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What would cause a fuel pump to "jam"?

Grandpa

NAXJA Forum User
Location
Atlanta
It's an '87 Cherokee. The fuel pump, filter, and tank and had recently been replaced by the PO. I drove it home yesterday and parked it. When I came back out to test out the new coolant and oil pressure sensors I had swapped in (recently did a gauge cluster swap from idiot lights to actual gauges), the cherokee wouldn't start. It turned over great, but the fuel pump wasn't priming. It started hailing, so I left it alone over night.

This morning I measured voltage at the pump, and it was receiving ~12V during priming and around 10.5V when cranking. So I pulled the pump to replace it.

Just for fun, I directly wired the pump to the battery to see if it was really, really dead. It ran! I ran it both directions, and it runs fine (slower in reverse, faster when wired correctly). I hooked it back up to the connector in the rear, just out of the tank, and it runs great now. The heck?

We're talking maybe 1000 miles on the tank, filter, and pump. I can't figure out what managed to clog it. Or jam it? Or whatever it. Could it be bad gas? Maybe just a fluke?
 
Leaning torwards dirty gas you probably had something clogged in it and when you went to pull it, it came out. Not good to run the pump long without pumping anything or you will burn it up. Other than that i love it when faults fix themselves lol.
 
XJ Stryker said:
Leaning torwards dirty gas you probably had something clogged in it and when you went to pull it, it came out. Not good to run the pump long without pumping anything or you will burn it up. Other than that i love it when faults fix themselves lol.

I love that too, but I typically file such things under the too-good-to-be-true category. I'll have to keep an eye on it for the next couple of weeks - and avoid that Chevron I just went to.

I never ran it for more than a couple of seconds. I figured if the pump doesn't like being run with a near empty tank, it definitely wouldn't like being run with no gas at all. :cheers:
 
Any tiny piece of dirt is a possibility. The bad thing is that while removing the pump you probably dropped that tiny piece of dirt back into the tank.
 
It could also be a mechanical failure in the pump itself. Electric fuel pumps are typically "vane pumps," using an offset rotor with several springloaded vanes in it (creating chambers of varying volume. http://en.wikipedia.org/wiki/Vane_pump)

If a vane wears down far enough, it could actually come loose when it's supposed to be a "max volume" - meaning that the vane falls out of the slot and the spring can now be free. The spring jams between the rotor and the housing, and the pump will then stop turning due to mechanical interference.

Vane pumps are usually pretty forgiving of dirt and contaminants, provided the intake is sized and strained properly (that's what the "sock filter" is for.) Most vane pump failures I've seen have been due to vane wear and spring interference. This is why lubricating oil pumps tend to be of the "gerotor" type - fewer moving parts, and more forgiving of wear in general (since they run in an oil bath, wear is minimised anyhow.)
 
Hi guy,

I wanted to resurrect this thread because I'm having a very similar issue. I have an 87 Cherokee and the original pump died. Turns out the original tank was very rusty, so I replace the tank, pump, and filter. I also wired the pump directly with a switch because I had some issues with the harness. Anyway, the new pump will run for around 10 minutes then just stop. It is getting 12 volts, but isn't doing anything.....no noise at all. Yesterday I pulled the pump and it would run outside of the tank, I made no changes at all, then reinstalled it. Hit the key and could hear the pump run immediatly and the Jeep starts right up. After driving around for 5-10 minutes the Jeep dies and the pump isn't running at all. Any ideas? Could it be over heating due to low fuel? If so why whould removing then reinstalling cause it to start working again? Any input is appreciated.

Thanks,
Jeff
 
It would be nice to check the voltage to the pump in the "jammed" condition to see if it actually has any. I realize that's not very easy at the time, but a temporary volt meter set up to do it could be ghetto fabbed to know about it instantly.

I just get this feeling something warms up and a hot short/open is happening, like in the ignition. Engine dies, pump has the line pressurized, no pumping.
 
TiRod said:
It would be nice to check the voltage to the pump in the "jammed" condition to see if it actually has any. I realize that's not very easy at the time, but a temporary volt meter set up to do it could be ghetto fabbed to know about it instantly.

I just get this feeling something warms up and a hot short/open is happening, like in the ignition. Engine dies, pump has the line pressurized, no pumping.

Good idea. I'll try and rig some leads to do this and watch the volttage while driving until it dies.
 
cotton said:
Good idea. I'll try and rig some leads to do this and watch the volttage while driving until it dies.


If you take small paper clips, you can straighten about half of them and use them as "backprobe pins" to check voltage in the connector while it's still connected...
 
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