How could this have happened? I have "only" 240k very pampered miles on my '91 300SE. She should have gone 300k at least!
Here's the story, and comments welcomed.

A couple years ago, static compression dropped a little on #1 only, and continued down to 155 last year. About that time I pulled the head to fix the infamous gasket oil leak. So, I had the head checked, and I looked over the cylinder bore, seeing zero evidence of any problem. No oil usage, clean plug, normal piston top carbon, etc. I bolted her back up and hoped for the best.

Well, she just slowly went on down to 105!!!! All others are 170-180.
I shot oil in the cylinder, and she popped to 200, indicating ring leakage.
I shot compressed air in, and could easily hear the hissing thru the crankcase (actually blew oil out the dipstick tube when I held my finger over the PCV tube).

So, gotta be rings/piston/cylinder - right?

I pulled the head (had it checked again) pulled the pan, yanked the piston, and could see NOTHING wrong! I did not have a bore mic, though, so just judged the bore by the mostly still-visible hatch marks and the total lack of a ridge at the top. Piston looked virtually new after soaking. So, I made the decision to brush-hone the bore, install new rings in the cylinder, and hope for the best. The only other option would be to pull the block and have it bored - ouch!

No luck - buttoned her back up and ran her 500 miles - 105 PSI - AAAAARG!

So, what am I missing here? After having the head professionally checked twice, and doing the dry/wet compression tests and homemade leak-down test, I'm convinced it's ring/piston leakage. The brush-hone quickly cleaned the bore up perfectly, piston looked virtually new, and the new rings fit well, and had good end-gap in the cylinder.
So, I'm assuming I must have taper or "egg shape" wear in the bore, enough to inhibit ring seal?
I do remember the #1 plug being a bit blacker than the others a few years back, when I was on my 2nd set of injectors. I had some starting & emissions issues and eventually replaced the injectors last spring - I wonder if I had a leaky one on #1 and it washed the cylinder wall for a few years, causing abnormal wear?

I'm on my way down the road to a complete rebuild, including boring all cylinders, but just thought I better run this by you guys before I jump all the way in and spend a wad of $$$ and a ton of hours.

DG

Can you borrow a cylinder bore gauge from a local machine shop?

This guy explains quite nicely where to measure for uneven wear:
Cylinder Bore - How to Measure.mp4 - YouTube


Edit: I don't know about the life expectancy of a typical 103, but i assume it should be more than what yours has.

Yeah, I need to get my hands on a bore guage, just to absolutely insure that I know the cause of the low compression. I would assume that if it took many thousands of miles to reach the current condition that it may not stick out like a sore thumb (broken ring, scored cylinder wall, etc).
One thing for sure, I'm not going back in there and fail to fix it!

DG

My m103 had valve problems that led to low #2 and #4 cylinder compression figures. With a recent valve job w/ new valves guides and seals, compression is back to 185 psi on all 6.

This was back in May 2012. Since then, I drove the car regularly. No more stalling or problems. Fuel economy is better, and the car passed emissions.

__________________
http://farm8.staticflickr.com/7271/7...144c3fc1dc.jpg

I had the same problem a year ago.
The engine suddenly lost all power, both on gas and LPG.

Since the old engine had a lot of miles on it and my other transportation was a motorcycle I bought a low mileage replacement engine for € 250 and jut swapped them.

Rob

Has the head been rebuilt before? If not it is due for at least valve guides and seals...

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

What kind of oil are you running??? Synthetic wont allow the rings to seat. Long oil change intervals can do the same.

You have e-mail.


.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
2003 Volvo V70

https://www.boldegoist.com/

There is a quick way to determine to cause of low compression.

Put some (not to much) thick oil in the cylinders and turn the crankshaft a few times to distribute it. This will temporarily seal the rings.
If you compression goes up it the rings leak, if it stays low it is the top end.

Rob

He said he did that in the original post. Also said he had the head checked.

I just bought an 89 300E, and stupid me, didnt check compression before buying. You would think that someone with 40 years of mechanic background i would know better.

Initially the car ran pretty good, just a miss at idle. No blowby even now. But after driving it home and tinkering on it it has gotten rougher, and a compression test showed almost zero compression. Oil in the cylinder didnt do anything which would indicate rings/piston, and I can blow through the adapter and clearly hear air in the crankcase, so I dont know if its a holed piston or blown rings, but its definitely bottom end related.

