Fuel Pressure problem - JeepForum.com

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post #1 of 49 Old 07-08-2014, 07:32 AM Thread Starter
pesnemesis
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Fuel Pressure problem

1996 Grand Cherokee V8
I replace the fuel pump and when I turn the key to the on position I hear the pump run but does not stop until I turn the key off and on a few times then the pump will build pressure.
If I try to turn the engine on while I can hear the pump running or trying to build pressure the engine will start then shut off due to lack of fuel.
any ideas what can be going on?

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post #2 of 49 Old 07-08-2014, 07:47 AM
94cherry
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Did you change the fuel filter?

Kiss my shiny metal bumper.
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post #3 of 49 Old 07-08-2014, 09:20 AM
Uniblurb
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^ agree on changing out the fuel filter when installing a new pump. What brand of fuel pump did you buy and some are known to have problems? Did you replace the whole fuel pump module or just the pump itself? Sounds like you need to connect a fuel pressure tester to it to see if you're close to the 49psi +/-5psi spec. Advance and/or Autozone has loaner ones but if you're having it stall for lack of fuel it's probably evident it's starving of fuel.

Too bad I didn't know you were replacing the fuel pump since this brand new Bosch 67646 fuel pump module for your 96, which is OE and comes w/a new tank seal, with free shipping is only $120. And e-herko is a reputable vender others here use.

http://www.ebay.com/itm/NEW-BOSCH-67...2512b8&vxp=mtr

Another thing you may want to try is change your tank fuel filler tube cap to new. These caps have an internal one-way check valve which lets air in but not fumes out. If it isn't letting air in then too much vacuum is created in the tank from the pump running and it can even collapse a tank. And it may also be causing the fuel to not run as it's supposed to. Good luck.
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post #4 of 49 Old 07-08-2014, 10:00 AM
jm8881
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Quote:
Originally Posted by pesnemesis View Post
1996 Grand Cherokee V8
I replace the fuel pump and when I turn the key to the on position I hear the pump run but does not stop until I turn the key off and on a few times then the pump will build pressure.
If I try to turn the engine on while I can hear the pump running or trying to build pressure the engine will start then shut off due to lack of fuel.
any ideas what can be going on?
How can the engine stall due to lack of fuel if the pump doesn't turn off?
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post #5 of 49 Old 07-08-2014, 03:27 PM
dusterdude
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I would think low volume being delivered to the injectors
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post #6 of 49 Old 07-12-2014, 08:08 AM Thread Starter
pesnemesis
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Well I didn't change the fuel filter but I will try that today, I didn't change it because I couldn't see how that would make my pump lose pressure.
The pump I bought is an airtex and I'm finding out that they are junk, I did replace the complete fuel pump assembly. I also found out my fuel pressure regulator is part of the fuel pump and I think it's failing.
My gas cap is new.
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post #7 of 49 Old 07-12-2014, 08:10 AM Thread Starter
pesnemesis
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Quote:
Originally Posted by jm8881 View Post
How can the engine stall due to lack of fuel if the pump doesn't turn off?
there are times it will stay on but can't throttle the engine due to the lack of fuel pressure
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post #8 of 49 Old 07-12-2014, 09:15 AM
Uniblurb
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Quote:
Originally Posted by pesnemesis View Post
Well I didn't change the fuel filter but I will try that today, I didn't change it because I couldn't see how that would make my pump lose pressure.
The pump I bought is an airtex and I'm finding out that they are junk, I did replace the complete fuel pump assembly. I also found out my fuel pressure regulator is part of the fuel pump and I think it's failing.
My gas cap is new.
Curious if when you installed the new pump you cleaned the tank out? There was a TSB about these plastic tanks creating some type of fine black residue causing problems in addition to the TSB for the 96's in particular on the fuel pumps overheating/failing.

