K series compression test, Jock7s cross post.

[Chucks]

Had a wee overheating episode on the ankle biter at the end of last year. I'm hoping it's nothing more serious than the expansion bottle cap however given the K series repution I'm being as cautious as possible. The car has been SORN's over winter so I've yet to get it to my mechanic to test the coolant. I have managed to do a compression test though, anyone care to give an opinion on my readings.

Cylinders 1&2, 9kg/cm3
Cylinders 3&4, 10kg/cm3

Don't have the gauge handy so can't give an exact conversion into real money but it was roughly 120psi on 3&4. So is a 10% spread in readings enough to be concerned about?

As noted my next check will be to get the coolant tested for exhaust gases though both water & oil look clean.

Cheers

Charlie

[robin]

First off, the unit must be kg/cm^2 as this is a pressure - kg/cm^3 is a density measure.

Secondly, 1kg/cm^2 is 14.2 psi, so you're looking at an average of about 135 psi, which is low but not terrible.

If you're only measuring in units of 1 kg/cm^2 you're bound to get a spread of 10% (the readings will only be 9/10//11).

Your low compression could be caused by all sorts of things, ranging from hairy cams (if the inlet is long duration then at cranking speeds you blow loads of the gas back into the inlet manifold before the inlet valve closes), to a cold engine, to worn (and cold) pistons.

Static compression ratio should be 10.5:1 or thereabouts, but this is derated to 7.5:1 because of typical inlet valve configuration, then you need to take into account the effect of heating the gas as it compresses, plus some other fudge factors, you'll get something like 13 bar or so - but the gauge will be relative to atmospheric, so that's 12 bar = 170-odd psi.

With a really cold engine, the gas won't heat much (the compression heat is lost to the liners, pistons and coolant). So you need to have run the engine for a while before doing the compression test, then hope to see 150-160psi I think; anything more is a bonus


The combustion gas->coolant leaks won't show themselves on a compression test, so unless you have any other reason to suspect the compression is out, you're probably not going to learn much more with compression testing than you already know.

Combustion->coolant leaks can and will cause overheating and a block tester is the easiest way to diagnose these, though not always 100% reliable (you get false negatives). If the overheating comes on only whilst giving it death and otherwise the car is well behaved, suspect combustion->coolant. Also if it loses a few mm of coolant from expansion tank each time the engine is switched off and allowed to cool, suspect combustion->coolant. If you keep getting new gas in the coolant whenever you bleed it out, suspect combustion->coolant.

Otherwise, it's just as likely to be the expansion tank cap.

Cheers,
Robin

[Chucks]

Cheers Robin.

Will get it booked in for a block test soon. Fingers crossed.

Thanks

Charlie

[robin]

In my experience you're better off doing the block test when it overheats, or at least once the temperature starts to behave erratically, so buy a block tester & keep it in whatever the se7en has for a boot Be careful removing the header tank cap - usual warnings about covering hands and face in boiling coolant and resulting injuries - don't remove straight away - give it 5 minutes and then remove it one 1/4 turn at a time until the pressure is out without the coolant starting to boil - if it starts to boil screw the cap back down and wait longer.

Cheers,
Robin