This thread helped me decide to give up on it. I gave some thought to trying to fix it, but I dont need any more headaches than I have already, so Im parting it out if anyones interested, I put an ad in parts. With the age and miles its not worth farting around with, not to me anyway. But the head should be good. Car runs down the road fine, doesnt run hot, no leaks or drips.

The car sat out back the last several months. I would start it up and move it occasionally to mow grass. No one wanted enough parts to make tearing it apart worthwhile, so I just sat on it. Recently I got a bug to put a motor in it and started looking. Everyone I found (as a parts car), I did a compression test on and found a weak cylinder or cylinders. On the last one, I actually saw vapor escaping past the screw in fitting? WHAT? I spent some time going back and forth between cylinders and found the tester wasnt seating well on the ones it read as low. Have I been condemning good engines?

Moral of the story, make darn sure your getting good sealing on EVERY cylinder, and never blindly trust your tools or your experience. Condemning a perfectly good engine because of a faulty test isn't helping you any.

But to finish the story... I decided it was time to just pull the head and look. If its junk its junk, and I can put the whole thing to rest. I decided to pull the intake as I figured if it was a bad piston, and I was masochistic enough, I would need it out of the way to jack the motor. So the head comes off and low and behold, no bad piston?

No, it has a burned valve. So then why was I hearing air escaping from the crankcase? Turns out, the exhaust gasket for that cylinder was blown out on the underside, and obviously the escaping air I was hearing was coming out from there and not the crankcase.

So, game on. I am taking the head to a machine shop and ordering parts. I'll be cleaning and scrubbing the next week or so, and putting her back on the road. The only reason this car had to sit so long was because of complacency. You can work on this stuff so long you begin to assume you know what your seeing and hearing, rather than always questioning, which is how you start out when your young. Once again I am reminded to measure twice (or three or four times), and cut once.

Just to correct an earlier thread, low compression that does not improve with a bit of oil added to the cylinder is top end/valve-related, not piston/ring related.
Mark

Well, I should have followed up on my original post.
MINE TURNED OUT TO BE A LEAKY EXHAUST VALVE. HOORAY!!!

To re-summarize:
- The head checked good at the shop.
- When I pumped air into the spark plug hole, I could easily detect it coming out of the crankcase.
- Squirting oil into the cylinder instantly raised the compression from 105 to 200.

But, as I was ready to give up, I decided to pull the exhaust manifold and run air thru the spark plug hole again - THE AIR POURED OUT THE EXHAUST PORT!

So, we hit the machine shop again with new exhaust valves. When the tech pulled the old valves he found evidence of uneven wear around the sealing edge of the offending valve, and we summarized that the valve would maybe intermittently seal in a static position, causing the false good bench test.

Anyway, new valves, and all has been good since spring. Running better than ever.

So, conclusions:
- the old tried and true tests are not always valid.
- believe half you see and none you hear (Marvin Gaye).
- the M103 bottom end is truly rugged.
- sometimes we get lucky.

Cheers, guys.

P.S. anyone want to buy a brand new set of 1st-over Mahle OE pistons & rings for a 103 - really cheap, like REALLY cheap.

Yup, I've had few go 'rounds with machine shops and their "testing" methods. For whatever reason some of them have decided that if you apply vacuum to the valve guides and the valves stay closed then the grind must be good.

I'm just too old school I guess. I don't accept assembled heads from them any more unless I see evidence of lapping compound and I usually request the heads to be disassembled so I can do a final lap just to ensure a good "ring".

Twice now while doing a final lap I've run across loose seats and this is from a decent machine shop I've dealt with for years.

My maxim has always been, "Trust no one else's work and double-check your own".

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Not necessarily. A blown/holed piston will read zero no matter how much oil you put into the cylinder. I made an incorrect statement. What I meant to say was I was losing pressure while blowing air into the cylinder at TDC with the valves closed, and it was NOT escaping at the intake or the exhaust (or so I thought, it WAS losing air past the exhaust valve).

My error was that I heard the air escaping, and never thought about a manifold gasket. The sound was "close", so assumed it was from the crankcase. If the loss had been just rings, oil should have raised compression some, or inhibited air loss. When it didnt, coupled with what I believed was loss to the crankcase, I assumed the piston was blown.

When I was young it was almost unheard of to burn a piston and i never saw them. I have seen so many holed pistons in the last decade or so that I guess I was a bit gun shy. Moral is, always follow through diagnosis in order, don't jump around, and never make assumptions.