With a clogged fuel filter your pump could keep running but not be able to build up the pressure you need to keep the engine running. Would also have to agree with you on the check valve in the assembly may be bad if you can keep turning it on/off to build more pressure. If you ever get a fuel pressure tester it shouldn't drop below 24psi after about 10-20 min of sitting after shut-down for testing the check valve. As already mentioned spec is 49psi +/-5psi for good fuel pressure.
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post #9 of 49 Old 07-13-2014, 07:00 AM
ZeeJay1997
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post #10 of 49 Old 07-14-2014, 06:27 PM Thread Starter
pesnemesis
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I can hear the fuel pump running every time I turn the key to the on position.
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post #11 of 49 Old 07-14-2014, 06:31 PM
ZeeJay1997
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takes about two minutes to try a relay swap. relays sometimes get carbon on the contacts and produce low voltage. the fuel pump will run on 6 volts, but it doesnt run fast enough to produce the pressure needed.
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post #12 of 49 Old 07-14-2014, 06:36 PM Thread Starter
pesnemesis
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Quote:
Originally Posted by Uniblurb View Post
Curious if when you installed the new pump you cleaned the tank out? There was a TSB about these plastic tanks creating some type of fine black residue causing problems in addition to the TSB for the 96's in particular on the fuel pumps overheating/failing.

With a clogged fuel filter your pump could keep running but not be able to build up the pressure you need to keep the engine running. Would also have to agree with you on the check valve in the assembly may be bad if you can keep turning it on/off to build more pressure. If you ever get a fuel pressure tester it shouldn't drop below 24psi after about 10-20 min of sitting after shut-down for testing the check valve. As already mentioned spec is 49psi +/-5psi for good fuel pressure.
the tank was very clean inside and I still gave it a good cleaning.
One thing I didn't mention, The ZJ is drivable. the problem is only happening when I let it sit for a few hours.
I'll get a fuel pressure tester and see what readings I get.
Thanks!
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post #13 of 49 Old 07-15-2014, 03:15 AM
Uniblurb
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Originally Posted by pesnemesis View Post
the tank was very clean inside and I still gave it a good cleaning.
One thing I didn't mention, The ZJ is drivable. the problem is only happening when I let it sit for a few hours.
I'll get a fuel pressure tester and see what readings I get.
Thanks!
Good deal in cleaning the tank out! Still wouldn't be a bad idea to change out the fuel filter, they're cheap, especially after doing work to the fuel system/installing a new pump module. The fuel pressure tester will help in diagnosing a bad check valve in the pump unless you have some really bad leaking injectors or a fuel leak somewhere else. Typically you would know if you have a raw fuel leak.

Believe what ZeeJay is saying is your fuel pump relay may not be passing through enough voltage to run the pump fully even if you hear it turning/running. Make sure your horn works and then flip the relay between horn/fuel since they're both identical while making sure they both work again.
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post #14 of 49 Old 07-16-2014, 02:54 PM Thread Starter
pesnemesis
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Quote:
Originally Posted by Uniblurb View Post
Good deal in cleaning the tank out! Still wouldn't be a bad idea to change out the fuel filter, they're cheap, especially after doing work to the fuel system/installing a new pump module. The fuel pressure tester will help in diagnosing a bad check valve in the pump unless you have some really bad leaking injectors or a fuel leak somewhere else. Typically you would know if you have a raw fuel leak.

Believe what ZeeJay is saying is your fuel pump relay may not be passing through enough voltage to run the pump fully even if you hear it turning/running. Make sure your horn works and then flip the relay between horn/fuel since they're both identical while making sure they both work again.
I'll change the fuel filter for sure
great idea on the relay switch test. I will try it.
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post #15 of 49 Old 07-16-2014, 05:57 PM
Uniblurb
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Originally Posted by pesnemesis View Post
I'll change the fuel filter for sure
great idea on the relay switch test. I will try it.
Actually that's Zee's idea and he taught to flip the horn relay with the other identical relays to check them. But my horn doesn't sound quite the same as his below!

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