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Unread 07-14-2015, 07:38 PM   #1
Paul80452
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1995 XJ Cherokee 
 
Join Date: Feb 2014
Location: Idaho Springs, Colorado
Posts: 10
Fuel pump noise and rough idle only after running for long periods

Hi,

I did a search, but couldn't find anything with these particular symptoms.

I have a 91 XJ 4.0 and the fuel pump has always been noisy after driving for awhile (45+ miles). I replaced the pump long ago and it did not fix the noise problem.

When I drive it long distances the pump will eventually begin to make noise. It doesn't necessarily cause any problems, but if I drive it even further, especially in hot weather it begins to idle roughly as well. Lastly, in hot weather over long distances the gas tank develops a pressure indifference (I'm not 100% positive whether it is pressurized or pulling a vacuum, but I think it is pressurized). I have tried a new gas cap and it doesn't seem to help the pressure indifference.

I've read about ballast resistors and pump noise, and it is quite possible that I bought a crappy pump when I replaced it, but is it possible that this is something to do with the mounting bracket in the tank or the venting system?

I made sure that the rubber end caps were on the pump when I installed it.

If you have any ideas thanks in advance.

Paul

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Unread 07-16-2015, 05:21 PM   #2
cruiser54
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1990 MJ Comanche 
 
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Quote:
Originally Posted by Paul80452 View Post
Hi,

I did a search, but couldn't find anything with these particular symptoms.

I have a 91 XJ 4.0 and the fuel pump has always been noisy after driving for awhile (45+ miles). I replaced the pump long ago and it did not fix the noise problem.

When I drive it long distances the pump will eventually begin to make noise. It doesn't necessarily cause any problems, but if I drive it even further, especially in hot weather it begins to idle roughly as well. Lastly, in hot weather over long distances the gas tank develops a pressure indifference (I'm not 100% positive whether it is pressurized or pulling a vacuum, but I think it is pressurized). I have tried a new gas cap and it doesn't seem to help the pressure indifference.

I've read about ballast resistors and pump noise, and it is quite possible that I bought a crappy pump when I replaced it, but is it possible that this is something to do with the mounting bracket in the tank or the venting system?

I made sure that the rubber end caps were on the pump when I installed it.

If you have any ideas thanks in advance.

Paul
Was the brand Airtex? Bypass the ballast resistor and see what happens just for fun.

Or, test the output from the ballast resistor while it's running. Oughta be 9 or 10 volts.
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Unread 07-17-2015, 12:22 PM   #3
Shortyyyyyy
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1988 XJ Cherokee 
 
Join Date: Feb 2009
Location: Ocean View, Va
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a cap that isn't venting pressure/vacuum can give you the problems you are having-- next time it's running like poop, pull the cap off and see if it begins idling better in a minute or two. The noise you are getting may also be from a pump that is working too hard to pull fuel from a tank that already has increased vacuum from the fuel used-- not the first thought, but always a possibility....
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Unread 07-21-2015, 06:22 PM   #4
Paul80452
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1995 XJ Cherokee 
 
Join Date: Feb 2014
Location: Idaho Springs, Colorado
Posts: 10
Thank you for the replies so far. I have switched the caps as well, because I know that they are supposed to vent at a certain pressure, but switching didn't make a difference.

I have a back-up balast resistor so maybe I'll change it out. I'll also try to run it with the cap off and see what that does.

In reality, I'm hoping to sell it this week (with the issues full disclosed), so perhaps I'll be passing this info on to the new owner.
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Unread 07-21-2015, 06:23 PM   #5
Paul80452
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1995 XJ Cherokee 
 
Join Date: Feb 2014
Location: Idaho Springs, Colorado
Posts: 10
Lastly, I did find the box for the fuel pump that I switched it with. I couldn't find an actual manufacturer on the Napa box.
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