Confusing Compression Test

[Mickywoosh]

I've just done a compression test on my T350 (knowing somethings up!!) and get some confusing results. Could do with some working out whats going on.

Cylinders 1,2,4 and 6 - all together at 150 PSI (Throttle closed)

Cylinder 3 - 95 PSI throttle closed

Cylinder 5 - 120 PSI throttle closed

So problems with Cylinder 3 and 5.

Put a small drop of oil down the cylinder to seal the piston ring and get the following results:

Cylinder 3 - 120 PSI throttle closed; 130 PSI throttle open

Cylinder 5 - 160 PSI throttle closed; 180 PSI throttle open

So I'm thinking that there is a valve guide/stem problem on Cylinder 3 but piston rings on 5.

Any thoughts appreciated before I work out if its a head rebuild or the whole lot!!! Expensive decision.

Cheers.

[GasMunkey]

Do a leak down test to pin point the problem

[SMGB]

Compression tests should be done with a warm engine and throttle wide open. I would expect a high performance engine to turn in numbers in the range 170 to 230 psi range. The recieved wisdom is that consistency is the big indication. You are right that a small amount of oil will produce bigger numbers if its the bores/rings that are in troudle. too much oil can mislead by raising the compression ratio though. A leak down test will tell you a lot as well, and is worth doing before you put a spanner on it as you cant then get the info. Whereabouts are you, I just had one done at Dave Pymm's garage here in South Bucks, but not every outfit can do this. it will tell you if its rings/bore exhaust valves/inlert valves leaks between cylinders on a per cylinder basis. Its worth checking valve lifts with a dial gauge as well, sporty regrinds wear pretty fast.

[Mickywoosh]

I'm living in Tring working in Beaconsfield so if you can let me know where Dave Pymm's garage is, much appreciated

[SMGB]

His business is on the Soho Mill Ind estate off the A4094 between Wooburn Green and Bourne End. You turn right at the Y junction just after Spade Oaks place.
He is a great guy and has held the MG Midget Challenge championship several times.
Only problem is he is usually booked out for a least a month. How much time does it take to get to the plugs on the speed six, he will need to know this to quote you, amd if its not difficult you may get in sooner. I will PM you some more stuff

Edited by SMGB on Thursday 22 November 09:36

[packman10_4]

Based on what you have already done it does not look promishing , better get your spanners ready ........

Id be after seeing between 180-220psi and pretty close to each other .....

[Geoff Ashcroft]

I wouldn't be too hasty with the spanners just yet. Piston ring gaps could have lined up on one or more cyls, giving a false reading (they do move around the piston when running). If it was mine, I'd do more checking to build up a more consistent picture of engine data with hot and cold compression tests and leak down tests.

[PuffsBack]

Mickywoosh said:

I've just done a compression test on my T350 (knowing somethings up!!)

What makes you think 'somethings up!'