The car dies within 5 minutes of starting. Idle will get progressively worse until it won't idle at all and you have to give it gas to keep it going - keeps getting worse until not possible to keep the engine running at all. Re-starting requires "flood start" - foot to the floor and crank like crazy - in about 30 seconds of cranking it may start, acting exactly like a badly flooded engine, cylinders slowly kicking in until it finally runs.

This problem was coincident with a leaking head gasket, and I have wondered whether coolant might have messed up the O2 sensor(s)... But I wouldn't think that a bad O2 sensor would be able to throw the mixture off so far that it would die...

What I have done:

Replaced crank sensor - no change.

Checked fuel pressure - ~45psi, and does not drop as the engine dies.

Removed exhaust to check for plugged cat - dies exactly the same (except much more loudly).

This vehicle has the "partial" OBDII (not fully implemented until 1996 as I understand...?) - does not show any codes.

Most likely either a bad coolant temp sensor, or a really bad vacuum leak. Has the wiring harness been replaced?

You need to build/buy a code reader so you can find out what the on board diagnostics can tell you. https://www.amazon.com/Mercedes-Diagnostic-reader-SL320-300SL/dp/B00H8SGV46 Not a recommendation to buy this particular one, but either buy a ready made one, or go to radio shack and buy the parts to make one.

I would first do a fuel pressure test . Then look in to electrics and its allways said, time and time again, do the easy parts first ---fuse chack and replace old fuses and --- earth lead contact to body and engine . Plus vacuum pipes for splits .

Scott, it would help to know more about the history of this vehicle. Have you owned it for years and this problem gradually happened, or did you just buy this vehicle recently? It seems to me that a new fuel filter would improve the idle. Check the condition of the spark plugs and ignition wires.

__________________
Fred Hoelzle

Is this hfm , or cis?

HFM