I have a 1998 jaguar XJR with 102,000 miles, I have been experiencing engine codes such as
P0304,P1316,and P0400. The main two codes the show up after clearing is P0304 and P1316.

I decided to do a compression test today after replacing spark plugs injectors coil packs many times but still the same code.

Compression
Results

L. R
85 .105
90. 75
90. 80
75. 65

Engine numbers
5. 1
6. 2
7. 3
8. 4



The compression tester I used was from Amazon, I am going to get another compression tester from the auto parts store to compare the readings to make sure it’s accurate.

I know these readings are pretty low it’s supposed to be 120 psi based on my research.
does anybody have any advice upon what I should do next or if I’m doing anything wrong


I did remove all spark plugs and coils and fuel pump relay. and car was started with throttlebody open.

Help!

Xalty, am I correct in that the 2002.5 my engines would still be x308 AJ27 engines, or are 2002.5 and beyond all x350 engines? Serial number would determine the cylinder number layout, correct? Interested because while I have an 01 XJR, I know it has a replacement engine thats newer than the car, but not sure of its date yet since I can't get a good look at the numbers on the side of the block yet. Just good info to know for potential future issues like misfires.

So cylinder 4 is misfiring and that is your lowest compression cylinder. I am sure
you know to check the wire, the boot, the plug and the coil to chase down the issue
you are experiencing. Coils do fail with age but I would first put in a new plug and
wire to alleviate that from the list.

Can a clogged EGR valve cause misfire in that cylinder?

I believe that all 4.0 engines are have the up to 2002.5 my layout. I remember reading it in a technical manual somewhere, I'll have to get it back out sometime.