Overheating

[Kit]

Missing, overheating. 1968 280 SL Rebuilt engine 20 years and 15 k miles ago by Metric Motors and Burlingame Mercedes. Mostly original, beautiful condition, appears well maintained, but hardly used for the last 20 years.  I purchased at auction at the end of 2024, but don't feel confident in the car. Before using, we had a D service. The only real issue we found was the wrong fluid in the manual transmission.  After running for an hour, the engine starts missing. More gas usually keeps it running, and it usually settles out.  We have checked the fan clutch, fuel lines, and fuel pump, fuel pressure, replaced the thermostat, and replaced the distributor with an electronic '123' distributor. This provided a better cold start and noticeable power increase, but the missing issue persists. My mechanic has the car. Any thoughts on what to look for> Thanks, Kit

[Lori]

What do the plugs show?

[Pawel66]

I am not a mechanic and I do not think shooting from the hip is effective, but perhaps...

On fuel - fuel flow rate at the end of return line (simple test) needs to be 0,7 - 0,8 liters in 15 seconds. Equally important as fuel pressure. All fuel filters need to be clean (fuel tank, fuel pump and the main one). Question from Lori - if the plugs are black and covered with sooth, it may be missing just because it is running too rich. Have you done the Linkage Tour and set idle right?

Electricity: have you pulled the plug connectors one by one to determine which cylinder is missing? If one - that is where to focus. I had it several times that Beru plug connector was faulty after car was standing for a while. Replaced it with Bosch. if all are missing at random, I would go back to fuel.

Also: run it in the evening in a completely dark garage - you will see where electricity escapes if it is an issue with electricity.

[WRe]

Hi,
I would check the fuel pressure (0.8 - 1.1 bar, min. 0.5 bar) and quantity (1 litre/15 sec, min. 30 sec). If possible, also while driving (hot engine - formation of vapour bubbles). Also the tightness of the fuel system. The injection pressure of the injection nozzles (15-18 bar, min. difference 3 bar) and their tightness are also important.
...WRe

[Wray]

Had a similar issue, car ran well originally but started to bog down.plugs were lousy. Replaced plugs . Fuel filter showed large build up of varnish and gunk. Drained tank added fresh fuel. Minimal relief. Removed sending unit and tank was very rusty. Replaced tank and not much better. Removed and cleaned the fuel pump filter and now run properly.

[Benz Dr.]

Missing, overheating. 1968 280 SL Rebuilt engine 20 years and 15 k miles ago by Metric Motors and Burlingame Mercedes. Mostly original, beautiful condition, appears well maintained, but hardly used for the last 20 years.  I purchased at auction at the end of 2024, but don't feel confident in the car. Before using, we had a D service. The only real issue we found was the wrong fluid in the manual transmission.  After running for an hour, the engine starts missing. More gas usually keeps it running, and it usually settles out.  We have checked the fan clutch, fuel lines, and fuel pump, fuel pressure, replaced the thermostat, and replaced the distributor with an electronic '123' distributor. This provided a better cold start and noticeable power increase, but the missing issue persists. My mechanic has the car. Any thoughts on what to look for> Thanks, Kit

Have you tried Water Wetter? It actually works.

[Kevkeller]

As others have said it could be the fuel filters. As the car runs for a while the filters might build up crap in them. When you shut the pump off the crap might drop off the filter so that when you start up again it runs good again until it builds up again.