Compression figures interpretation

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ms500

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Jun 18, 2008
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I just did a compression test in my continuing efforts to fix the slow starting on my 300TE-24, here's what I got;

1/ 110 PSI
2/ 100
3/ 105
4/ 115
5/ 130
6/ 130

These seem very low to me, I'm not totally convinced of the accuracy of the tester I used but I hoped it would give me a rough idea.

The car takes quite a few cranks before it will start, but once started it will run OK, it doesn't cut out or stall and it seems to have plenty of power, although I haven't driven a known good one for comparison.

Other issues I've noticed is disappearing coolant (though no visible leaks) and a bit of oil that has come through the breather hose into the air filter box. There doesn't seem to be any oil in the coolant or coolant in the engine oil.
 
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That does sound very low to me also.

I would have expected the readings to at least be similar, 100-130PSI is quite a range.

Wouldn't you be looking at compression readings to be in the high 100's. I think my 2.5-16 engine measures about 175-180PSI accross the board.

The fact that your engine seems to be losing coolant would lead me to investigate for a faulty cylinder head gasket - the 300-24s are prone for it IIRC. And from memory, if they're not caught quite quickly, they've been known to wear out the bores where the water enters?

Will
 
That does sound very low to me also.

I would have expected the readings to at least be similar, 100-130PSI is quite a range.

Wouldn't you be looking at compression readings to be in the high 100's. I think my 2.5-16 engine measures about 175-180PSI accross the board.

The fact that your engine seems to be losing coolant would lead me to investigate for a faulty cylinder head gasket - the 300-24s are prone for it IIRC.

Will

Yes, I would of thought about 165-185 would indicate a healthy engine. Some have said that with compression figures in the low 100's would prevent the engine from running at all.

The car has done 204k miles; there is a lot of history including regular servicing at MB dealers up to about 180k, but I haven't seen any evidence of the Head Gasket being changed.

Edit: Just remembered, the throttle wasn't open when testing, I've heard that this can make a difference.
 
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An engine could/would still run with compression in the low 100s, deffo.

Might take longer to start, and would certainly lack power. I guess it comes down to how and why the compression is low - ie, blow-by through the rings/bore, leaking valves, head gasket leaking, etc.

Was the engine warm when you did the test? You should really have tried opening the throttle as well I would think.

You can try and test for bottom end (rings) by putting a small amount of oil into the bores from what I recall.

Will
 
yes I think 100 to 130 30% is a big differance plus very low, Head Gasket + valve seats.did you put some oil in the bores and re test to see if the compression is better, if so worn bores/rings may be the fault. once did a head job on a Datsun compression was all over the place refaced the head reground valves new hed gasket after 180,180,180,180,went like a dream.
don't play with cars know just drive them.
 
Guys how do you test something like this? its quite interesting.. (sorry about ur engine tho ms500).
 
An engine could/would still run with compression in the low 100s, deffo.

Might take longer to start, and would certainly lack power. I guess it comes down to how and why the compression is low - ie, blow-by through the rings/bore, leaking valves, head gasket leaking, etc.

Was the engine warm when you did the test? You should really have tried opening the throttle as well I would think.

You can try and test for bottom end (rings) by putting a small amount of oil into the bores from what I recall.

Will

I did the test when the engine was cold (ie hadn't been started since yesterday). Maybe I'll try it again with the throttle open to see if it makes much of a difference.

I would be quite surprised if it was worn bores / piston rings, from what I heard the bottom ends of the m103 / early m104 engines were supposed to be pretty much bomb proof and capable of lasting the life of the car.
 
Agree; more likely to be a head gasket if you are losing coolant too. You will probably find that some of the valves will need replacing at 200k miles. You need to weigh it all up - removing the head is a 10 hr job in MB book + £100 ish for the gasket set + £ss for valves + seats - it will start to add up to a big bill I am afraid.
 
Agree; more likely to be a head gasket if you are losing coolant too. You will probably find that some of the valves will need replacing at 200k miles. You need to weigh it all up - removing the head is a 10 hr job in MB book + £100 ish for the gasket set + £ss for valves + seats - it will start to add up to a big bill I am afraid.

I might have a word with my local independent; he's done a few things for me before, his rates are very reasonable and I've never had a problem with the quality of the work.

Either that or I'll cut my losses and look for another 300TE.
 
Re test it and then perform a leak down test if all isn't well.

I suspect some ring wear mixed with valve leakage.
 
Re test it and then perform a leak down test if all isn't well.

I suspect some ring wear mixed with valve leakage.

Highly likely to be h/g problems as well with loss of coolant.
 

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