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

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DrDKW