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Unread 08-26-2010, 07:44 PM   #1
AdvisingElf
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1999 Grand Cherokee idle problem - odd

My brother-in-law bought my daughter a 1999 Grand Cherokee as a high school graduation present. It seems in great shape, but it has an odd idling issue.

When the gas tank is above half-full, it starts and idles fine. At about the half full mark you have to give it a little gas to start, and you have to keep giving it a little gas or it'll die.

I've heard that you can have problems with some cars if you don't fully crank the gas cap down, but this is just hearsay. Does the fuel pump pressurize the gas tank? It would have to work harder to pressurize a tank with more airspace in it. This is just a WAG. Anyone have any ideas?

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Unread 09-02-2010, 06:42 PM   #2
AdvisingElf
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Is there anybody out there?
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Unread 09-02-2010, 07:38 PM   #3
billzcat1
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There may be a problem at the tank where the fuel pump draws in fuel. I had a similar problem years ago on a vintage car and it was a fuel pickup problem due to a rusty tank. It's unlikely that your tank is rusty (since its plastic) but it may have dirt and grit in there.

The fuel tank doesn't get pressurized by the pump, but there are vacuum/emissions components drawing vapors from the intake and fuel system. If your cap is defective, it can cause a check engine light but usually won't affect running. Regardless, the cap is like $10 and they do commonly fail. It can't hurt to replace it.

WAG = Wild-*** Guess?
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Unread 09-02-2010, 08:31 PM   #4
WiCkeDuDe
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Fuel pump is dying and has a hard time when the tank is less than half full or the strainer is plugged up on the pump. Check your fuel pressure.
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Unread 09-02-2010, 11:04 PM   #5
warrenpeace24
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If I remember right Jeep has a TSB on this I think it was a o-ring on the fuel pump that is easily damaged
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Unread 09-02-2010, 11:41 PM   #6
U2Fans
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Make sure all of the vacuum lines are in place and tight... I had an idle problem on my '99 and it was due to a loose/disconnected vacuum line. It also was the culprit for my cruise control not working problem.
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Unread 09-03-2010, 08:19 AM   #7
AdvisingElf
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WAG = Wild-*** Guess?
Correct.
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Unread 09-03-2010, 08:19 AM   #8
AdvisingElf
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Originally Posted by WiCkeDuDe View Post
Fuel pump is dying and has a hard time when the tank is less than half full or the strainer is plugged up on the pump. Check your fuel pressure.
Thanks.
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Unread 09-03-2010, 09:47 AM   #9
rm2001wj
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Quote:
Originally Posted by AdvisingElf View Post
My brother-in-law bought my daughter a 1999 Grand Cherokee as a high school graduation present. It seems in great shape, but it has an odd idling issue.

When the gas tank is above half-full, it starts and idles fine. At about the half full mark you have to give it a little gas to start, and you have to keep giving it a little gas or it'll die.

I've heard that you can have problems with some cars if you don't fully crank the gas cap down, but this is just hearsay. Does the fuel pump pressurize the gas tank? It would have to work harder to pressurize a tank with more airspace in it. This is just a WAG. Anyone have any ideas?
With fuel injected engines, "giving it a little gas" is actually giving it a little air. Pressing on the accelerator pedal open up the throttle so that more air goes into the engine, and the engine computer controls the injectors which put in the gas. Bottom line is that you have an air problem, not a gas problem as far as the idle is concerned. Most likely the Idle Air Control (IAC) needs cleaning or replacement. Next most likely would be a worn Throttle Position Sensor (TPS). The connection with the fuel level in the tank is only a coincidence.
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Unread 09-03-2010, 11:19 AM   #10
AdvisingElf
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Originally Posted by rm2001wj View Post
With fuel injected engines, "giving it a little gas" is actually giving it a little air. Pressing on the accelerator pedal open up the throttle so that more air goes into the engine, and the engine computer controls the injectors which put in the gas. Bottom line is that you have an air problem, not a gas problem as far as the idle is concerned. Most likely the Idle Air Control (IAC) needs cleaning or replacement. Next most likely would be a worn Throttle Position Sensor (TPS). The connection with the fuel level in the tank is only a coincidence.
In searching for info on this problem the IAC and TPS came up. I will service those, but I doubt the fuel level thing is a coincidence. We've run it a while and it starts having trouble idling at about the half empty level, and it goes away as soon as you fill the tank. I actually hope you are right, because I don't want to have to pay to get the fuel pump replaced.
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Unread 09-03-2010, 11:44 AM   #11
rm2001wj
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If there was a fuel pump problem, it have to show up under conditions other than when the engine is idling. Idling requires less fuel and less effort on the part of the fuel pump than anything else, except possibly rolling down hill with no pressure on the accelerator pedal.
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Unread 04-24-2013, 12:41 PM   #12
scottphxaz1
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FYI: Today, my 99 jgc 4.0 would only start with my foot on the gas pedal, let off to idle and it would stall out. It ran perfect with my foot on the pedal. I opened the hood, wiggled the wires to the ECM, and it started and ran fine. fingers crossed
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