Compression test results

[BI8759]

I think my 1098 is running on three cylinders. There is also an irregular knocking noise coming from somewhere even at idle. I have checked the ignition and reset the valves. No difference so did a compression test this morning: throttle open, plugs out and engine cold. The results were:

1=150, 2>140, 3>110, 4<150

WIth oil:

1=165, 2=190, 3=140, 4=190

Plugs 1&4 looked in good nick, 2&3 were oily.

Would I be right in assuming the rings have gone on cylinder 3?

Sorry if this is a silly question - I can follow instructions but I don't have the experience/skill for the diagnostic bit

Many thanks in advance.

[bmcecosse]

None of them are 'great' - but #3 is certainly suspect...off with it's head! If the valves are ok (grind them in anyway) then pop up #3 piston for a good look at the rings.

[BI8759]

Many thanks. I'll take a look.

[chickenjohn]

you may have a cracked ring on No 3.

[bmcecosse]

The pressures all rise about the same amount when oil is added - so it's more likely to be a valve... The head has to come off first anyway!

[chickenjohn]

yes, agreed, most likely to be a valve as comp on no 3 would have gone up with oil in the bore.

[BI8759]

I've taken the head off today and there was a ridge of carbon/oil in the thin strip between pots 2&3. There was a lightly scored ring about an inch down from the top of pot 1. No lips at the top or vertical scoring anywhere.

I have a good mpi head here so after cleaning put that head on, did the tappets, and the results are:

1=160
2=170
3=170
4=175

wet:

1=175
2=185
3=190
4=205

so, if 3 is now in line with the others can I assume that the rings are okay or have I introduced another variable buy changing the head???

Many thanks

[bmcecosse]

Have you sunk the valves on the MPi head - or do you have pockets in the block??? Otherwise - bent valves ahead.....

[BI8759]

I'm not going to run it with that head on - it's a refurbed head so I know it has no issues. I thought maybe I should use it for testing while I'm cleaning my one up. My theory is that if I know those valves are okay, then that takes one variable out of the equation. I think the results I got today show me that the rings on no.3 are okay but I have been wrong more than once this week - you should ask my electrician

[bmcecosse]

Yes the bores/pistons/ rings are fine by that test - but the exhaust valves may be dangerously close to the block! Don't run the engine with that head on!!