Whilst giving it a good service I decided, (foolishly I suspect), to carry out a (dry), compression test. Well, whilst I'd got the plugs out and the inlet manifold off, why not. What could possibly go wrong? The test was carried out with a Clarkes compression tester, using the rubber push-down bung attachment as none of the other adaptors would fit, even though they were the same diameter and screw pitch.

Cylinder 1 = 85 psi
Cylinder 2 = 110 psi
Cylinder 3 = 90 psi
Cylinder 4 = 130 psi
Cylinder 5 = 105 / 110 psi
Cylinder 6 = 110 psi

Concerned about the low results for cylinders 1 & 3 I carried out a second compression test on those two cylinders. Just incase I hadn't pressed the compression tester down hard enough, or I'd got it at a funny angle.

The second run gave the following results;

Cylinder 1 = 85-90 psi
Cylinder 3 = 90 psi

Looking at the engine head-on, (radiator end), the left bank is cylinders 1,3,5. The right hand bank cylinders 2,4,6.

I know I might be clutching at straws here, but when I first started the compression test, the bowl I was using to catch the fuel from the fuel rail, (injectors removed), slipped, spilling about 500-600ml of petrol right down cylinders 1 & 3. I sucked as much up with a syringe as best as I could. Like I said, I'm pretty sure I'm wrong, but is there any chance the low readings from cylinders 1 & 3 could be something to do with that? Bore wash perhaps.

More to the point, if any of you chaps were to hazard a guess, what would be your estimate as to the root cause of low compression on two neighbouring cylinders? For what little it's worth, the spark plugs on all six cylinders looked in equally aged condition, weren't sludged up and black or anything. In fact all six electrodes / tips were a nice dark grey colour with a small amount of brown 'burnt' around the bottoms of the ceramics.

Assuming that 110 psi is the norm, is a 20% drop on two of the cylinders something to be 'really, really' concerned about?


In advance, many thanks.

 

I suspect that you conducted the compression check because the vehicle likely had some performance issues. In any case, 85 & 90 PSI on cylinders 1 & 3 would definitely be low. Perhaps some engine additive might at least partially restore the compression on those cylinders.

 

How many miles on the car and what year ?

 

Sorry. My apologies. I thought, (incorrectly), that I had included that information.

Early 2004 V6. 92'500 miles.

 

When I removed the plugs the other day, the plug from cylinder six was soaking wet in oil, as was the spark plug 'well'. Closer inspection of this plugs tip showed that only half of it was oil soaked, suggesting that the oil was coming from the top of the plug, not the bottom, and suggesting that only half of the tip was oily because of the angle at which it was retracted from the spark plug well, causing the oil at the upper part of the spark plug to run down. I was 90% certain that this was in issue caused by a partial failure on the innermost cam cover gasket, (as there were at the time no other symptoms of mechanical problems). However, I decided to do a compression test just to be on the safe side.

As it turns out, cylinder 6 was fine, (110 psi). 1 & 3, as you can see, are not.

The thing that is baffling me is that there are no other obvious signs of a problem.

All six of the spark plugs showed signs of age, but also the tips of each looked evenly 'aged', coloured with no signs of burnt oil residue.
The engine pulls nice and cleanly, very strongly, revs sweetly and freely, doesn't overheat, even in very heavy rush hour traffic, doesn't lose any oil or coolant, doesn't smoke, no signs of any 'mayo' on the expansion tank cap or the oil filler cap, no signs, as far as I can see, or oil or coolant leaks on the block. Everything appears to be fine except for this worryingly low compression.

of course the million dollar question is, why? Why is the compression so low on two neighbouring cylinders?

When I was doing the compression test I knocked over the bowl that was catching the fuel coming out of the fuel rail straight into cylinders 1 & 3. About 500-600ml (1 pint). Could the low results on those two cylinders be caused by bore wash?

 

Does this help:

https://workshop-manuals.com/jaguar/...ions/page_951/

I believe that the professionals inject oil and check again.

If you are 90% sure that the oil is from the cam cover why are you doing a compression test?

IMO just fix the oil leak and enjoy your new Jag especially since it's running like a dream.

BTW have they fixed that wonky spire yet?

 

Yes it is, greatly.

Many thanks. I hadn't seen that page before. The bad news is that it confirms what I suspected, but didn't want to admit, (HGF I suspect).

I'll do a 'wet' test in the morning and see what figures that brings up.

 

Don't say I didn't tell you

Just to make sure, did you perform the compression test at fully open throttle? That's absolutely mandatory to get representative peak values that tell you something. If the throttle is cracked open just at idle angle than the start pressure in the cylinder is so low that also the peak pressures are of course too low.

If you did test with wide open throttle (WOT) then all your cylinders are far too low. Best one with 130psi is 8.9bar for my SI-mind, which is far too low for an engine with a compression ratio of 10.5
Also the spread of the cylinders is huge with the lowest cyl. 1 being 35% below the highest cyl. 4
If these measurements have been done properly (@ WOT) then you will probably re-enact my old 320i experience and rip the engine apart to change the piston rings which are the main suspects (stuck in their coked grooves).