Compression Test Results - What does it show?

stuu

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Hi, I got a compression test done for my 1992 2.0 190e and the results show:

1: 200 PSI - 13.78 Bar
2. 200 PSI - 13.78 Bar
3. 170 PSI - 11.72 Bar
4: 160 PSI - 11.03 Bar

The Haynes manual says there can be a maximum difference of 1.5 Bars between each cylinder. Cylinder 1/2 have over 2 Bar of difference between cylinder 3/4 which would indicate somethings not right?

It's over the 1.5 bar maximum compression pressure, haynes manual says it could be a camshaft lobe?

What could it mean guys, any advice/help?

Thanks!
 
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Ian B Walker

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I take it that was a dry test with the throttle fully open. Do the same again with a wet test (a tablespoon full of oil in each cylinder) and compare the results. If the readings all seem similar then I would look at piston rings. If you have the facility to do a leakdown test, that would be my next option. It would tell you where the cylinder leak was coming from.
 
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stuu

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Yes it was a dry test. I got a friend to do the dry test but not sure if he has the facilities to do a leakdown test. Is 200 psi not very high?

Should the values be around 180psi?
 
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stuu

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But with over 1.5 bar difference in the cylinders and the first 2 being over 13 then there's definitely a fault somewhere ya reckon?
 

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Why did you feel the need to conduct a compression test?

BTW, I'd ignore the cam lobe stuff. As per post #2, wet test and if rings are OK then either valves not sealing at their best or incipient head gasket failure between cyls 3 and 4.

What mileage has it done?
 
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stuu

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Why did you feel the need to conduct a compression test?

BTW, I'd ignore the cam lobe stuff. As per post #2, wet test and if rings are OK then either valves not sealing at their best or incipient head gasket failure between cyls 3 and 4.

What mileage has it done?
139K miles done, need to check when the head gasket was last changed.

Cars lacking power and getting around 18mpg. I've basically tried everything else and changed lots of parts, the only thing I didn't get done was a compression test.

Where can I buy the valve seals?
 
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stuu

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I'll see if my mechanic can do a wet test and he should know whether there was a new head put on it a few years ago as the guy before who owned the car used the same mechanic.

So there's little possibility of it being the camshaft lobes? Is there anything else it could be apart from the valve seals or head gasket?
 

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Cam lobes shouldn't affect compression at cranking speed, but visual check should be possible.

Valve sealing problems are addressed by removing the head and grinding/lapping them. Head removal is the opportunity to address that and the head gasket.

My understanding is that these engines are prone to failure of the valve stem seals. Which of itself shouldn't lose compression, but oil passed by them may lead to a build up of deposits that will impair performance and hence economy. An outside chance deposits could affect compression at low cranking speed - more likely if valve stems have gummed.

Ascertain the ring condition and if they are OK a head rebuild is probably in order. I had a 200 with the same engine with 230,000 miles, no ring problem and 24mpg with a carb.

You might get away with just the stem seals and some fuel detergent - saves head removal.
 
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stuu

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Cam lobes shouldn't affect compression at cranking speed, but visual check should be possible.

Valve sealing problems are addressed by removing the head and grinding/lapping them. Head removal is the opportunity to address that and the head gasket.

My understanding is that these engines are prone to failure of the valve stem seals. Which of itself shouldn't lose compression, but oil passed by them may lead to a build up of deposits that will impair performance and hence economy. An outside chance deposits could affect compression at low cranking speed - more likely if valve stems have gummed.

Ascertain the ring condition and if they are OK a head rebuild is probably in order. I had a 200 with the same engine with 230,000 miles, no ring problem and 24mpg with a carb.

You might get away with just the stem seals and some fuel detergent - saves head removal.
Thanks for the informative answer! Are the steam seals expensive and can they only be purchased from MB?
 

Ian B Walker

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Thanks for the informative answer! Are the steam seals expensive and can they only be purchased from MB?
No they can also be purchased from people such as Euro Car parts. But I personally prefer genuine Mb ones.
 

Bellow

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As the man says, cheap. I would give them a go - MB parts as per above - and get some aggressive fuel treatment into the tank. If things haven't improved in a couple of hundred miles, then consider head removal.
Do watch for head gasket failure though. Misfiring will be the first sign of it failing between cylinders. If you are merely down on power and economy though, I would put it on hold - for now.

