Help me interpret compression/leakdown! - PeachParts Mercedes-Benz Forum

#1 11-02-2006, 01:17 PM

My car started running very rough last tuesday. I took it to a good indy and they did a compression and leakdown test. Could somebody help me interpret the results?

Compression test: # 1-210 #2-95 #3-170 #4-225
Leakdown Test: #2-90% Leak-intake valve

The shop wants to do a valve job on the head, but I want to be as sure as possible there is nothing else wrong with the engine before I give them the Ok. Are these results enough to be reasonably sure I don't have lower end work in my future?

cheers,
dan r.

EDIT: Car has 174k miles, original head, replaced timing chain and tensioner at 90k

david s poole · #2 11-02-2006, 06:21 PM

what they are saying makes perfect sense.

Engr693 · #3 11-02-2006, 08:56 PM

Quote:

Originally Posted by Dan Rotigel

Compression test: # 1-210 #2-95 #3-170 #4-225
Leakdown Test: #2-90% Leak-intake valve

Notice the difference between the 4 cylinders. That 95 reading is out of spec. all four cylinder pressure readings should be close to each other in value. Not exact but close. Say within 15% or so (don't know the exact % that is correct)

Did they do the compressions test again after squirting oil into the cylinders? If not, by doing so it could bring the 95 reading up. If it does, I would suspect you may have compression ring problems on the piston in that #2 cylinder. Number 3 is a little off too, but I would be more worried about #2.

Merkey · #4 11-02-2006, 10:11 PM

Yep, makes sense.
A compression test is a quick start, but only tells half of the story.
When you do a leakdown test it tells you pretty well the other half.

They would be able to hear the hissing of the pressure escaping back into the inlet manifold, so telling them where the leak is.
If it was a compression ring or holed piston, the pressure would vent into the crankcase....
If it was a blown headgasket into the cooling system... the pressure would go into the cooling system... bubbling would be heard.

And a Exhaust valve would be heard in the exhaust!

I hope its just a valve.
Good luck!

Matt L · #5 11-02-2006, 10:45 PM

Quote:

Originally Posted by Engr693

All four cylinder pressure readings should be close to each other in value. Not exact but close. Say within 15% or so (don't know the exact % that is correct)

20% is considered acceptable for most engines. Closer than that will result in a smoother and more efficient running engine. You won't get all of them to be exactly the same.

#6 11-03-2006, 08:10 AM

Thanks all,

I didn't understand how the mechanic could tell if it was the upper or lower end that needed workd-now I do. What I don't want is to have them open up the engine and say 'oops, this is going to be another 3k, sorry!' I'll call the shop today, talk it over with the owner and most likely go ahead and have the work done. 2.5k for a valve job, assuming no valves are shot (most likely one will be!), they also put in a new tensioner. Sound reasonable?

cheers,
dan r.

t walgamuth · #7 11-03-2006, 08:18 AM

they are supposed to be within 10% of each other. is number three leaking past the valves too? if not then the cylinder is suspect. seems most likely to be valves i suppose.

is this the 16v you are taliking of?

the compression values are very high for a gasser. the combustion chambers must be full of carbon.

tom w

cjlipps · #8 11-03-2006, 10:44 AM

Ditto Tom's comments. As a rule of thumb you can use the compression ratio to ballpark what ideal compression should be. A 10:1 CR with standard atmospheric pressure of 14.7 psia would yield 147 psig if everything was good. Not an exact representation by any means but it serves to illustrate that 210 psig for a gasoline engine compression reading is horrendously high. And even if your gauge is faulty and #s 1 and 4 actually have normal readings, #3 is bad and #2 is dismal.