So I bought this car at the dealer auction in Manheim PA.

Car passed post sale inspection apparently incorrectly. The car is an 02 xk8 - 93k miles.

Car has a distinct miss. Its noticeable at idle, and the car generally feels sluggish.

The car had a P0307 cyl #7 misfire code, which the auction inspector diagnosed as 1 new ignition coil and spark plug, said the repair was less than $500 and told me it was not arbitrable.

So here's where I'm at.

Replaced coil and plug, made zero difference. So obviously it's beyond that. Code immediately comes back after clearing.

We did a compression test on that cylinder and it's showing 75 psi. There was no oil in the plug well.

My brother seems to think it's a head gasket, but I'm not convinced. No coolant loss, no smoke, no external coolant leak, car runs and idles at proper temperature. No other sign of a head gasket failure.

Where would you guys go from there? I'm thinking fuel delivery - possibly a bad or clogged injector?

I'm on the fence as to what to do with this car....

All thoughts would be appreciated.

Thanks in advance,

George

 

Low compression will cause a CEL and misfire, according to the Jag OBD it's the following
worn cam/broken valve spring
fuel delivery pressure low/hi
fuel injectors leaking
spark plug failure/incorrect gap
ignition module
coil

Did you check fuel pressure?
As the coil and plug are new it's down to the rest. The compression should be double that, so maybe a look at valve clearance on the cylinder.
Is the plug wet, or can you smell gas in the cylinder after running? this would almost rule out a faulty injector

 

That compression is worrisome. Are you opening the throttle when you check it? Can you check another cylinder to validate your test?

 

I wasn't there for the compression test. My brother did it.

I'm not sure if he left the throttle open or not. I'm not at the shop except for Saturdays.

What exactly do I need to tell him as far as a compression test procedure.

Thanks again,

George

 

So I called my brother and asked him if he had the throttle open when he did the compression test, which was met with a snyde " I know how to do a compression test" so I guess that's a yes.

He basically asked me how far I want to go with the car. In terms of do we diagnose it throughly and fix it and sell it, or do we take it back to the dealer auction and sell it as is and cut our losses.....

Food for thought.

Take care,

George

 

Just thinking here... are you "certain" that you replaced #7? If memory serves, the right bank is 1 through 4, and the left bank is 5 through 7. Am I right on that, guys?? Double check me. That would be the second to the last plug on the driver's side. One thing you can do is to run the engine, and disconnect the coil wires one at at time until you find the one that does not change. When you find that one, swap coils with one that fires correctly and see if the problem moves with the coil. If not, you may have a bigger problem in that cylinder. In any event, 75 psi is not good. But, I am wondering if it will actually show up as a "misfire"? Don't know about that one. There was a time when we saw a great many of these with carboned up top ends. Some would get so fouled up that they wouldn't start cold. Basically a lack of compression. But once we got them running by putting oil in the cylinders, we could run some top engine cleaner to help clear things out. Confirm that cylinder is at fault, and maybe try some top engine cleaner before you give up on her. Frankly, we usually find a bad coil. Probably why the guy tried to sell that idea to you.

 

We replaced the coil and plug on #7 - driver's side 3rd from front of engine. No change.

What I need to get a better understanding of is this.

HOW does the OBD system determine WHICH cylinder is misfiring. It's consistently pointing to #7, but if it's not ignition related, then how does the car know. We have 4 o2 sensors, and a knock sensor. What else would let it pinpoint the cylinder?

Take care,

George

 

crank and cam position sensors? low compression leaves a slower cylinder as there's no power stroke.

Maybe whip the cam cover off and check the valves. Just in case...