Before I change the $113.00 fuelpump relay,I understand there are other things that can cause the relay to shut off the fuel supply. I have taken out the relay and jumped the power pins and the car runs fine no rough idle or stalling when warmed up! but I have sen on other posts that the Idle speed controll unit,a/c compressor controll unit,tach,Cis-e controll unit all can cause the relay to cut out. Any sugestions on further trouble shooting? I have already replaced the fuel filter, coil, plugs, wires, cap, rotor, no help! only removing the fuel pump relay and jumping the pins eliminates the problem!

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

It's pretty clear it's the fuel pump relay, $113 is not too much to spend, how much was your last Annual ? Don't forget, IDENTIFY,VERIFY,FEATHER

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Jerry

2004 SL500 Black 55K Miles
2002 s500 Black 90K Miles
1986 300E Gold 233K Miles
1966 Twin Cessna 320D
1989 Jeep Cherokee 233K Miles
1994 Ford Van E150 399K Miles

I would love to just change the fuel pump relay,but being aircraft mechanic I took the relay apart which is dated 1994(my cars an 88) there is no burning of the contacts on the relay contactors,and the circuit board looks good very clean like nothing got hot. the problem only happens when the car gets up to operating temp and only at idle. so its something in the idle circuit that starts acting up when at operating temp.

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

What exactly is the problem? Does the engine stop completely when warmed up, which would suggest the relay doesn't like getting hot, or is it stalling and running rough?

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1987 300e manual 250,000 km (sold)

It stalls when warmed up when turning the steering wheel or comming to a stop. if I drive with 1 foot on the brake and 1 on the accelerator to keep the car out of idle mode it wont stall but you can hear the fuel pump rpm going up and down , and also the rpm's fluctuate you can restart the car as long as you depress the accelerator pedal. it does not matter if the a/c is on or off either. I think after I check the amps the pump is drawing I will rplace the relay,but I have my doubts.

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

I went thru this recently - M103 motor. Car would stall once fully warmed up.

Turned out to be a tired OVP. The OVP powers up the idle control circuit.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Almost certainly the idle control valve not functioning correctly for the car to stall like that.

It's possible that the valve is gummed up and sticking internally, much more likely to be the electronic control system. First one to check would be the OVP, but it could also be a soldered connection gone bad in the FPR or, in my experience, in the ECU itself.

I'd like to know why the fuel pump rpm was fluctuating up and down though, anyone have any thoughts on that?

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1987 300e manual 250,000 km (sold)

If jumping the pins at the relay socket solves the problem, and it's the original relay, it's likely that the relay is at fault. I think the original part that came from the factory had cold-solder problems, which resulted in hairline fractures that occasionally gave open circuits. You can try carefully resoldering all of the solder connections, and see if that fixes your problem.

If that doesn't seem to help, you can remove the idle control valve and spray some air intake cleaner into it until the solvent runs clean, then lightly spray some WD-40 into the valve, and reinstall it. That may help cure any stickiness resulting from fuel system residue.

For what it's worth, my car would occasionally stall coming down to idle, and every once in awhile, it would just not start for a couple hours at a time. Replacing the fuel pump relay fixed both problems.

Same with my car. I assumed that the relay was weak and would fail when hot because it would always restart if you let it sit for a while. New FPR and problem solved.

Jorg

I bought the fuel pump relay for my 94 E320, from the Dealership for around $20.

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Zafar
94 E320 58000 Miles

Thanx for all the good advice. I will try cleaning the idle controll valve out. its the original from 1988. the fuel pump relay shows a date of 1994 so its been replaced and it looks good internally. also I plan on checking the amp draw on the pump at pins 7-8 see if its higher then 8 amps then I know the pump is frying the relay when it gets hot. see ya john

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

Clean the idle control valve by removing, putting a thumb (or something else) over one end and spraying in a good amount of carb cleaner, then capping it and shaking it for 30 sec or so, then dump. repeat until it comes out clear -- this will take a while!

Stalling while coming to a stop is the idle control valve failing to re-open at the correct time. Also check the throttle position switch for correct operation, as it will also cause stalling if the switch does not close at idle -- running mixture is wrong for idle, too lean I think.

It's possible the idle control valve faulty, too.

If the control valve hoses aren't nicely pliable, replace them (manifold side is a PITA, but doable) -- this will cure a number of ills, including curing rough idle by putting the overlean mixure back into control range.

Then, if you can cure the rough idle by jumpering the relay, replace it -- new relay cured a stalling problem with the TE, but that was only while driving.

Note that a bad OVP relay will cause all the same problems -- if yours has a single fuse on top, it's overdue for replacement.

Do also check the current draw on the fuel pumps (there are two). If one is bad, it will cause all sorts of problems when it gets hot.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!