poor compression results from my 280SE

[s class]

So, I have been suspecting that my 280SE's engine is near the end of its life, even though it runs well and pulls strongly.

Did a compression test today :

Cyl 1 : 600kPa (87PSI) dry, 800kPa (116PSI) wet
Cyl 2 : 800kPa (116PSI) dry, 850kPa (123PSI) wer

Cyl 4 : 300kPa (43PSI) dry, 400kPa (58PSI) wet.

I didn't do the other cylinders - too disheartening. 

I think that overhauling only cylinder head will be a waste of time.  Thoughts?

[koan]

Any idea what the figures should be?

Big change in #1 dry/wet suggests it needs more than a head job.

Test them all.

koan

[TJ 450]

That's a bit average.

Unless you have burnt valves, I think you'll find that the bottom end is the culprit. Is it burning a fair amount of oil?

I'm about to do a compression test on my 450... I just drove it today and lets just say that it is a bit average.

Tim

[s class]

given that it is a high compression euro spec engine, it should have in excess if 140psi.  So cyl 4 at 43psi is like 25 to 30% of spec.   

[oscar]

Damn.  I'm surprised it runs well with that much variation.  How's the idle?  I wouldn't have thought number 4 would fire at all until your up high in the rev range.   

[s class]

Idle is very lumpy.  The engine rocks from side to side. 

[koan]

given that it is a high compression euro spec engine, it should have in excess if 140psi.  So cyl 4 at 43psi is like 25 to 30% of spec.   

But that compression loss on 4 is all (or mostly) down to valves.

koan

[s class]

True, koan.

I am not really sure what to do at this stage.  It is becoming more and more apparent that soon I have to do something about the engine.  It has been previously bored.  I am not sure what size, but I think it is already on second oversize.  I can sleeve, but there is a problem with this.... the head and block face have already been skimmed substantially.  Once a block is sleeved, it must be faced again, and the general feeling is that this can't be done with my engine.  The possible escape is if I can get a good head to rebuild that has never been skimmed, then just maybe I can skim the block again.   

I really do want to keep the original block. 

Recently, the car has not seen much open road driving.  I am intending to take a long trip (500km) at 120km/h tomorrow, so I plan to do a more thorough compression test after that.  I am hoping I can convince myself the rings/bores are passable. 

If I can convince myself that it makes sense, my preference at this stage is to leave the block and pistons as they are, but I would like to do up another head to put on this engine, and replace timing chain etc etc. 

[s class]

OK, so on Sunday we took a long trip to a game reserve and back, totally about 470km.

This morning on the way to work, I thought the car felt more lively, and decided to re-check the compression.

              DRY          WET

cyl 1     830kPa     865kPa
cyl 2     825kPa     875kPa
cyl 3     640kPa     800kPa
cyl 4     700kPa     760kPa
cyl 5     740kPa     795kPa
cyl 6     745kPa     835kPa

dry and wet results generally differ by 30 to 50kPa, except for cyl 3 and 6 which are a bit worrying.

Ideas?

[oscar]

I'm surprised #4 jumped from 300 to 700 dry

Here's a couple of thoughts.  A few years ago I took my 350 in to my mechanic who put some light oil into the cylinders and let it sit overnight, said it would "free" the rings up.  Not sure exactly what he used. I can't remember the compression readings before or after except they rose when I measured them a few days after the 200km trip home.  Whatever oil you used for the first wet test might have had a similar effect. 

Secondly, cold v's warm engine compression.  When I was thinking of saving the 280S and had low compression on 6 mainly, I did some google research and came to the conclusion that compression testing should be done whilst the engine is cold to expose issues since expansion of pistons and the cylinders inwards closes any gap that might have been there before and therefore masks problems.  Just wondering if both sets of tests were done under the same conditions, say first thing in the morning whilst the engine was cold.

EDIT:
I've been doing compression testing wrong so far as the manual says "check compression pressure at operating temp".  Googling the subject a few people say at operating temp too because that's where the engine will be running most of the time.  Fair point, however, although the manual's figures are for a hot engine, I still reckon a cold engine test is valuable for reasons mentioned earlier.  Afterall, an engine's supposed to run smooth during startup and after warming up.

[koan]

I'd get the head done, at your SA prices it won't cost the earth. Number 3 is not the best but I would put it out of my mind.

You did it warm and with throttle to the floor?

I should check the figures on my 6.9 after the heads were done, but don't seem to find time.

koan

[s class]

Yes, both tests were done with a warm engine.  koan, I tried with the throttle open and closed, and actually found no difference. 

Its the first time I've ever done a compression test on this car.  My thinking was always that the car was running well, so why worry - ignorance is bliss you know. 

Now however, there are issues of uneven running, and a bad smoking problem on the overrun, so I thought I needed to investigate. 

Oscar, the oil I used for the wet check was ordinary engine oil.  I don't think that contributed to the change in compression results.  I believe it was because the car had only slogged in traffic the last few months, and the rings would have been gummed up.  The long trip would have assisted to free them. 

[s class]

OK, its 18 months on, and my 280 engine is now past its best.  In the intervening period, the head gasket has developed a leak between one of the combustion chambers and the coolant passages.  Probably it is blown. 

The head is not in he greatest condition after 520 000km, and I've been on the lookout for a better head.  Well, finally I found a suitable donor car.  Also a late spec 1980 280SE with only 195 000km, and no history of engine work.

Towed it home with the 'new' 350 this morning :

bugger - photos will come when photobucket decides to co-operate. 

[TJ 450]

A parts car is usually the best value option in my opinion... Well done.

I'm interested in seeing the pics.

Do you reckon you'll overhaul this new engine if it checks out OK, or simply swap it in?

I would be interested in hearing your thoughts on the dreaded cam box oil leak issue. I need to fix that on my 280S because the clean engine is now getting messy... Has this ever been a problem with your 280SE?

Tim

[s class]