Fuel Management

I have an issue with my 1970 300SEL 6.3 (10901812004242) that is driving me nuts. When cold starting, it starts quickly and runs about 20 seconds then quits. If I restart and hold foot on throttle, it seems to run OK for a little bit, then starts to miss and will quit unless I give it substantial throttle. If I can catch it before it quits, it will run roughly for a little bit then clear up and run OK for perhaps 20 or 30 seconds, then it does it again. If I persist and keep it running for 15 or 20 minutes using my foot on the throttle until it is really fully warmed up, it will run fine and not exhibit this behavior.

I hooked up a timing light and a fuel pressure gage so I could monitor while it was doing this. So far I have had the timing light on cylinders 1, 5, 6, and 7 and I have seen no indication of a problem with spark. The fuel pressure stays between 11 and 14 psi the whole time, though I might be seeing a little shudder in the needle during the time when the engine is missing.

I am a little suspicious of leaking cold start valves, or perhaps issues with the power to the solenoids on the back of the injection pump, but I don't know how to figure this out.

For background, the car sat outside from about 2011 until it was rescued in 2019. I bought it in 2020 and started bringing it back from the dead. I believe that it exhibited this behavior when I first started the car in 2020 (in May) but it went away and ran fine then and in December when I was next working on it. This behavior returned in April 2021, with no obvious reason. The car received a new fuel tank, fuel pump, fuel filter etc, and I flushed the system through the return line to check that the fuel was clean. I also put ATF in the fuel filter upon first start in 2020 per the guidance in this forum and checked and renewed the oil level in the injection pump. Also replaced the spark plugs and had to replace the primary set of points since initial startup was running only on 4 cylinders.

Your help and sage wisdom would be appreciated.

Sounds like an issue with the warm up device on the MFIP. If it starts fine cold the CSV and rack solenoid should be good, they supply an instantaneous shot of fuel to start the engine. Then the wax thermostat in the injection pump should control the mixture as the engine warms. There are many threads in this forum that deal with it if you use the search function.

Basically it involves an element that acts on the pump rack to regulate fuel and an auxiliary air slide to regulate air during the warm up phase. The wax thermostat, immersed in coolant, reacts with coolant temp eventually bypassig the warm up process.

Hopefully others will chime in with reference material.

300SEL 6.3 #2723, my first classic Benz3.5 #8659, my second.2 to go...

Thank you for your thoughtful response.

As an update to my original post at the bottom, I started the car yesterday after fixing a different problem and it did NOT exhibit these symptoms. The only obvious difference was that I had filled the tank with fuel. With the indicated normal fuel pressure, not sure what’s going on but I’ll try and keep the tank near full for a while and see what happens.