mechanically challenged - PeachParts Mercedes-Benz Forum

gdemoss01 · #1 10-22-2007, 04:44 PM

O.K., is it possible to check engine compression with a pressure gage on an engine no longer in the car? How?

Also, is it possible to just replace rings, gaskets, etc and have a valve job done and wind up with an engine that is likely to last 100K miles or more?

617.950 engine

Thanks.

rrgrassi · #2 10-22-2007, 04:48 PM

The complete engine (still assembled) would need to be on a stand, and you would have to spin it using a rigged up battery and starter connections.

As far as just rings, gaskets, valve job, etc, it would all depend on the conditiion of the pistons, cylinder sleeves, bearings, etc.

Old300D · #3 10-22-2007, 05:03 PM

You are not likely to get good results if the engine has sat for a while without oil. Unless you first crank it a while to get the cylinders lubed up well. But it will still not be as good as if you had just shut it down, and it's still warm.

Wodnek · #4 10-22-2007, 05:54 PM

Using an oil can, squirt some 10 w or 20 W motor oil in each cylinder. Turn the engine over by hand a few times, this will help seat the rings so you can get better results.

The other questions I doubt anyone can answer without seeing the engine.

Whiskeydan · #5 10-22-2007, 07:24 PM

Quote:

Originally Posted by Wodnek

Using an oil can, squirt some 10 w or 20 W motor oil in each cylinder. Turn the engine over by hand a few times, this will help seat the rings so you can get better results.

The other questions I doubt anyone can answer without seeing the engine.

I doubt the rings will be un-seated. Adding oil will give a false (high) compression reading if the rings/sleeves are worn.

I'd pull the injectors, check to see if it turns by hand several revolutions then crank it with the starter to get the oil pressure up. Then check compression.

Or, just do a leak down test on each cylinder at TDC.

300SDog · #6 10-22-2007, 07:48 PM

Just replacing the rings and doing valve job strikes me as entirely kosher if bores aint deeply scored. Look for soft metal Swedish aftermarket rings designed for high mileage ring replacement without cuttin into bores.

Piston slap and bottom end crank bearings going south on Mercedes is practically unheard of. Unless the engine has been abused, taken off road 20 yrs, driven overheated long time, run out of oil etc.

gdemoss01 · #7 10-23-2007, 10:57 AM

Quote:

Originally Posted by rrgrassi

The complete engine (still assembled) would need to be on a stand, and you would have to spin it using a rigged up battery and starter connections.

As far as just rings, gaskets, valve job, etc, it would all depend on the conditiion of the pistons, cylinder sleeves, bearings, etc.

This engine originally was swapped out due to bad oil leak ( 1 qt/350miles) that seems to be from a bad head gasket.

It has no starter on it, in fact all accessories including the starter were swapped to the other engine. It had 188K miles on it when removed.

So I guess you're telling me a leak down test is the only way to determine compression? The engine is sitting on an engine stand and it still has oil in it and it's been under cover in a garage.

Thanks for the info so far.

300SDog · #8 10-23-2007, 05:20 PM

Quote:

Originally Posted by gdemoss01

This engine originally was swapped out due to bad oil leak ( 1 qt/350miles) that seems to be from a bad head gasket.

It has no starter on it, in fact all accessories including the starter were swapped to the other engine. It had 188K miles on it when removed.

So I guess you're telling me a leak down test is the only way to determine compression? The engine is sitting on an engine stand and it still has oil in it and it's been under cover in a garage.

Thanks for the info so far.

Heck, if you're workin on blown headgasket longblock then why bother with compression test? Pull the damned head and visually inspect condition of bores.

And d'ye know how many miles been driven on blown headgasket? Knowin diesels, it coulda gone thousands of miles in this condition. Worst thing about blown headgaskets is coolant mixes with oil, scores wrist pin surfaces and spins bottom end bearings too. Even at just 188k miles it's probly a boat anchor not worth rebuilding. Sounds like junk to me.

junqueyardjim · #9 10-23-2007, 06:43 PM

Contrary to what rrgrassi thinks he saw, a blown head gasket will not cause or be the cause of using oil. There is really no engine oil in that area of the engine. It is below, doing its function on the bottom end and it is above, oiling the the cam and valve assembly. There has to be a passage of course, either internal or external to get that oil to the head, but in all the years I have spent in a shop, I have never or heard of a head gasket blowing into the oil system. There can be seepage of the gasket and it might look a little oily, but I think the only thing it can be is unburned fuel and water from poor combustion.

300SDog · #10 10-23-2007, 07:42 PM

Yeah but do NOT jump all over rrgrassi, he's talkin about Ford 460's anyhow.

Otherwise yer right, blown HG wont generally wont compress into oil and coolant galleries - but oil and coolant WILL seep into combustion chambers. Seen this myself with M180 inline 6. Meanwhile coolant bein more pressurized than oil fills the sump through entering oil passages and provides false readings on stick as if oil level aint dropping.

gdemoss tell us the truth, what exactly are you workin with here? Was the engine run outa oil and/or driven how long with blown headgasket? Gotta pull the head to inspect bores first thing. What the hell are you waitin for? Pulling cyl head on 616 is easy. Skip the compression test says me.

Meanwhile diesels do NOT rebuild easy. 22:1 compression ratio with 350 lbs per jug you understand. Backyard build might last 60k miles if engine's been severely abused. Personally, I will never buy an MB diesel where seller boasts of "rebuilt" engine. Gimme an original decent running Stuttgart engine and i dont care how many miles it's got.

rrgrassi · #11 10-23-2007, 11:33 AM

Was the head gasket ever replaced?

gdemoss01 · #12 10-23-2007, 12:24 PM

Quote:

Originally Posted by rrgrassi

Was the head gasket ever replaced?

Don't know the answer to that, but I would guess (for no good reason) that it hadn't. Is there anyway to tell? Probably not, I quess.

junqueyardjim · #13 10-23-2007, 04:40 PM

A head blown gasket will cause a number of different problems, but not using oil. That just doesn't fit. It might have a blown head gasket, but using oil has to be either the engine is burning it because it is worn out or has a tremendous oil leak. I would guess that it is very worn out and a major overhaul would be the only way to go. Rings and a head gasket won't do it.

rrgrassi · #14 10-23-2007, 05:04 PM

A blown head gasket can allow oil to leak. I have seen that on a Ford 460. That said, a blown head gasket can also show up as low compression on a cylinder or two. It depends on what actually happened when the gasket blew. Overheating will cause them to blow also. You really need more info on that engine before spending money on it. You may be better off buying a different used engine.

Magoo · #15 10-26-2007, 01:28 PM

I think you will have to get a starter rigged up on there somehow. There is just no other practical way to spin the engine that I can think of...
I suppose the leak down test would give you some indication of cylinder condition. I've never done it though.