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  #1  
Old November 15th, 2009, 11:00
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xj88superjeep xj88superjeep is offline
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Fuel pressure question....

My 97 is running lean and idling like crap and guzzling gas since I put on the new exhaust... I rented a fuel pressure gauge and tested it so can anyone tell me if the needle should be fluttering between 39-49 PSI at idle? By fluttering, I don't mean fluctuating, I mean fluttering like an insect wing so fast you can hardly see the needle...

Starting out, I purged the fuel into a gas can and started at zero PSI.

Key on no start = 10 PSI

Key off = Stayed at 10 PSI

Start/Idle = 39-49 PSI (fluttering)
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  #2  
Old November 15th, 2009, 11:07
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Re: Fuel pressure question....

When I bring the RPMs up, the needle settles in at 45 PSI, but as soon as it drops to idle, the needle is fluttering as stated above...
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Old November 15th, 2009, 11:34
JJacobs JJacobs is offline
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Re: Fuel pressure question....

What exactly are you testing for? You say poor fuel mileage and running lean, those don't go together. If the fuel pressure stays in spec at full throttle driving then the pump and filter are fine. 97 is a returnless system with the regulator all the way back in the tank so you can see some needle fluctuating depending on the gauge type.
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Old November 15th, 2009, 12:54
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xj88superjeep xj88superjeep is offline
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Re: Fuel pressure question....

I understand what you are saying. Makes no sense to me either... CEL tells me it is running in constant lean condition... gas milage is like 50%... Rough idle when in closed loop... exhaust stinks... I was testing fuel pressure to try to find if that was causing the rough idle and / or making the computer over-compensate the timing or something similar causing the code. I'm basically hunting for a gremlin that was hiding until I just put the new exhaust on. If the fuel pressure is ok jumping like that, then it just passed that test, and I will move on to the up-stream O2 sensor.

EDIT: The gauge is an expensive loaner, not a cheap harbor freight knock off.

Last edited by xj88superjeep; November 15th, 2009 at 12:58.
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  #5  
Old November 15th, 2009, 12:58
JJacobs JJacobs is offline
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Re: Fuel pressure question....

So it has lean codes stored in the computer then?
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Old November 15th, 2009, 13:11
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Re: Fuel pressure question....

Yes...
PO171 to be exact...

Says either O2 sensor bad, Ignition misfire, or fuel injector problem...

I was checking fuel pressure because I know all plugs are firing, and this was a free test compared to a 60 dollar O2 sensor. (I already threw a downstream sensor that I bought with the new exhaust and a new MAP sensor at it so I didn't want to plunk down another 60 clams if not needed...) (Looks like it's needed)
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  #7  
Old November 15th, 2009, 14:47
joe_peters joe_peters is offline
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Re: Fuel pressure question....

Look, something is amiss in the fuel system. You should read 49 psi with the engine running, NOT a fluctuating pressure. If the pressure is dropping then the injectors can't do their job--deliver enough fuel to the engine.

Repair the fuel pressure problem and your codes should go away.

Why your mileage is in the crapper with a lean condition is that the PCM is trying to compensate by increasing duration of the injectors' firing.
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  #8  
Old November 15th, 2009, 15:29
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Re: Fuel pressure question....

Thanks Joe, I have fuel pump sitting around here because the sending unit has been bad since I got it and I picked up a used one before I knew it was a factory recall. I am going to swap out the fuel pump tomorrow and see where that road leads. I also picked up the o2 sensor, but will return it if the pump swap fixes the issue.
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Old November 15th, 2009, 16:33
JJacobs JJacobs is offline
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Re: Fuel pressure question....

Quote:
Originally Posted by joe_peters View Post
Look, something is amiss in the fuel system. You should read 49 psi with the engine running, NOT a fluctuating pressure. If the pressure is dropping then the injectors can't do their job--deliver enough fuel to the engine.

Repair the fuel pressure problem and your codes should go away.

Why your mileage is in the crapper with a lean condition is that the PCM is trying to compensate by increasing duration of the injectors' firing.
Read it again Joe, it's not dropping from what I read.
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Old November 15th, 2009, 17:02
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Re: Fuel pressure question....

Well, if it is supposed to be a costant 49PSI at the rail at idle, then it is dropping. It is fluctuating between 39-49. It only gets a steady pressure of 45 if I hold the throttle open. as soon as it drops to idle, the needle buzzes between 39-49 again. If this constant drop to 39 PSI every time I let off the gas is throwing the computer into a state of compensation then it very well could be the problem. Either way, it's been crying for a new pump/sending unit anyway, so we will find out tomorrow if it makes a difference.
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  #11  
Old November 16th, 2009, 03:50
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Re: Fuel pressure question....

it is suppose to be 49 psi +or- 5psi all the time. i would say you have a weak pump especially at idle. mike
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Old November 26th, 2009, 20:50
The Weez The Weez is offline
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Re: Fuel pressure question....

I am searching for the same monster in my 97. Fuel pressure at idle fluxates between low 30's and upper 40's as quick as you can move your finger. Once I throttle up the rpm's, the pressure stabalizes.

When driving, my jeep hesitates and stutters for the first 15 minutes of mixed driving then begins to drive normal. I am kicking a CE code for the O2 sensor bank 1 although both sensors were just replaced and it is blowing the 15 amp O2 fuse. My mpg is also bad.

Needless to say, I'm in the same spot trying to make the same decision. Should I replace fuel pump and fuel filter? Jeep got 226,000 miles on it. Like the original post, I appreciate some input.
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  #13  
Old November 26th, 2009, 21:14
joe_peters joe_peters is offline
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Re: Fuel pressure question....

Well, both are in the tank.

The system is designed for 49 psi constant.

The usual failure mode for a pressure regulator is HIGH pressure, on a renix that can hit 95 psi (30/31 with vacuum to the regulator, 39 psi w/o vacuum applied) just by pinching off the return line.

Whether you should replace the entire pump assembly is up to you, but as you have to drop the tank to do it, and have 226K miles on it, I would recommend it.
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Old November 26th, 2009, 21:16
joe_peters joe_peters is offline
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Re: Fuel pressure question....

Quote:
Originally Posted by xj88superjeep View Post
Well, if it is supposed to be a costant 49PSI at the rail at idle, then it is dropping. It is fluctuating between 39-49. It only gets a steady pressure of 45 if I hold the throttle open. as soon as it drops to idle, the needle buzzes between 39-49 again. If this constant drop to 39 PSI every time I let off the gas is throwing the computer into a state of compensation then it very well could be the problem. Either way, it's been crying for a new pump/sending unit anyway, so we will find out tomorrow if it makes a difference.
Happy ending?
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Old November 26th, 2009, 23:02
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Re: Fuel pressure question....

CEL codes, poor gas mileage, engine bucking (blown fuses), hmmmm, they all point to a bad front O2 sensor or damaged wires/loose connectors to the front O2 sensor.


In general, the fuel pump does not just go bad in a day or two. It is one on the least likely suspect for the given symptoms. Fluctuating fuel pressure should be expected in a non-return fuel system. The pump has to cycle on and off to deliver fuel. The fuel pump in question may not be as good as a new pump, but if you are finding 49 psi +- 5psi at higher rpms, I wouldn't worry about the idle pressure readings.
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Last edited by Tim_MN; November 26th, 2009 at 23:08.
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