Valve stem seals harden with age as much as wear. They are little things that you could slip all 8 over a Biro and still have room for the same again.

Are they available from MB separately though? They are always contained in a head gasket kit.
 

312 Sprinter

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Thanks for the informative answer! Are the steam seals expensive and can they only be purchased from MB?
Valve stem seals won't affect your compression reading. Their purpose is to stop oil getting sucked in the combustion chamber through the valve guide when the engine is on the overrun; engine braking down a hill for instance.

The valves themselves seal against the valve seats in the head.

As a rule of thumb the compression readings should be within 10% from highest to lowest.
 

Bellow

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As the man says, cheap. I would give them a go - MB parts as per above - and get some aggressive fuel treatment into the tank. If things haven't improved in a couple of hundred miles, then consider head removal.
Damn and blast!!!

I've just remembered stem seals are a head off job.
Apologies for misleading you.
Price a complete head gasket kit.
 

Bellow

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Incidentally a tell tale sign of stem seals is smoky start up, esp from cold.
Worn rings will burn oil all the time and lead to high oil consumption.

If you are seeing smoke on start up but oil consumption is still reasonable, then stem seals are suspect. Not all engines do it but most will.

PS. Plse post and let me know you have seen post # 16.
 

Will

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M102 8-valve engines seem to let go of the head gasket seal on the rear two cylinders it seems. I reckon many of these engines this age/with this sort of mileage already have early signs of HG failure before the conclusive mayo-in-oil and oily coolant, overheating, bubbling in the header tank etc. Ie, perhaps suffering slightly poor starting/spluttering in the mornings, and a little loss of power rather than exhibiting the usual signs of catastrophic failure - they 'soldier on' for quite a while.

Logic being that they're the furthest away from the coolant pump, and hence if the coolant changes have been neglected the waterways can sludge up and become blocked. This in turn can lead to localised overheating and the HG seal can become damaged as the coolant leaks into the bores. Being at the back of the engine, the rearmost cylinders also suffer from less airflow.

From what I have seen, early detection is quite important, as if left leaking for some time the head (and block) mating surfaces can become corroded, weakened and damaged. Have a look at this for example - a 1993 1.8 M102 head that I actually replaced in the end as I wasn't comfortable with the corrosion in the waterways:

http://www.mbclub.co.uk/forums/engine/82861-lack-coolant-changes.html

Personally, I think that those compression readings are a bit suspect, and I would diagnose a little further. A proper 'leakdown' test as suggested would be a good idea to see if you do have cylinder leakage, and more importantly - where it is going.

*If* it turns out that you're looking for a head overhaul, on the positive side it is a pretty easy job as they go - and most of the parts are cheap/easy to find. A complete gasket set, stem seals, head bolts etc should come in at around £100 - if you can DIY then it's not the end of the world :)

You can see some pics of the head rebuild I did at the end of this post:

http://www.mbclub.co.uk/forums/members-gallery/86629-1993-190e-le-brilliant-silver.html

After this work, the engine started perfectly, ran like new - with lots of power and great fuel economy :cool:

Most satisfying :)

Will
 
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stuu

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Excellent stuff guys.

The car would hesitate slightly when starting up and it drinks oil as well as petrol! I'm not sure how much power its lost but it was tested and is slower than an MK1 Clio 1.2 8V (0-80)

Will, is this what your talking about the head set?

Mercedes Benz - Euro Car Parts UK

What do you use to de-coke the head etc?

Incidentally a tell tale sign of stem seals is smoky start up, esp from cold.
Worn rings will burn oil all the time and lead to high oil consumption.

If you are seeing smoke on start up but oil consumption is still reasonable, then stem seals are suspect. Not all engines do it but most will.

PS. Plse post and let me know you have seen post # 16.
Bellow, rings and steam seals - Are they the same thing? I have little knowledge when it comes to this type of engine work!
 
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Will

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Haven't bought gaskets from ECP before but they should be ok. Worth checking the price of an MB set though, might not be as expensive as you think.

That kit is a complete set for a head gasket swap (HG + intake/exhaust manifold gaskets and seals for temp sensors etc), but it does not include the valve stem seals or cam cover gasket.

You can measure the head bolts to see if they need replacing, there is data on WIS that specifies the maximum stretch length.

Will
 

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