Hello folks,

I have a euro W126 280S with an engine rebuilt 80 000km ago. This is a fact.

My compression test show 11.3 bars +/- 0.2 bars for 5 cylinders and ONE cylinder (number 3) at 8 bars . Add oil, it gets up to 9.5.

My first reaction was : broken ring . But then I thought: why did this happen ? It seems unlikely tp me.
The plug tends to oil foul in that cylinder but not really oil soaked, just dark - humid

The coolant looks ok if a bit smelly. Could this be a head gasket without a leak into the coolant ? The idle is rough but the engine turns over quite smoothly at some speeds.

Could a valve be cracked ? I had to install thinner pressure pieces to get the valve gear to be in spec. Maybe a valve was damages...

Any tips/ diagnostic approaches appreciated before I decide to pull the head.

Cheers

Denis
Lion sur Mer
France

If a valve is burnt or has a small crevace in it, you will lose compression and you will be able to hear it. If an intake valve it will push air back into the intake manifold. If its an exhaust it will be distinctly audible in the exhaust note. A burnt valve will show up more at idle than at speed, too. and if its a leaky exhaust valve it will make the car run a little hot.

oil in the cylinder can sometimes seal a leaking valve too.

Good luck.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

What Tom said. You need a leak down test, or introduce some air under light pressure and listen for the leak. If it's a bad ring you'll hear it in the oil fill hole.

I also say leakdown test. If it hadn't been for that particular test, I'd have been totally lost in finding the dead cylinders in my m110. Mine was rebuilt when the frame had 221k miles on it because it was burning excessive oil.

I did clearance checks once i had torn it down. The skirt to wall was way over spec by about 400%. The valve stem seals were hardened and leaking. The valve guides were just slightly over. Some valves were also out of spec and had to be replaced.

80k km isn't a lot of miles for an engine like that. I'd say you should check for oil consumption. I'd doubt its rings at so many miles. When researching my own, I had found that many people did the valve stem seals more frequently than anything else.

It can be done with the head on the car, but if you're going to do anything to the head, I'd just go the full nine yards. stem seals, valves as needed, guides all around, reangle the valves and valve ports, and check the block and head mating surfaces. That'll get you holding pressure again.

-Troy

__________________
- - - '77 federal 280e - 222k miles
- - - '85 380se - 129k miles
- - - '07 Chevy truck
- - - '79 Datsun 280zx with '82 turbo motor
- - - Currently looking for a clean 190e with less than 160k miles.

Thanks guys,

It's like you say...I suspect that T walgamuth is right. I will do a leakdown test on the weekend. If it is a cracked valve, I will stop driving this car after another 100 km getting it back home - it's parked in Deauville at this time.

If it's cylinder head overhaul at 80 000km, I suspect that the valves were never adjusted since the rebuild by the PO and that at least one has burnt.

Cheers

Denis
Lion sur mer
France

Yeah, at 80K that engine is barely broken in. They are known for doing about 400K miles and will match up with the legendary durability of the 616 617 diesels, though the valves will probably need some attention before the bottom end.

If it really is that low miles a ring could be stuck too.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I guess so Tom but why would a ring be stuck in a very clean rebuilt engine ? Of course maybe the rings were improperly installed but why 80 000 km before aproblem...I'm betting on a cracked valve
I'll keep you guys posted on the leakdown results.

Cheers

Denis
Lion sur Mer
France

Strong waves and gusty winds on the English channel this morning (dog walk requirement as it were)...