Lately, my 1991 420SEL is starting to stall. I could go a week and run fine, but then the next day, the car will stall three times in an hour. It's even stalled while cruising at 40 mph! When it does stall, it'll start right up.

Could the fuel pump be going bad and is there a way to test it?

Welcome to the wild and wonderful world of w126 sudden stalls.

There are a myriad of issues that can cause this. You could spend a day searching and reading posts on this subject right here on this forum.

Here's the SHORT list of possible causes:

1) Vacuum leaks
2) Fuel delivery issues (pump, filter, screen, etc....)
3) Electrical issues (OVP Relay, bad grounds, fuel computer, etc...)
4) Ignition system issues (coil, cap, rotor, etc...)

If you need help from this forum, you should be prepared to provide all sorts of info such as:

1) Work history of the vehicle (Which parts have already been replaced?)
2) Overall running condition (does it run perfectly other than the stall or does it have other issues such as rough idle?)
3) Conditions under which the stall will occur (outside temp., driving conditions, etc....)

Do a search on "SSS Sheet" there is a little troubleshooting sheet that somebody created to help narrow down the possibilities.

To answer your question, yes you can do a fuel pressure test to see if your fuel pressure is adequate and stable.

__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition