable.

I've had a vacuum/smoke test performed and no leaks where found. So my next thought is the coil packs as I now have 143K miles on the clock, so most likely that one of them has an issue.

I have no warning lights at all and no fault codes registered either, no matter how far or how hard I drive. There aren't even any pending codes.

I have found how to test the coil packs with a multimeter for OHMs but I see the coil packs on our cars have 4 pins and not 3. Does anyone know what OHM reading I should expect to see from a correctly working coil pack and across which pins ?

Thanks,

Chris

 

If you want to do it get a proper tester. An ohms figure is not much use for the COP type.

 

hey chris...for what is worth,had a similar problem with my 95 xj6....all the ohm readings were the same on all 6 packs, 3 or 4 had signs of aging, cracks and carbon dust. cleaned them and used heat shirnk that I bought at the electrical supply house......$ 30.00 dollar repair...problem went away.....hope this helps

 

Ebay sells 8 coils from China, with a lifetime warranty, for less than a hundred dollars delivered.
They have been on both my XJ's for several thousand miles now, with great results.
The stock ones were still working, but for the price, the spark plug tune up, with cars both approaching 100,000 miles, included new coils.
Old ones are still in the trunk, probably ought to take em out next time i'm in there.

 

Hi all,

Apologies for not responding sooner but I've been out of action for a while. Thanks for all the responses and suggestions.

I haven't got around to investigating this further yet. The car has now started to run a little rough from a cold start and will sometimes slowly lose revs and if you try to rev the car the revs get lower instead and then the car sometimes stall.

Thanks,

Chris

 

I'd use OBD live data (e.g. via elm327) and see what the fuel trims show - many threads on how etc.

 

Thanks JagV8, I'll look into that and see how I can set it up. The car has finally just thrown a fault code of P0172 but that seems to have many causes. I'm convinced I have a vacuum leak somewhere or my MAF is faulty possibly.

 

Have those eight intake manifold gaskets been replaced?

 

P0172 is a rich code so an air leak is not likely. The jag doc give all the possible causes.

 

Good point wingrider. No they haven't. Although I've had a smoke test done already and nothing was found but I'm going to get it done again in case.

 

Smoke will not show you much, if anything, if its the intake gaskets.
Its sucked into the motor, & out the exhaust if my thinking is right.
My car always ran great, passed the tail pipe emissions test with very, very, low numbers.
No more tail pipe test after that year past though, so i was happy after fixing things, and the light stayed off..

 

so has the issue been solved? I have an XJ 5.0 SC with the exact same issue you described. I tried changing the fuel filter, transmission fluid and filter, and intake hose. Still the issue persists.
​​​​​​​would appreciate your feedback!

 

Since it is only on one bank I would look at leaking injector or coil misfire on right hand bank. Both will give a rich running code. And cause uneven running if intermittent. May even lean to the leaking injector since on acceleration even more fuel is added and floods out the cylinder.