How much to do piston/rings on a W123 3.0L Turbo diesel? - PeachParts Mercedes-Benz Forum

Douge · #1 04-26-2009, 07:36 PM

I found a car in good condition with bad compression on one cylinder and wonder what the ballpark is to tear it down and rebuild it?

Trying to determine if its worth the save. Not sure I have the skills or tools to do it myself but if you have some insight into the job itself, I would be interested to know what it takes.

TIA

Doug

t walgamuth · #2 04-26-2009, 07:56 PM

First thing is to do a leak down test to be sure its not just a valve out of adjustment.

If its a ring its best to think of just going through it and doing whatever needs doing. This if done professionally would cost from 3 to 5000, at least.

#3 04-26-2009, 08:00 PM

If the engine really has low compression, after adjusting the valves, its cheaper to buy a used engine than pay somebody to rering yours.

Douge · #4 04-26-2009, 08:35 PM

Also, the car has 166K and has substantial documented work history.

Without any further testing here is what I know. The car will start and run, PO states that the car started running rough and he drove home and stopped the car, he later drove about 15 miles round trip to the Mercedes dealer who performed a compression test and reported that one of the middle cylinders didn't have compression and suggested it had a bad valve, bad piston, broken rod etc... and they could not determine the problem without a tear down. He declined and bought a new car. He states that the car does run and actually when revved runs relatively smooth, better than at idle or low rpms. He says there is not any clattering that would suggest a catastrophic failure.

As you suggested, this could be a bad valve spring or even simpler a very out of spec valve clearance issue?

I will search on the leakdown, but if you have a sec and can it explain it that would be great.

Thanks for the help!

lietuviai · #5 04-26-2009, 08:42 PM

A broken rod can be easily determined without a tear down as there would be a definite knock. Any dealer that would say that about a broken rod is a complete moron.

Douge · #6 04-26-2009, 08:50 PM

Quote:

Originally Posted by lietuviai

A broken rod can be easily determined without a tear down as there would be a definite knock. Any dealer that would say that about a broken rod is a complete moron.

I agree.

But that seems par for this particular dealer. I stopped using them years ago.

GRIESL · #7 04-26-2009, 10:21 PM

A real leakdown test involves exact pressure and accurate measures of time elapsing and all that sort of stuff. But here's the poor mans leakdown that can at least get you pointed in the right direction: Get yourself a harbor freight compression kit for diesels and get the fitting that goes into the injector hole. Go to a plumbing supplier (not Lowe's or HD--they won't have the combination of fittings) to get that adapter hooked up to a 1/4 npt, and get a ball valve. Get a hold of almost any size compressor (because you don't need a whole lot of air) and plug her up to the suspect cylinder on that cylinder's compression stroke (I did mine with the cam cover off to ensure I knew the valves were closed), and with the compressor pumped up and then turned off open up the ball valve and shoot a small volume of air in there--not too much 'cause I hear you could push the piston down with the air and you don't want to do that. If you see you're losing pressure on the compressor, listen for where the air is escaping--intake, exhaust or crank case. IIRC, if you hear hissing in the crankcase like in the chain galley, then it's rings and/or piston; if you hear it in the intake, then it's likely an intake valve; and if you hear it in the tailpipe, then it's an exhaust valve. But don't be fooled by making an exclusive ruling on a valve. I think it's possible to have a faulty valve AND and bad rings and/or piston. The only way to know for sure if you have bad valves is to pull the head, but you still wouldn't know for sure you didn't have bad rings. But I would imagine you'd have enough visual evidence to see if the pistons were cracked. And I highly doubt that bad compression rings alone would cause a zero reading in compression. I would say if you get the guy down enough and you enjoy the thrill of hunting down and fixing old cars, then go for it. If you're looking for an immediate commuter, I'd pass and keep on looking. These cars are a lot of fun, but they can also consume a lot of time and money.
Good luck!

JonL · #8 04-27-2009, 01:21 AM

Could be a can of worms, but it sounds more like a burnt or bent valve than bad rings.

ricardobs84 · #9 04-27-2009, 01:40 AM

You can pull a complete engine from a junkyard for $100-$500.

t walgamuth · #10 04-27-2009, 06:31 AM

Quote:

Originally Posted by JonL

Could be a can of worms, but it sounds more like a burnt or bent valve than bad rings.

x2. Bottom end woes would not likely come on all at once. A broken valve spring could though.

Douge · #11 04-27-2009, 12:43 PM

I got a little excited about it last night but agree that it is definitely a can of worms and internals are not my thing yet.

Knock on wood though, my luck has been brilliant and if it held it was probably something easy/cheap. Can't have them all though.

JonL · #12 04-27-2009, 03:01 PM

I think part of your good luck comes from being wise about which risks to take.

winmutt · #13 04-27-2009, 04:25 PM

My $250 engine came with a car attached. Not perfect compression but well with in specs.