#3@120/145 Psi
 

Sad day.
While I was R&Ring the suspension I compression tested #3 (the others give no indication of being bad and I was on limited time. Will retest and test the others tomorrow.
120PSI dry, 145ish with about 2.5cc of oil.

Thoughts? I just finished the ball joints, UCA, and Tie rods. Also: Other than fuel economy what are the downsides driving with a bad cylinder?
-nB

When was the last time you adjusted the valves?

And did you decompress all other cylinders pulling GP's or injectors when you tested #3?

That won't affect the readings.

Valves about 5Kmi ago (8 months?) by an indy I trust, and no, the other cylinders were not decompressed (would it matter that much?)
-nB

Just think of it as a 616 turbo. As long as it starts and runs, I'd keep driving while I looked for a low mileage replacement engine. (after confirming it's not maladjusted valves).

I would blow some air into that cylinder to see if perhaps just a valve problem. That is after making sure there is still clearance at the valve adjusters on that cylinder. It does not matter if someone was at the valve adjustments some time ago you still have to check it.
A valve job is not all that bad if you know how to get things done. Or if not just doing the one bad valve itself is not a particularily good ideal but possible. It is not always actual bore/piston/ring problems.
The air test could also help establish if it was a head gasket leakage problem as well. Any air bubbles showing up in the radiator while running?

I suspect it is not valve adjustment, as the shaking at idle was an issue with the car ever since I got it (and the PO noted it before). Even immediately after the valves were adjusted it would shake like this.

I assume that aside from fuel economy I'll go through motor mounts and air cleaner mounts quickly, but is there anything else to watch out for?
Also, since the numbers didn't climb much with the oil, I'll assume that it's likely in the top end. How possible is it a head gasket Vs, valves? There is a little pressure in the cooling system...
-nB

Bring it by Nik and Garretts.... They have a battery of stuff I am sure they can look for and help you narrow it down.

OK, All Numbers (PSI, using injector bore):
320-320-100-340-280
No change on the 100 with oil in the cylinder, didn't check the 280 as there is not enough time (got a slot at 1:00 for laser alignment)
phoey this isn't wonderful, though I guess there's little to lose by burning veggie oil now :o. Good news is that the prechambers all look ok.

Any thoughts other than replace the engine? The chassis is in awesome shape, as is the front suspension (assuming I made no grievous errors)...
Well, wish me luck that the wheels don't explode on my way to the alignment shop (thread showing on the drivers side).
-nB

Again if you can establish it is just a leaky valve it is not all that bad to deal with. By application of compressed air to that cylinder and just listening at the tailpipe and intake you will know one way or another.
Of course the valves have to be closed with the engine at top dead centre for that cylinder. Also if your head gasket is getting a little tired you will get to change it as well.
Piston, rings or bore damage are the real terrors. A valve problem on a mercedes is far from uncommon. A five cylinder engine with one cylinder down is not all that much fun.
The head gasket probably is not really the total problem as at high pumping speed you would get certain effects like the coolant being blown out of the system, overheating and perhaps other external evidence, etc.
I would take the time to find out what it is before deciding if you want to repair it or not. In fact since most your compression numbers are not all that bad it might be well worth it. Unless you do not want to get your hands dirty that is.

It'll have to wait for another day. I've blown my budget for a while on suspension/steering/tires. If it is ok for the engine to drive this way for a while, then I'll likely tackle it this summer/fall. I'll be bowing out of the NorCal diesel drive (again) and leave the car as a work commuter only (about 30Mi round trip) and drive it easy.
-nB

I can think of no reason it will hurt to use it. That low cylinder might be running once revs pick up anyways. The cylinder leakage is a time thing. At higher revolutions there may not be enough time for the compression to leak off as it does at your cranking and idle speed.

Like other's have said {repeatedly} you need to verify your valve clearances. Even a knowledgeable mechanic could have accidentally adjusted an intake to the exhaust spec, or visa versa, or even a lock nut was loose.
You need to check them!

Yes I understand that. I plan on checking the valves as soon as I can get a valve cover gasket in (but it too will have to wait for at least a paycheck or two). I assume at that time would be a good time to do a leakdown test (I can use the GP holes rather than pulling the injectors right?) Just need to get/build a compressor to GP compression tester adapter. My compressor will only do about 120PSI (that good enough?).
I ended up going over budget on the suspension by about $300 (mostly tools and tires).:o
I used to be nervous about doing the valve adjustment, but after the suspension and compression test, I don't think there's much beyond pulling the head or rebuilding the transmission that can overwhelm me any more:P
-nB

just clean and reuse the gasket, they are pretty beefy. Ive reused many times with no leaks.

If the piston rings were aligned for some reason you would get very poor readings. Try driving for a few hundred and measuring again.