I've been getting email help from MBDoc. Here is a summary. His comments are preceded by >>:

My wife has a 2003 C230 Sport Coupe (hatchback) mit kompressor. The car has had a disappointingly expensive repair history, and we only just bought it in late-2006. The current problem is a misfire driving the Check Engine Light. The code is on cylinders 1, 3 and 4. The car "chugs" at idle. It does it more with the engine warm than when it's cold.

I saw on the Mercedes Shop forum that you said there were several service bulletins about this problem.

This 2003 car has just under 50,000 miles. I'm a handy guy with access to tools and tired of being taken by these repair shops (on the last repair I think $300 of the $500 bill was unnecessary work).

Are there any easy things I can check without a diagnostic computer?

>> I would start with a compression test. Several of the bulletins on that engine are about BURNT valves. Have the compression tested first.

I will. Thank you. This car has me really scared. I bought a subscription to AllData but the information available there is useless. For "troubleshooting" all it gives me is a list of codes. I already know what the codes are!

0301
0303
0304
and now also
0172

>>That list provides a start to a better diagnosis. The 1 (P0 172) is a fuel trim code & most likely causing the engine to run lean, causing the misfires. There is a bulletin on a vacuum hose that breaks & causes that type of problem. Possibly that is the problem.

Thank you. That IS very helpful (and easier to fix). Is there one particular vacuum hose that gives the trouble mostly?

Thank you for your great help. Here's what I've found...

The compression on the rear cylinder was 160 PSI. The three forward of it were each 155. That sounds pretty good to me. Do you agree?

When we took the plugs out, the front two were sitting in clean engine oil. There was oil on the stems of the ignition coils too. When I extracted the plugs, the threads were full of oil but the tips were dry. I think that somebody must have spilled some oil while they were adding a quart. I cleaned it all up.

There is a vacuum valve of some kind on top of the engine. I disconnected the line that feeds it and I hooked that to my suction indicator. First I can tell you that the check valve before it is good. When my wife blipped the throttle, the indicator went down into the low 20's and stayed there. I had a heck of a time getting the hose off the gauge. I moved across to the engine-side of the check valve and the suction was about 15 in Hg and wavering at cold idle. When my wife blipped the throttle, it went over to the boost side of the dial and then down to the low 20's and stabilized again at about 15.

The Check Engine light extinguished.

Then the trouble started.

I previously took this car to the shop for a code P0128 and P0172. The shop replaced the temp sensor and the thermostat and flushed the coolant. Well first-off, I should not have approved that whole action. If I had taken a moment to think, I would have known that the thermostat had to be fine because I'd been watching the temperature from the dashboard and it was fine. But I didn't think and it's my wife's car so I said, "Go. Do it."

Well, that repair was $500 and $300 of that total was the thermostat and the labor to replace it. The trouble? For one thing, we found the old temperature sensor rolling around in the windshield wipers. It had lodged in the rain drain at the base of the windshield. For another thing, my wife thought that the main coolant hose at the front of the engine "didn't look right" and when I touched it, it popped out and coolant sprayed everywhere. The shop didn't get it all the way into the clip. I'm sure it would have come out in the next few drives.

Needless to say I am NOT happy with the shop.

So I fixed the hose and the clip and she started the car again. While we stood there I guess the engine warmed up because it started surging again going chug-chug-chug-chug and shaking. It's only at dead idle. If she presses the gas (even only a little) then everything's fine.

What's my next move?

Now more details: Now wife says it’s worse when idling with the air conditioning on (under a load). We’re going to see if pressing or releasing the brake pedal makes a difference and laso see what the new fault codes are.