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johnflight1 05-23-2005 08:31 PM

88 300e 124 Fuel Pump Relay????
Before I change the $113.00 fuelpump relay,I understand there are other things :confused: 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!

JerryWagner 05-23-2005 10:10 PM

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 :P

johnflight1 05-24-2005 07:28 PM

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.

simmo300e 05-25-2005 06:40 AM

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?

johnflight1 05-25-2005 07:14 PM

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.

Mike Murrell 05-25-2005 07:28 PM

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.

simmo300e 05-26-2005 01:04 AM

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?

anthonyb 05-26-2005 03:08 AM

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.

89-300ce 05-26-2005 11:25 AM


Originally Posted by anthonyb
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.


zafarhayatkhan 05-26-2005 12:07 PM

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

johnflight1 05-29-2005 08:17 PM

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

psfred 05-29-2005 10:31 PM

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.


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