No Start, Erratic Fuel Pump - PeachParts Mercedes-Benz Forum

mpolli · #1 01-20-2011, 11:50 PM

Bad fuel pump relays are pretty common. Did you try a search?

porkface · #2 01-21-2011, 12:40 AM

the only "regulation" for the fuel pump is the MAS relay, which controls the pumps, ac compressor and has other functions. there is no pressure switch to shut off the pumps. the shutoff is because of a timer built into the relay to prime the system. the pumps stop until an ignition signal is received by the relay and then the pumps run again.

you do need to know what the fuel system pressure is when the car doesn't start. needs a special set of gauges to test. i can't relate to WHY the pumps are cycling 3 times and then no start. sound like a relay and/or another problem. keep in mind, you need to know if there is spark when it won't start. the MAS relay has no control over the ignition. good luck, chuck.

Ferdman · #3 01-21-2011, 03:55 AM

Jeff, the stumbling/stalling upon driving a short distance after a cold start could be due to the Oxygen Sensor. When was it replaced last?

I had similar symptoms on our 1991 300CE that a new Oxygen Sensor resolved. Best to check codes before throwing parts at the problem.

tvpierce · #4 01-21-2011, 10:12 AM

Quote:

Originally Posted by Ferdman

Jeff, the stumbling/stalling upon driving a short distance after a cold start could be due to the Oxygen Sensor. When was it replaced last?

I had similar symptoms on our 1991 300CE that a new Oxygen Sensor resolved. Best to check codes before throwing parts at the problem.

O2 sensor was replaced within the last 20K miles... so that should be OK.

Mike Murrell · #5 01-21-2011, 11:07 AM

Had to replace my fuel pump a few years ago - situation different - warm/hot weather.

When it warmed up - temps over 80 degress - car would shut down after 15 miles of driving and that was after starting out cold in the A.M.

Let the car sit for an hr. or so and it would restart and drive for 3-5 miles B4 dying again. Let it side overnight and again - 15 miles B4 croaking

Not saying fuel pumps only die in warm weather - just making a comment.

I had a situation similar to the OPs - turned out to be the OVP.

#6 01-21-2011, 01:57 PM

You could check the coolant temperature sensor. Yours should be a 4 pin I believe. You do a 2x on them. 1&3 & 2&4. +20C 2.28-2.72koms, +80C 290-364Koms. If it's out of range it could cause you trouble when cold there. It's a quick easy check with a vom. You could also check the intake air sensor 2&3 for a reading. Both up front on the 2.3.

tvpierce · #7 01-21-2011, 10:11 AM

Quote:

Originally Posted by porkface

the only "regulation" for the fuel pump is the MAS relay, which controls the pumps, ac compressor and has other functions. there is no pressure switch to shut off the pumps. the shutoff is because of a timer built into the relay to prime the system. the pumps stop until an ignition signal is received by the relay and then the pumps run again.

you do need to know what the fuel system pressure is when the car doesn't start. needs a special set of gauges to test. i can't relate to WHY the pumps are cycling 3 times and then no start. sound like a relay and/or another problem. keep in mind, you need to know if there is spark when it won't start. the MAS relay has no control over the ignition. good luck, chuck.

OK, the fact that the pump is timed by the relay helps me understand the system a little better. So as mpolli indicated above, I need to at least take a look at that relay.

I haven't been able to troubleshoot spark and/or fuel delivery because of the intermittent nature of the failure. When I'm at home -- where I have a fuel pressure gauge, and an assistant to turn the key while I work under the hood -- the problem doesn't present itself.

tvpierce · #8 01-21-2011, 09:52 AM

Quote:

Originally Posted by mpolli

Bad fuel pump relays are pretty common. Did you try a search?

It would seem if the Fuel Pump Relay were going, it the pump would intermittently not activate. In this case it activates every time, and when I'm experiencing the no-start, it activates multiple times.

Does that seem logical?