E30 2litre M20 compression

I have a 1988 320i auto buar tc cabrio. my ex girlfriend borrowed the car to drive 40 miles down the motorway to pick her dad up from the airport. on the way back she said the car lost power and a bit of steam
was coming from under the bonnet She pulled over immediatly, of course (or she said she did) got the car home attempted to start it and it wouldnt start. I checked all the obvious things like fuel and spark which was all fine. I then came to the conclusion that the head gasket must have blown, so i replaced it along with the timing belt. Well needless to say it didnt start, so i did a compression test on it cylinders 1-5 were between 5.5 and 6.5 bar and cylinder 6 was at 9.75 bar. does anybody have any ideas what could be wrong? could it be the valve stem seals? Or is it more serious and the piston rings are gone? if i need to replace the piston rings can i do this without removing the engine from the car? All help much appreciated! THANKS
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all i know if pistons get damaged, your going to have to start searching for a new engin. But it doesn't sound like piston damage, do u thrash your car heaps? as in, going really fast and making the engin work harder.
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You can rule out the valve stem seals, as the valves are closed during compression.
Or is it more serious and the piston rings are gone?
Try doing a compression test after putting about a teaspoonful of oil down the plug holes. If there is an improvement, it would indicate the problem is with the piston rings. If there is no change, check the valve clearances. If they are OK the head will have to come off again to check the valves and valve seats.

That might be possible, but I'm but familiar with that engine. No doubt someone else can answer that question. Mike.
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Mike G wrote:

thanks alot did compression test with oil and i had an improvement so that means there is a problem with piston rings. also i managed to get the engine running and there is a loud banging noise coming from the head. any ideas?
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Sorry. I haven't a clue. I've seen engines, that when stripped down, have had the head of a valve break off, and others with bits of broken piston in the bores. They would certainly cause banging, but since you've already had the head off, I don't see either of those being the cause. I reckon you'll have to take the head off again to find out. Mike.
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Isn't this just as likely to be "just" a head gasket?
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To check if it's valves or pistons causing the low compression, inject about a tablespoonful of oil into each cylinder - one at a time - and check the compression again. The oil will help seal faulty rings etc, but make no difference to leaking valves or head gasket.
According to my data the M20 should have a cylinder pressure of 10-11 bar.
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