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Old 07-15-2012, 10:05 AM
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Angry 2003 jeep grand cherokee runs rough

Howdy guys,
I just got a 2003 Jeep grand cherokee 4.7 4WD for my wife to throw miles on for work. I have been getting it ready for service, and now have a problem.
I did a half-crank the other day, (cranked but didnt get it started the first time) and ever since then, it does not want to crank on the first try, when it does crank it runs rough, then if you give it gas it bogs down even more. Eventually after 10 seconds it will allow you to give it enough gas to idle normal, but it wants to bog when you start moving, then that smooths out. It starts getting a lot worse mileage, and if you suddenly need power (jam on the gas) it literally does nothing. If you jam it all the way, it will start shuddering. I had the guy at orielly's use the diagnostic machine on it and it said both banks were running lean. (he suggested o2 sensors)

I did change the fuel filter/ boost pump assembly when I got it because I heard a loud hissing coming from the air line that goes into it. I figured the connection was bad. Turns out it sounds the same on the new one. The hose looks fine, but still hisses. I have heard that this could lead to a rough running engine. Figure Ill crank it and spray it with brake kleen to see if it affects it.
Any Ideas?
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Old 07-15-2012, 04:22 PM
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fuel pump?

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Old 07-15-2012, 06:34 PM
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Re: 2003 jeep grand cherokee runs rough

The engine vacuum is used to lower the pressure in the tank, to prevent gas vapours to escape. If you hear something hissing, then there is some kind of leak. Remove the fuel cap and see if the hissing stops.
The fuel pump has a strainer under it, but the filter/regulator is outside/in front of the tank. To see if you have a fuel pressure problem, just press the schrader valve on the right hand bank fuel rail (ignition off and engine cold, to prevent fire). There should come fuel under pressure out of this valve. If not, change first the filter/regulator and check again. Best is to put a fuel pressure indicator on the rail to check if you get around 45 PSI (when i'm not wrong).
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Old 07-15-2012, 09:53 PM
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Re: 2003 jeep grand cherokee runs rough

This was literally 1 turn of the key to the next, so I would be leary of it being the fuel pump, but I will still keep it on the list.
Since I posted, I found a pretty serious vacuum leak near the tank. I cut the effected area off and reattached it to the steel line... hiss disappeared. I thought this was the problem, but alas.... the fight continues. Idles rough and it stalls whenever you push the gas over 1/2way down. I will try the schrader valve to see if it has pressure with the engine off. I guess I should get a fuel pressure tester if its not too much. Can you get them at Oriellys? or ABC auto parts? I've seen some things that might suggest a PCV valve? Thoughts?
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Old 07-16-2012, 12:56 AM
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Re: 2003 jeep grand cherokee runs rough

Not sure if its worth to spend money on a fuel pressure gauge, you will probably use it only once. If fuel is coming out of the schrader valve under reasonable pressure, i would not bother too much anymore.
If you don't know the service history of this jeep, i would also change the air inlet filter and have a look at the plugs.
I don't think that the PCV valve could cause this problem. I have run my jeep with the PCV line disconnected and open and also with the line closed, but it will decrease the idle until the idle air control steper motor compensates for the extra air or less air and then it runs perfectly normal again.
The O2 sensors mentioned by the mechanic could be the culprit, but it would be strange if the O2 sensors of both banks have the same problem at the same time.
I would start with the fuel filter/regulator and check again.
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Old 07-20-2012, 06:00 PM
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Re: 2003 jeep grand cherokee runs rough

Howdy,
Just got home off the road. Jeep has been sitting for 5 days. I hit the schrader valve. No pressure. I am guessing my filter/regulator is bad. It was from Oreillys. I have heard of one instance that someone put an aftermarket filter on that was identical, but it was not a regulator, so they had to get an OE unit. But it worked for a few days, so maybe its just bad?
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Old 07-20-2012, 09:15 PM
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Re: 2003 jeep grand cherokee runs rough

Well... swapped the store brand filter with a Wix and whaddaya know... problem appears solved. Finally got all the power outta that sweet 4.7. If anyone else pops up with this, I will be sure to let them know to stay away from the cheap brands and go OE or Wix. I am thankful for all the help from you guys.
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Old 07-20-2012, 09:58 PM
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Re: 2003 jeep grand cherokee runs rough

Good to hear that it was a simple fix. I heard though that some other members had some strange noises coming from under the jeep after changing the filter/regulator with a Wix one.
But as long as it worked for you, its ok.
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