Compression too high.

1994 Astra 1600. Belongs to colleages mum. Cambelt went bending valves for No 2 and 3 cyls. No appaent damage to pistons. Head was crack tested, skimmed and rebuilt professionally, with 2 new guides, all stem seals, and then refitted by son, with new gaskets, bolts etc. No apparent wear on bores. Car has only done about 60k.

Car started and appeared OK. On test drive clouds of oil smoke, which didn't clear even after driving for a couple of miles.

Head removed, and cylinders contained puddles of oil. Head was sent back, as son reckoned the stem seals must be leaking. Came back said to have been checked and found OK. Head was refitted with a new gasket set and bolts. No improvement. Still smoking heavily, especially under acceleration.

His mate checked compressions and said they were too high. I doubted this, so checked it with my own compression tester. 270psi on all cylinders! About twice as much as I would have expected. Checked tester and found it to be reasonably accurate.

Looking down the plug holes, the pistons are oily but there doesn't appear to be enough to explain such a high pressure. I've never come across this situation before, and I've rebuilt a few engines in the past. Doesn't necessarily explain the excessive oil burning problem, but it's something that needs explaining.

So far nearly £500 has been spent, but typically, 'mum' wants the car fixed rather than scrapped, as it's been so reliable in the 8 years she's owned it.

I am completely at a loss to explain the problem. Any ideas of what might be causing such high pressures, and also where the oil might be coming from? TIA. Mike

Reply to
Mike G
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I would get another tester to double check the reading. A normal reading should not be higher than about 170, even massive skimming would not be liable to raise it to the required 17 to 1 you appear to have.

The oil is almost certainly coming down the guides, either the outside of them (very likely ) or past the seals or through a further cracked guide or even a cracked head.

Take the head off and fill the area above the valves with oil, it should become apparent where it is falling through.

Best bet would be to get a s/h head from a breakers yard, preferably hear it running first.

Mrcheerful

Reply to
Mrcheerful

Valve timing correct? Valve clearances? Hydraulic tappets or normal?

Pressure so high - lots of oil in bores, causing higher pressure - possible gasket failure, or even cracked oil way?

My bet is on valve clearances, then maybe a tooth out on the timing belt. Excess compression could be getting past the bores, pressurising the sump, then blowing oil all over the place, hence lots of oil in the wrong places. Alan.

Reply to
A.Lee

I'm certain it's not my compression tester. I work in an engineering shop. The reading on mine matches the pressure guage fitted to the shop compressor, and also the reading using another guage connected to the airline.

This was our thought after it was fitted the first time, which is why it was sent back to be rechecked. The Co that did it, reconns heads professionally. The sons mate, who checked and stated the compression was too high, works in a garage who have used the Co regularly over a few years. I know they could still be at fault, but it seems unlikely, especially after having had it sent back as possibly faulty.

He's a bit reluctant to do that at this stage, as it would mean another head gasket and another set of head bolts. It seems to be developing into a money pit.

As it turns out it would have been cheaper to just fit a s/h engine.

I still can't get my head round how the compression can be so high.

Thanks for the suggestions though. Mike.

Reply to
Mike G

Think about it. As I said in my OP there is very little oil in the cylinders. Assuming no leaks, the max pressure is governed purely by the compression ratio. Valve gear or timing can't have any effect on compression pressures, except reduce it if the timing or valve clearance is hopelessly incorrect. Mike.

Reply to
Mike G

There shouldnt be any visable oil in the cylinder bores , this might sound stupid but the exhaust system doesnt have a blockage does it , this would certainly cause a back pressure to build up and throw your readings out

Reply to
steve robinson

Is there *any* possibility they've sent back the wrong head? To get twice the expected pressure you'd need to have half the final volume at TDC (I think) which implies something drastically different about the piston/head conditions at that point.

Could the head have already been skimmed and is now way smaller than it should be?

Reply to
PC Paul

The small amount of oil in the bores shouldn't affect the maximum compression readings by any great amount. Mike

Reply to
Mike G

That's a thought I had, but according the son, it is the same head with the right casting number.

To get

I agree, but what the hell is it? :-)

Also crossed my mind, until I realised a helluva lot would have to be machined off the head to get a pressure of 270psi. Mike.

Reply to
Mike G

Of course it can. At cranking speed there is plenty of time for gas already drawn in on the inlet stroke to go back out of the inlet valve before it closes after BDC. The more you advance the cam timing, bringing the point of inlet valve closure closer to BDC, the higher the cranking pressure. This is all explained in detail in the article on compression testing on my website.

The excess pressure is probably blowing oil up into the inlet manifold from where it's getting drawn back into the cylinders. There's a very simple rule to apply when a problem manifests for the first time with no apparent cause. Who was the last person to tinker with the engine? In this case your friend's son. Get someone to check what he's doing when refitting the head.

Reply to
Dave Baker

Assuming everything you say is correct and the excess oil in the cyls is very small it is a bit confusing. however I do have a low compression engine (8.5:1) and see 190psi purely from a squirt of fuel into the cyls, let along ANY excess oil? I have 170psi when dry and cold. On a non-turbo, higher compression engine I would suspect 270 isnt massively high....

I'd personally pull the head again if I liked the car, I suspect you're not seeing most of the excess oil (pistons slightly domed, oil slips to sides?) and the combustion chamber is very very small at TDC - wouldnt take a vast amount of oil to stop all blowbye and take up a large percentage of the chamber volume?

Reply to
Coyoteboy

Err no, if the exhaust valves actually close at Bottom dead centre then the compression when cranking is way higher.

Reply to
Duncan Wood

Granted, but how can it go higher than the theoretical maximum, which would be what? 150psi?

But where's it coming from, if the valve guides are good, and the stem seals have been replaced?

There's a very simple rule

He's actually just a work colleague. He wanted my advice, when he asked to borrow my compression tester. I'm not really involved, but I became curious when I saw the test results and he described what had been done. Mike.

Reply to
Mike G

As high as 270psi? I would have thought the maximum pressure that can be reached, even under those cirumstances would only be around 150-160psi. Mike.

Reply to
Mike G

Nope, it's an adiabatic compression, not an isothermal one, so the peak pressures higher than CRxatmospheric pressure. Given the Assume an engine where the Inlet valve opens 15deg BTDC & closes 45 Degs ATDC, time it

15degs out & you're not far off your ratio.
Reply to
Duncan Wood

I understand what you're saying, but I can't believe it's possible that the rise in temperature can account for such a high reading. I've worked on many cars over the years, but the highest CC pressure I can recall seeing before was only around 190psi.

As far as the valve timing is concerned, the son reckons the tickover and engine performance is about the same as it was. It's the smokng that's the problem. Mike.

Reply to
Mike G

It would be mnuch easier if you read the article rather than me type it all out again in here.

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Up the breather pipes from the sump into the inlet manifold.

Reply to
Dave Baker

Or even the inlet valves.

Reply to
Dave Baker

Blocked oil scraper ring holes? Had a Mini that started burning oil. Ignored it until it was leaving a red arrows style smokescreen down the motorway and using a pint of oil in 40 miles. It went from a puff of smoke on the overrun, to this in a matter of weeks. I had a spare head, so I did the seals on it, got new valve inserts put in and reground the valves. Plonked head on car and............ sod all difference.

Reply to
gazzafield

Surely with a compression appraching 250 psi when running, plus the oil in the pots, it's going to pink LIKE HELL!?

If its running normally but just smoking, then i'd bet the oil control rings are shattered / stuck? How long has it been stood whilst the work has been done?

Tim.

Reply to
Tim..

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