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Old 02-08-2009, 07:29 PM   #1
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intermittant power to fuel pump

Problem on my grand cherokee.
My fuel pump dies periodically, a few days later it'll start working again. I replaced the fuel pump but that didn't help. I checked power when it was working and got a good 12 volts. I check fuel pressure and it stays steady at about 32psi with my gauge.
When it doesn't work there's no power to the pump and obviously no pressure.
From what i understand the computer controls when the pump runs but as long as the engine is running the pump should be running.
Would it hurt to run an alternate power source from the ignition to an auxiliary switch to the pump so when I hit the ignition it sends power to a switch I can turn off and on.
Or can anybody think of a reason I would be losing power to the pump?

This thing has been driving me nuts.

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Old 02-08-2009, 07:35 PM   #2
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you need to know if you have spark and injector pulse when the pump power is lost. do not rewire the fuel pump. possible crank sensor problem or the like if you are losing more than just fuel pump power. if you are just losing fuel it may just be a relay.

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Old 02-08-2009, 07:49 PM   #3
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I swapped the relays around and still got the problem.

How would I check for spark and injector pulse?

I can dribble gas down the throttle body and it'll start so I figured it was just a fuel problem.
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Old 02-08-2009, 07:59 PM   #4
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Could a crank sensor problem cause surging at idle?
I get surging every once in awhile, usually when it's cold but sometimes when it's warm where it'll rev between 500 and 1100 rpm's
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Old 02-08-2009, 08:22 PM   #5
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i used to think crank sensor worked or didn't, run or not. but i've had a couple that caused weird driveablity for a short time, then no start, and not restart till it sat.

but if you poor gas in the throttle body and it runs, its getting spark. a spark tester would be better though, they are around 12 bux and its just a test light, but it has a spark plug boot on one end to fit on the plug, and the other end looks like a spark plug, the light flashes with the spark. you need to have nice constant flashing. there's special noid lights for pulse, but you can check for power pulsing with a regular test light also.
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