Hi there,
I've got a 1986 240DL wagon (4cyl, auto trans), and while back it started acting like it was missing really bad (like it was getting fuel for a second, then not, and repeating that). It wouldn't always do it, but then it got to the point where it did, and I knew it wasn't safe to drive anymore, so I parked it. I replaced the fuel filter thinking maybe it was gummed up: didn't fix it. Checked/cleaned all fuses, that didn't do it, and today I just replaced the main fuel pump relay ($55!!) and that didn't fix it. I've taken jumpers to the fuel pump and make it work that way, so it's not the problem. Does anyone know what could be causing this/how to fix it?

 

Check the connectors to the fuel pump itself. Mine were corroded and crumbled off when I went to clean it up. Be careful when you do so, when I tried to get the nut off to reattach the wires, the post snapped - I had to replace the pump.

If that's not it, I would start checking wire continuity from the pump back.

 

your car has /two/ fuel pumps, one in the gas tank, another underneath. they are both powered at the same time, but the one in the fuel tank is on its own fuse (4), which means there's a simple trick for testing them both separately...

remove fuse 4. take a paperclip bent so you can reach from the spring contacts of fuse 6 to fuse 4. fuse 6-10 are *ALWAYS POWERED*, even when the car is switched off. if you jumper from either side of fuse 6 (or 7,8,9,10) to the LEFT contact of fuse 4, you should power the MAIN fuel pump, and hear it run. if you jumper from fuse 6(7,8,9,10) to the RIGHT contact of fuse 4, you should power the in-tank pump, which can be somewhat harder to hear.

replace fuse 4 after completing these tests

the wires to both fuel pumps are yellow red, and they run straight from the fuse panel next to fuse 4 to the corresponding pump.

now, if the fuel pumps DID run in the above test, then put fuse 4 back in, remove the fuel pump relay, and jumper from pin 30 (always-powered) to pin 87/2 (yellow red wire), and this should power the fuel pumps, again, you should hear them.

if that also works, then things are getting a touch more complex. The ECU tells the fuel pump relay to trigger when it sees timing pulses coming from the engine via the ICU. when the engine stops turning over, the pulses stop, and the ECU shuts the pumps off a second later.

 

Thank you both very much! I'll try the tests, and post the results. I really appreciate the feedback!

 

Pierce, I performed both tests you suggested and heard nothing from the fuel pump.. :/

 

then its probably the wiring between the fuse panel and the fuel pumps, since you said you were able to jump power directly at the fuel pump and it ran.

 

Ok, so ,I've done some more work/testing. I pulled up the carpet and found the wires running to the fuel pump, took em off the pump and pulled em up through the floorboard. My dad and I did continuity tests from the wires at the battery to the ends/connection points. They passed the tests. Does this mean the fuel pump is bad, or could it still be a wiring problem?

Also let me know if I should post pictures/if that'd help.

 

you earlier said ...
?

 

To clear that up: I used a jumper box, so that's 12 volts guaranteed. When I turn the key to the "on" position, I only read about 2.5-3 volts at the connections. How much voltage should be running back there/ is it possible that somehow the fuel pump is gummed up or something and less than 12 volts won't run it?

 

the fuel pumps are only powered when the motor is actually cranking over, not when the ignition is turned on. there should be 12V across the wires to the pump at the pump when its powered..

 

Alrighty, went outside, hooked v/o meter up to the connections (after first testing/confirming that the battery itself gave 12 volts ((11.82 specifically))), cranked engine and it read crazy negative numbers (like in the 100s). Switched wires, gave crazy positive numbers (again in the hundreds, and constantly varying). What does this mean?

Thank you so much for your help/patience so far! I can't tell you how much I appreciate it!

 

11.8 is pretty close to a dead battery, you should put it on a battery charger overnight before doing any more cranking.

If your volt meter is the typical digital volt meter with autoranging scales, you should lock the range down to a 19.99V kind of range so it doesn't bounce wildly, odds are you were seeing 100s of millivolts rather than 100s of volts. while cranking the battery voltage can drop to like 9V even with a fully charged battery as the starter is pulling a huge current load off it.