Recently our 450SL (48,000 miles) started demonstrating an engine miss at idle and a cold acceleration stumble/hesitation. The engine misses until the temperature comes up to above 60 C on the temperature gage and then it works and drives fine. It is most noticeable when accelerating when the engine is cold. Previously the engine started and ran fine at all temperatures. The car has a new (~1 year old) acceleration warm up compensator on it so I suspect that this is not the cause of the engine miss. It appears that there is some part of the enrichment circuit that is not happening correctly which goes away once the engine warms up.

Two questions:

1) The car has the original plug wires and cap/rotor. Could this be causing this problem? They appear to be in good shape and there is no evidence of any cracking or burning in either the rotor or the wires. The humidity here in Wisconsin is not bad so I do not suspect that this is the cause.

2) Is there an order that I should follow in trying to diagnose this problem? Several items that could contribute towards this would be the thermosensor, the oil temp sensor and/or the water temp sensor. How do I check these or should I check something else first?

Appreciate your help with this. It's a great car (once the engine is warmed up!) and would like to resolve this prior to the short summer driving season.

First, sensors do not cause misfires.

Misfires are caused by things that affect only single cylinders. My guess is that you have a spark plug resistor bad. This is easily seen with a secondary ignition oscilliscope. That is the first step!!!

If you don't have one or know how to evaluate it, you can spend a bunch for a set of wires. But of course it could be other things.

I am trying to find a reputable mechanic in the area that can help me troubleshoot the problem. I agree, I can dump a ton of money into plug wires, cap and rotor and still not fix the problem. You mentioned a spark plug resistor that could be bad. Would this cause the car to miss when cold and resolve itself once the car has warmed up? Where would I find this resistor and is it something that I can test with an ohmmeter? Could this be an issue with a vacuum leak? I have been having trouble with the Climate Control Servo recently and the problem seems to coincide with the engine missing.

Appreciate all your help. I generally try to do all of my own service since I enjoy working with cars and find that the work is rewarding and (sometimes) relaxing. I have screwed up a couple of times but learned from it each time I do it and want to continue to learn. What a great hobby!

Unlike domestic radio suppresion ignition cables, the Germans use real copper wire for their plug wires and have ceramic resister ends (at the spark plug and dist cap) that does the RF suppresion. These open circuit quite often and are the single most common cause of single cylinder misfires on these engines. The open circuit voltage is obvious on a secondary ignition scope. The problem can even be seen when the symptoms aren't evident.

There are many other causes of single cylinder misefires but identifying the cylinder is usually the first step.

Appreciate your knowledge and your help. As stated, I am trying to find a reputable mechanic in the area. Will let you know how it turns out.

P.S. Just bought a Climate control servo from Parts Shop (ouch !!!!). I know that this was the culprit since you had to bang on the old one to get it to work. Working on Mercedes might be fun but it sure isn't cheap!