What is engine compression telling me?

[domaso]

Hi,

Sorry if this is an easy question, but I'm not really a good mechanic, so I thought some smart guys here could help me

I did a compression test on old Volvo (1998, 2.0, petrol, non-turbo). I did both "Dry" and "Wet" tests (by "Wet" I mean adding a spoonful of oil into the cylinder before measuring).
So, here are the results (cylinders 1-2-3-4):

DRY : 170psi – 170psi – 170psi – 170psi
WET: 170psi – 170psi – 245psi – 170psi

I would understand if cylinder-3 would show low compression on dry, and 170psi on wet test - it would indicate worn rings. But now dry test kinda shows All-OK on all cylinders, but it's way to high on wet test. By the way, manual says the normal pressure is 189-218psi. Yes, 170psi is quite low, but the engine has 190,000 miles on it so I guess it should be expected. On the other hand the manual says 218psi should be the highest pressure on good condition engine, while I get 245psi on an old engine.

I'm a bit baffled here... Anyone has an idea what could be the problem in cylinder-3?

P.S. I did the wet test twice on cylinder-3, and it showed 245, so I'm pretty sure it's not the gauge.

 

[domaso]

One more thing - when testing cylinder-3, the compression seems to be the same (170) for a while, but after a few additional turns it jumps to 245.

 

[miningman]

I would say you are worrying needlessly. Look at it from the other direction. Having even pressure across all four cylinders is a strong indication nothing is wrong. And to have 170 psi I would think is excellent..... so excellent that I might question the accuracy of your gauage. At normal atmospheric pressure of 14.7, a reading of 145 -150 psi implies a compression ration of about 10:1. Is you engine really set up with 11.5 :1 compresion pistons?? The 245 reading is an anamoly. Leave it at that.

 

[domaso]

245 also seems unreal to me...
The gauge is new, I don't think it's faulty, at least not that inaccurate to show a 30% bigger value, plus I did repeated tests, and this is only happening on cylinder-3...
Maybe it's carbon build-up in cylinder-3 reducing the cylinder volume, hence the bigger compression?

Regarding the compression ratio/pressures, here's what the manual says:

Displacement: 1948 cc
Bore: 83.0 mm
Stroke: 90.0 mm
Compression ratio: 10.5 : 1
Compression pressure: 13 to 15 bar (189 to 218 psi)

 

[Compositepro]

Most likely you added too much oil to that one cylinder.

 

[Lou Scannon]

Assuming there are no errors in your method, the variation you are seeing in cyl 3 is very odd. It makes me wonder if there is an intake valve timing (i.e. early closing) issue that comes and goes.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[domaso]

Could be.. But wouldn't it cause higher pressure on dry test too?

 

[patprimmer]

Not if it comes and goes and was gone when you did the dry test.

It sounds like an experimental error to me. Even closing the inlet at BDC should not change it that much, especially with a short duration cam as one would expect in an old Volvo as it came from the factory.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[MikeHalloran]

The wet test spoon should be a teaspoon, not a tablespoon.


Mike Halloran
Pembroke Pines, FL, USA

 

[domaso]

I've must've added too much oil then.. However I still don't think it would have caused 30% higher reading...
I'm planning to use engine "revitalizant" claiming to improve compression in cylinders, and after couple thousand miles I'll repeat the test using teaspoon of oil this time, I'll see if there will be any difference.

 

[patprimmer]

You don't need to add anything. There is nothing wrong with your compression unless the engine is running rough or burning oil and smoking.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[ivymike]

oooooh, mechanic in a can...

 

[domaso]

noooope, mechanic in a TUBE...
I'm trying this because a friend of mine with exact same engine witnessed reduced engine noise/vibration and increased performance/dynamics. I am myself am a big sceptic regarding these products, but my engine is old/high-mileage, so I thought what the hell.. (this is why I measured the compression in the first place, so I can compare later if the claims are at least partially true...).

 

[patprimmer]

No additive can put metal back into the bores or rings. Your piston to bore will not decrease, your bores will not miraculously become straight and round, your ring end gaps will not decrease and your ring side clearance in the pistons will not decrease.

Most likely your oil will become thicker which may reduce oil burning and reduce smoke and reduce noise, oh and reduce engine power and increase fuel consumption, but hey, it's your car and your money.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[domaso]

"No additive can put metal back into the bores or rings. <...>"
I myself am on your side too, however believe it or not the "revitalizant" claims contrarily - it says it adds ceramic-metal (not metal) and rebuilds worn areas at atomic level using nano-technology. I know, it sounds too good to be true, and I'm sure it won't work so miraculously. However I'm willing to experiment, since the chemical only cost me $100, and I'm not going to rebuild that engine anyway.

 

[Tmoose]

I'd repeat the compression test for a baseline.

 

[domaso]

Well, I already did the test twice (same results), but I guess it won't take long to do it one more time just before the first stage of revitalizant treatment.

 

[dgallup]

Only $100! What a waste. The FTC comes along and whacks these guys with a cease and desist order and some small fine every once in a while but there are a dozen scam artists waiting to take their place.

Yes it's your money and your engine and you are free to do what ever you want. But do you want the rest of us to know that you have been bilked?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[berkshire]

When I sold my Volvo Station wagon it had 450,000 miles on it and the compression was about 165psi on all four cylinders. It was still running well.
Just run the thing until you get fed up with it, or your wife demands you get a newer car.
B.E.

 

[domaso]

Call me whatever you like... I don't feel bilked as $70 is engine oil with stage-1 revitalizant (I needed an oil change anyway), remaining $30 is two tubes of revitalizant for stage-2 and stage-3.

I will know if I have been bilked after the full 3-stage treatment and compression test. As I've said I'm doing this mainly because of curiosity and my friends positive feedback.

P.S. this company sells products in many countries for more than 10 years now, I think they would've been whacked by now...