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I reolaced fuel pump, will not run


 
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rodneyjt24
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Joined: 19 Jul 2005

PostPosted: Tue Jul 19, 2005 4:33 pm    Post subject: I reolaced fuel pump, will not run Reply with quote

Hello, everyone. I have a 1988 cherokee laredo, 4.0 straight six. The fuel pump was beginning to whine all the time so I replaced it. Now it will start and run for a few seconds and then shut off. The only mistake that I made was not getting the polarity correct on the pump. It is correct now. The pump does run and there is some pressure in the rail, but it will not stay running. It was not acting this way before the replacement. Should the pump run all the time? Can anyone suggest a fix for my problem?
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ThePhantum
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PostPosted: Tue Jul 19, 2005 5:35 pm    Post subject: Reply with quote

OK, when you turn the key on the fuel pump will only run until the pressure comes up. Once pressure has been established, the pump will cut off. Now if the Jeep will not stay running, the first thing to check is pressure at the rail. It should be between 30-40psi. If it's low, then you have a bad regulator or the replacement pump is bad.
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rodneyjt24
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Joined: 19 Jul 2005

PostPosted: Tue Jul 19, 2005 6:57 pm    Post subject: fuel pump Reply with quote

Ok. I am thinking that the regulator is bad because the old pump used to run all the time. And from what I can tell the new pump runs constantly during the short period that the engine runs.
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ThePhantum
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PostPosted: Tue Jul 19, 2005 8:51 pm    Post subject: Reply with quote

To test the regulator, put a gauge on the rail and turn the key to run (don't start the engine). The pump should kick on, bring pressure up and cut off. This should all happen within a few seconds. Then turn the key off. The pressure should stay up for at least 5 minutes. If it drops off before then, the regulator is bad. If the pressure comes up slowly and when the pump cuts off pressure immediatley drops, again, it's the regulator.

If the pressure does not come up high enough (and/or the pump runs continously), it could still be the regulator...but the new fuel pump might be faulty as well. Low pressure could also be a kinked fuel line someplace or a clogged pickup or filter.

I know spec for pressure on my 97 is 49 psi. I think on older 4.0's (pre-HO) the pressure is a little lower.
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rodneyjt24
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Joined: 19 Jul 2005

PostPosted: Tue Jul 19, 2005 10:58 pm    Post subject: fuel pump Reply with quote

I will try this in the morning. I have a new regulator just in case. By the way, I have never heard the pump come on unless the engine was cranking. I will have to look into that also. Maybe a relay?
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rodneyjt24
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PostPosted: Thu Jul 21, 2005 1:05 am    Post subject: fuel pump Reply with quote

Ok, I think I found it. The new fuel pump was faulty. The regulator works just fine. While I was at it I installed a new TPS. The engine will start and run but it idles at about 2K. So I will try the old TPS and all should be back to normal.
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