28 replies to this topic

[westlin]

I tested the compression on my sons 1275 cooper. The outputs I got are as follows.

cylinder 1 120psi
cylinder 2 110psi
cylinder 3 110psi
cylinder 4 130psi

Can any one advise if I have any issues from these pressures. All four rose quickly once engine cranked.

I think it is burning oil, its also leaking as well but that's another job.

Thanks

Stephen

[Craig89]

There pretty good cylinder pressures. Going from that, if it's burning oil it may well be valve stem seals or more likely worn valve guides

[Cooperman]

Those figures, whilst not very different for each cylinder, are a bit on the low side.

The first question is were the measurements taken whilst cranking with all the plugs out and the throttle(s) fully open?

If they were, then I would get a leak down test done to check the piston ring condition.

It is unlikely to be a valve seating issue as that normally just affects one cylinder where an exhaust is on the way out. Consistently low figures do indicate bore wear.

Also try running the engine fully warm at tickover with the oil filler cap removed. If it 'chuffs' smoke out that is an indication of worn bores.

[westlin]

Yes was donecwith all plugs out and throttle fully open. How do I do a leak down test? And does it look like issue with all 4?

Thanks

[sonikk4]

Add a drop of oil in each cylinder then carry out the compression test again, if the pressure increases then its the rings. If nothing changes then the valves need to be lapped in again. However this can also be down to damaged seats / valves so a strip down will be needed.

[westlin]

Thanks ill give it a go

[carbon]

Westlin - if these are compression pressures correctly measure then I think you have issues.

 

For guidance, my 1293 running about 9.5:1 CR was between 185-190 psi. This was measured after getting the engine up to full working temperature.

 

The last 1275 engine which I had readings down as low as 110-130 psi (when measured 'cold') turned out to have all of the top compression rings in multiple pieces, and bores were very worn. Mind you, it had been used hard...

[westlin]

Great! Ill check the oil approach first. It's running great though. From the mot history its done about 135k. The previous owner had the block apart because he had work done to the crank, but anything more than that I dont know, so I dont know if the rings would have been changed at that time.

is there a documented set if pressures, haynes has not got them.

[westlin]

Just dug out the old paperwork from when I bought the car 2 1/2 years ago. It had a crank regrind, new pistons and rebore. Im running in with light oil as recommended. Would that make any difference.

it was not run after the rebuild as far as im aware, if it was the rest of the car was not up to it.

[westlin]

Ok re done the test again this morning both with and without the oil in the bores and here is the results.

Cylinder initial with oil

1 120 psi 130 psi
2 105 psi 115 psi
3 108 psi 112 psi
4 130 psi 135 psi

this tine I let the pressure stablise before noting it down. As I mentioned earlier in the thread and I dont know if this makes any difference, it has been rebored and new pistons and has only done as far as im aware about 60 miles since being changed.

let me know your thoughts.

thanks

[fwdracer]

Light oil? Not sure what you mean. Do you mean light grade (10w40?). Check the Api rating on whatever you've put in as a start point...

As it is a fresh build/running in scenario I've another suggestion.

Running in should generally be done on something with a low/poor APi rating (C or D), if lubrication is good the rings never bed in and this could be the source of your low compression readings. Run in on cheap, low grade 20/50 and then move to something decent once the period of running in is complete. And don't be afraid to give the engine some load/beans to really get the thing run in - extend rpm.

It may be hard to imagine, but even your "light" oil might be too good.

Max variation across the cylinders should be 10-15psi.

Edited by fwdracer, 22 December 2014 - 12:46 PM.

[sonikk4]

Mmm a low increase but even then these are low figures. If it's all new then it could be down to the rings not being bed in but I would still expect to see them higher than that.

Get some more mileage on the bottom end but if the figures do not rise then it could be the bores are glazed and will need re honing.

[westlin]

Im using morris goldern film sae 30 as recommended following a rebuild. Ill see how it goes after alot more miles.

thanks

[fwdracer]

Morris golden film sae 30 - APi CC/SD. This is suitable. Go and give the car some grief - extend the rpm, use wide open throttle from low down and really load the engine. Sounds as if your rings haven't bed in yet.

Edited by fwdracer, 22 December 2014 - 01:28 PM.

[jaydee]

Headgasket gone between 2 and 3 (typical 1275 - use a BK450 HG).

Poor rings seal, i dont know if you measured bore clearance and tolerancies but for now check mixture, if too rich then rings will never bed in properly due to bore wash.