Time for an update. I bet you guys thought I had this fixed by now.

I sent the MFI pump off to Gus Pfister and asked him to set it to factory settings. He said it was way too rich. I reinstalled it, timed it correctly and went on a test drive. It ran way too lean with lots of popping and no power.

I checked fuel pressure at the return port on the filter console and found 2.5 psi. Pressure directly at the electric fuel pump outlet is 3.5 psi. Spec is 11.8 +/- 3.0. The pump part number is 0580970001 so it's the correct one, not the lower pressure pump for carbs.

Seems like I've found my problem. Before I send the electric pump off to be rebuilt, is there any chance the pump could be ok and the pressure regulating check valve on the filter console is the problem? How would I check that?

I just bought one of these injector testers from mercedessource.com

Loved it!! Easy to use and really helps in seeing the spray pattern and cleaning due to its pressure.

http://forums.pelicanparts.com/uploa...1467401566.jpg

do you have an AFR instrument? That will tell you a lot about how it is running.

Also what ignition do you have? Stock CDI of old kettering?

I have found that a good CDI....modern one....really makes the CDI perform well.

You could try pinching off the return line to the tank, if the pressure at the pump outlet goes up, it would indicate an issue with the pressure regulator(restrictor?)

ALso I'm sure you've done this, but there's always the chance that the lines have been miss-connected here or there in the system. The MFI fuel system is a bit of a mystery overall, especially the short return line from the third nipple on the pump. I think there is supposed to be a restrictor in that line that could be missing.

In my car (under construction) I am going to try to ditch the MFI fuel pump and go with a smaller, lighter, cheaper and simpler TBI fuel pump, but keep the fuel filter console since it will allow me to retain the cold start solenoid, and keep the original look in the engine compartment.