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Member Poll
How many miles on your W124?

0mi - 50,000mi
0mi - 50,000mi
5% [89 Votes]

50,000mi - 100,000mi
50,000mi - 100,000mi
10% [185 Votes]

100,000mi - 150,000mi
100,000mi - 150,000mi
21% [388 Votes]

150,000mi - 200,000mi
150,000mi - 200,000mi
25% [455 Votes]

200,000mi - 250,000mi
200,000mi - 250,000mi
18% [337 Votes]

250,000mi - 300,000mi
250,000mi - 300,000mi
10% [185 Votes]

300,000mi - 350,000mi
300,000mi - 350,000mi
5% [86 Votes]

350,000mi +
350,000mi +
7% [121 Votes]

Votes: 1846
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Started: 03/14/2011

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» W124-Zone.com » Technical Issues
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MAS/fuel pump relay or crank position sensor
Savannah Buzz
93 300TE, 298,000 miles. After driving or warming up, will stumble and cut off. Will re-crank and run maybe 20 seconds. Started after installing used fuel pump and new fuel filter, but took about a day of use before it started. Will let me crank up cold, run up to 3000 rpm, etc. But when warmed up cuts off. When it coughs and cuts off, can hear fuel pump buzz for a second or two. 13.4 volts on battery.

Re-installed original fuel pump (it was used as well, from previous owner, but not as loud as used pump I got from scrap yard). No change in die off. Length of time to die gets shorter, but is not a predictable number, like 20 seconds, etc. Varies quite a bit.

Swapped out OVP - mine had corosion on prongs and across the base, which I also cleaned up.

Removed, inspected, and replaced MAS module, did not see any corrosion or cold solder joints.

Fuel filter is new, Purolator, and big end toward rear of car.

Since the fuel pump can be heard running for a second or two after engine dies, does that rule out the MAS/relay module? Would that point more toward crank position sensor?

Thanks in advance!
Savannah Buzz
New pump arrived , installed, same problem. Attached fuel pressure gauge to fitting on fuel distributor. Fuel pressure 5.5 bar, about 70 PSI. Run at 2000 - 3000 rpm, and in a short time, pressure starts to drop, when it hits about 30 PSI the engine stops but you can hear pump still buzzing for a few seconds.

Just discovered that wagons (this is a 300TE) have a second pump, but it is not attached to regular pump, it is mounted in front of tank in that mystery blob of plastic. So will pull that apart in the AM, enough shade tree and partz hunting for one day.

Went to salvage yard, looking for MAS module which has fuel pump relay. They asked if I meant the "black one", well, yes, that's the one. Their owner has a W124, and needed one. He went looking and found that every one from every Mercedes on his yard had been snatched....

Update - rear pump appeared to be original, the side pump had been changed once already. So swapped out rear pump (that banjo bolt was frozen and the whole fuel line from rear pump to side pump had to be pulled out, and a vise used to free up the banjo fitting). You can remove the hose without taking any other parts off. Once replacement pump fitted much quieter, and PSI at fuel distributor is 72 PSI (with my leaky adapter).

Diagnosis - one pump was failing, dropping fuel PSI to 30 ish, car would shut off, but you could hear the working pump still running for a few secods until MAS unit shut it down. Morale: previous owner should have changed both pumps. But pumps can make noise for a long time, it justs gets a little louder and you turn up the radio.....

Pump source for new was Rock Auto, $67, not Bosch.
Edited by Savannah Buzz on 08-09-2012 09:37
have very similar issue with my 1990 300e 2.6. have 99 percent determined it is in tank screen. when i wait to restart, the small amount flowing through is enough to pressurize system, but as pumps are starved fuel pressure drops and engine stumbles and dies.
1990 300e 2.6 132k
1983 380sel 124k
1982 300sd 325k plus
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Their owner has a W124, and needed one. He went looking and found that every one from every Mercedes on his yard had been snatched.
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