Dying 300E
 

A friend has a 1991 300E-2.6L-CIS. Last week she drove it to her friend's house and it died.
I went over to look at it. Checked the rubber connections on the air intake boot and idle control motor (another friend had problems there on a 300SE). All the rubber was intact and connected, car started and ran fine for 10 minutes but stalled out on the way home. Restarted and stalled several times then ran fine for the next 10 minutes until home.
I replaced the overdue fuel filter. It was slightly clogged. Noticed there were TWO fuel pumps plumbed in series.
First question - will failure of ONE of the two pumps kill the car?

Checked the Library's Mitchell repair manual and read that 91 and later 300E have a MAS contol unit instead of a fuel pump relay but manual says car only has single pump!
Went back to restart car - started and ran OK for about a minute then died. Cranked motor for 1/2 minute and got a few pops but no running. Tried it again and it started and ran perfectly. Opened the MAS control unit to inspect solder joint/connections for visible damage - no damage found. Put it back together and car started and ran fine. Went back later to check fuel pressure and found I didn't have the correct adapter for my guage. Decided to run car anyway to see what happened. Car started, ran and drove perfectly for the 45 minutes that I ran it. As a new MAS control unit is around $400, I'm not going to just stick another one in there. I'm getting an adapter for my fuel pressure guage. Meanwhile, is there anything else I can check or do I need to wait until it dies permanently?

Behind the battery, near the MAS control unit, there's another small relay with a fuse on top. Can I / Should I check that as well?

Thanks in advance and Happy Motoring, Mark

That small red top relay is the OVP (overload/overvoltage protection relay)...DO check the fuse on top.

Certainly smells like fuel pump(s).

Try this low-tech diagnostic method:

When it is not starting, try placing a hand on each pump while someone turns the key to the on position. You can feel if they are operating for a few seconds.

I say use hands because my digital voltmeter is too slow to register a voltage within the short interval they operate.
I suppose you could also use a test light...

If one fails to operate, give it a firm whack with something, and see if it kicks in. If it does, you know the culprit.

Update: The fuse on the OVP relay checked out OK and I ran/test-drove the 300E for about 45 minutes, for two days in a row, without problem. Previously, it hadn't died long enough to be able to check it out more thoroughly. And NO ONE here in Yuppieland has METRIC fuel line fittings with a 14 mm male thread to make an adapter for my fuel pressure guage. Will need to search my local salvage yards and cut one out of a junker.

Or maybe I'll let her leave it to the local MB dealer!

Happy Motoring, Mark

OVP maybe bad

UPDATE!

After a few days break to move on to other projects and think about that dying 300E, I went back 2 days ago to start/run/see if the problem would return. The owner had tried it one time and it died 3 feet out of the carport but restarted long enough to go back in.
I started the car. It ran perfectly for 5 minutes then started missing. Ran long enough for me to get the distributor cap dust cover off in time to see pretty little sparks jumping from the wire boots at various points before it died. Also a little spark
jumping from the coil wire to the aircleaner mounting. After it died, I had the owner crank the engine. Good strong spark from the coil wire but nothing from the plug wires. Yesterday, I replaced cap, rotor and wires and went on LONG test drive - car ran fine. Keeping my fingers crossed!

Happy Motoring, Mark