compression reading - erroneous because of oil leakage ?

An engine with low compression will not be showing a falsely high reading because of piston rings letting oil past. (unless we are talking of something esoteric like a radial engine)

You would only get a massively high reading if the combustion chamber is full of something (eg fluid or coke). Oil leakage past the rings is irrelevant to the reading. An improvement in reading by putting a SMALL amount of oil in the cylinder MAY show that the rings are worn.

A better test is to air pressurise the cylinders with the valves shut and find out where the air leaks out, this can show up blown head gasket or cracked head/liner, leaking valves, worn bores/rings etc etc.

What are you testing and why?

Mrcheerful

His car with the silicon and glycol in the oil samples when he had them tested?

:-)

Mike P

It'll could possibly show a small improvement with a very small amount of oil dribbled into the cylinder if the rings/bores are worn. I can't see how it would create a much too high reading unless you've got a hydraulic lock (cylinder full of water/oil/crap etc) going on. I had a Saab 900T16 with a blown head gasket, occasionally it would fail to turn the engine over because one of the cylinders was full of water..

Mike P

Which isn't a helpfull indication anyway, it depends on the engine.

Not to any relevant extent

It nelps seal the scratches. If you've got enough oil going past the piston rings to manage that normally then you wouldn't need a compression test.

Dang, I didn't notice the originator, Mind you it could introduce a raft of new queries about compression, such as the height above sea level, air density, phase of the moon, sun spot activity etc.

Mrcheerful

What is your altitude above sea-level?

Or, on a more serious note, is this a petrol or diesel engine?

Andrew

Hello,

A lot of replies, and I'm glad to see some interest. Yes, it is the same engine as with the oil analysis, namely a Mitsubishi/hyundai (or daihatsu)

1,3 16v petrol engine. It's just that I should feel happy about high compression readings, but are they realistic ? I had the hi-gauge compared to a professional tester and readings corresponded very well.

As I said, readings are 16 kgs of compression rotating the engine for some 4 seconds, which should only be 13 kgs or so. How come they are high ? I don't imagine too much coolant entering through the head gasket or so. The car is handling badly when accelerating, slightly shaking and not revving up smoothly. Only when I run it in fourth gear it is performing well, it accelerates smoothly. I can't describe the engine performance this easily in text, sorry.

Rgds,

Peter

Sounds like a misfire under load to me. Do you have any of those mobile mechanic type people over there (you're in Belgium right?) who have all the diagnostic equipment - here in the UK we have (or had) Computa-Tune. They've sorted out similar problems for me in the past - might just be a dead plug or lead. Here , at least a few years ago, they usually had far better diagnostic equipment than garages and could find strange problems easily.

Mike P

Sounds to me like an almost terminal failure of your engine block. Symptoms include silicon and aluminium particles in the oil, and usually acompanied by pinking. If I were you I would take the car to the nearest scrap yards and leave it. Hope this helps.

He'd only buy another one...

Chris

on 28/08/2008, peter supposed :

No, that will be quickly burned off.

Oil deliberately injected into the bores and not burned off will help seal around the wear in the pistons, thus improving compression for the test. The idea is to prove it is worn pistons, rather than wear in the valves and their seatings in the head. If the compression doesn't improve with the addition of the oil, obviously the engine has (or also has) valve leakage problems.

"Harry Bloomfield" schreef in bericht news: snipped-for-privacy@NOSPAM.tiscali.co.uk...

Thanks, Harry.

This is - with all the other posts and comments, the engine which was probably overheated when someone changed my coolant liquid, and got the engine to run for half an hour without sufficient coolant in the engine ( he had the engine run stationary and filled up the radiator slowly, he added at least half the volume of coolant this way. While there was a bolt meant to let the air out when filling the engine with new coolant, without having to run the engine at all while filling. It shows that engineers make good designs, but they were ignored by this garage. )

Peter