I have a 1998 Jaguar XJR that I've been working on as a project car, and I consistently get P0304 and P1316 misfire codes.

Here's what I've done so far:

1. Replaced all spark plugs and coils.
2. Ran fuel injector cleaner through the fuel system.

Despite these efforts, the codes still return. The car runs and drives fine, though I think it might have a slight vibration when accelerating, but I'm not completely sure.

I performed a compression test using two different gauges from different brands and found low compression on cylinder 4.

Here's a summary of my next steps and observations:

- I attempted a piston soak with B12 and Chem-Dip, thinking carbon buildup might be causing the issue. After letting it sit for a few days, the codes still returned, and the compression readings remained low.
- I added oil to the cylinder walls, which temporarily increased the compression. Here are the compression readings for all cylinders, both dry and after adding oil:

1. 110 psi (dry), 150 psi (oil)
2. 100 psi (dry), 160 psi (oil)
3. 110 psi (dry), 150 psi (oil)
4. 80 psi (dry), 120 psi (oil)
5. 140 psi (dry), 160 psi (oil)
6. 110 psi (dry), 140 psi (oil)
7. 120 psi (dry), 160 psi (oil)
8. 90 psi (dry), 130 psi (oil)

FYI ( Engine was at operating temp while doing these test)


The car has 103,000 miles. It still drives fine, but I'm concerned about the persistent check engine light and "restricted performance" message. I also added an engine additive called Restore during an oil change to help with compression, but the problem persists.

Given these symptoms, could this issue be related to piston rings, cylinder walls, or the timing chain? The engine does not make any rattling noises.

I'm looking for advice on whether it's safe to continue driving the car in this condition and what my next steps should be. I'm hesitant to replace the engine due to the cost and complexity of installation, and I'd hate to part with the car in otherwise good condition.

Any insights would be greatly appreciated.

 

What spark plugs you're running?
you replaced all coils too?
Secondary tensioner what about it ?
Vacuum leak could make multiple misfiring code check the intake and it's hoses vacuum lines
i hope your plug seal doesn't leak because it can cause misfiring too

 

Actually this is very strange I thought that cylinder 4 had low compression, but I had the diagram confused. Cylinder 4 actually has good compression, but is still misfiring.

cylinder 5 has low compression but I’m not getting misfire code for that cylinder it is on the opposite bank.

I have Bosch spark plugs. I am going to change them again today.

I inspected the tensioner while doing a valve cover a few months ago. It looked like it was in good condition. I’m pretty sure they were updated. But I couldn’t really tell..

 

Bosch 7957 working great for our engine
Check the plug seals it leaks oil
I would inspect the coils while engine running remove the coil bolts and pull them one by one if you hear ticking sound and the engine low rpm then coil and spark and injector are fine on that cylinder if you pull coil and hear ticking but no lower rpm then probably the spark is toasted . I prefer to do this while engine kinda let's say 100f I don't like to hear the cooling fan kick high speed it's headache

 

The only thing adding oil to the cylinder will do is temporarily seal the piston ring area. And if that brought your compression up…then leaky rings or scored walls is the issue. But, that won’t throw codes. I’d diagnose the codes issue, and temporarily ignore the compression.

 

OK, great I’ll try that. I’ll keep you posted.

 

it seems that you have two problems.
Cylinder 4 misfiring, and low compression on 5.
For the misfire, switch the coil to an adjacent cylinder and see if the misfire moves with it. If so, bad coil. If it stays, the coil is good but there might be some faulty wiring to it. Sometimes coils are bad right out of the box.
The low compression sounds like rings or cylinder wall since it improved with oil. It might get better or worse but you can run the engine with no real damage.

 

I’ve tried replacing all coils, spark plugs, but no different same code. Could it be a injector problem?

 

Hi 98Jagxjr,

Sorry to join your thread late. Yes, the #4 injector could be clogged, leaking, or failed. One clue is to view your Short-Term- and Long-Term Fuel Trims with your scan tool. A leaking injector will cause the engine to run rich and the Engine Control Module to reduce fueling in response. As long as the fuel trims don't exceed a 25% variance, rich or lean diagnostic trouble codes will not be triggered.

Jtestrake's suggestion of swapping coils to rule out a bad new coil is a standard and definitive test. Also inspect the coil wiring carefully for any damage. The wire insulation can become brittle with age and flake off, leading to shorts. The supply voltage should be the same at all the coils, so it's easy to test for a loss of power to the coil.

Oil on the spark plug threads in the cylinder head adds resistance to the ground path for spark and can cause misfires. To resolve this, I use a long handled "bottle" brush with brass bristles and throttle body cleaner to clean the threads in the head.

Your low-compression cylinder could have a valve problem. If you have access to an endoscope with a right-angle mirror, you may be able to see if a valve is burned or not closing due to carbon buildup. You can also inspect the cylinder walls for scoring. Inexpensive USB borescopes are now available that interface with your phone. Check Amazon for options.

You can run the engine with a low-compression cylinder, but you don't want to run it with a misfire. The unburned fuel will destroy your catalytic converter and may cause other expensive issues.

Cheers,

Don

 

leakdown now

 

I am running fuel injector Through the fuel tank and driving it so I’m still waiting to see what happens to see if that’ll help.

Another code I have I forgot to mention is P1646. I know it’s a passenger side upstream oxygen sensor. Will this cause my restricted performance and cylinder form misfire or no. Will the cylinder 4 misfire be separate?

 

On 1998/99 AJ26 engines, P1646 is related to the Fuel Pump 2 Relay:

You need to address this before suspecting anything else.

 

Yeah, Injector cleaner will not solve that problem.