6.3

I changed the oil on the MFI pump. I replaced it with a heavier oil 20W50 because that's what I use. Where cold starts took two 7 seconds cycles to fire, a new condition manifested where it takes 3-4 minutes to fire. I cleaned and resealed the cold start valves. I verified the relays were operational. I scoured this board and found a case for the MFIP wax thermostat warm up regulator might be sticking. The screws were fairly beat up and the threads were very rusty. One was only finger tight. The rubber hoses look fairly new and are MB branded. The clamps are correct. After cleaning and lubricating with penetrating oil, the warm up regulator button in the pump moved much easier. Reassemble. Turn the key. It starts almost instantly. Had to adjust the idle air half a turn, but otherwise a happy engine. Amazing.

The problem comes with reassembly. I can't get the brass thermostat housing to sit flush against the pump. I thought maybe the screws needed to be tightened a little bit at a time, alternating sides. Nope. I thought rapping it lightly with a mallet might help. Nope. I thought maybe the thermostat had backed out of the housing a little bit. Nope. Everything I try results in an even gap of about 3mm between the thermostat housing and the pump. Can't be good. Can't be right. Maybe it was like that before I disassembled, but I hadn't noticed. Maybe the thermostat is the wrong part number? Maybe this housing is the wrong part number? Maybe there is supposed to be a gasket? I'm lost. Any ideas? Anyone have the height dimension of the correct thermostat housing?

Hello.

Here is a picture of pump as I got from Bosch Service for comparison.

For the wax element see pictures at Mercedes Injection Pump Thermostat - fits 230SL & other models (authenticclassics.com)

There must be one copper seal washer between the housing and the element. There may be an optional shim between pump body and the element.

Please share pictures of your installation for comparison.

600 #796 (1967 US version)

300SEL 6.3 #1285 (1969 EURO version)

300SEL 6.3 #5346 (1971 US version project)

Thank you for the picture. That helps tremendously. It appears that the thermostat housing doesn't fit flush by design. The center circle is flush while the two tabs have a gap. Mine is no different than the way yours appears (apart from shiny parts!).

I'm still fighting the cold start issue. I have to floor the gas pedal while cranking, and it will fire on the second crank. I'm thinking about adding the extra relay to the cold start system. If that doesn't work, I guess I could start over with correlating the MFIP to the air valve. Timing is dead on. Injectors were serviced 5k miles ago. I've got 2 clicks left on the idle adjustment until full rich. I never have black smoke. The car runs great once it starts. This condition started after I did a C service, including changing the MFIP oil for new. I went with a heavier oil. I've checked the fluid level twice and its still good. Any thoughts?

I suggest thread "6.3 Cold/Hot Start Issues -Tutorial". The most important during normal temperature start is voltage and fuel present at two extra intake injectors during cranking.

600 #796 (1967 US version)

300SEL 6.3 #1285 (1969 EURO version)

300SEL 6.3 #5346 (1971 US version project)