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  #1  
Old June 11th, 2006, 13:45
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blaisetd blaisetd is offline
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Low RPM stumble/poor idle

Yes, i have done extensive searching. I'm having low RPM stumble and poor idle (stalls also). I am also hearing what I think is backfire through the intake at certain RPMs. I have a 1990 DD XJ with a 4.0 and AW4. It has a Rusty's intake, MSD ignition box and a flowmaster. The current mileage is 192K. I rebuilt the engine around 176K. With the rebuild i replaced fuel injectors, oil pump hoses, water pump etc. Replaced CPS recently/before current problem started. Since problem started Iíve replaced fuel filter, cap, wires, plugs, battery, adjusted TPS to 82% input/output (was 77%), checked push rods and gave the valve springs the once over and did a compression check. Compression was between 145 and 155lbs on all cylinders. Do good compression test numbers definitely rule out any valve problems? Unplugging EGR vac lines produces no change in idle. Haven't come up with any vac leaks either. Not sure if its related, but the fuel gauge started reading incorrectly around when my problems started. Anyone got any suggestions, Iím about to throw in the towel and bring it to the stealership.
Thanks in advance blaise
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Old June 11th, 2006, 13:54
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Re: Low RPM stumble/poor idle

have u checked your fuel pressure?
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  #3  
Old June 11th, 2006, 13:59
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Re: Low RPM stumble/poor idle

i will check the fuel pressure. thanks
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Old June 11th, 2006, 17:49
jeepr jeepr is offline
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Re: Low RPM stumble/poor idle

And fuel filter.

I had a bad plug once (right out of the box) would do the same thing. Because the plugs were new, it was the last thing I checked. Get one of those laser temp gauges and check each exhaust at the head and see if one is colder. (or you can do it with a spray water bottle if you cant find a temp gauge).
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Old June 11th, 2006, 20:06
4xBob 4xBob is offline
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Re: Low RPM stumble/poor idle

You didn't mention the oxygen sensor- when mine went, the engine stumbled at idle and gas milege declined.
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  #6  
Old June 11th, 2006, 22:46
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Re: Low RPM stumble/poor idle

Hello again. Changed the O2 a few months back. The fuel filter was one of the first things i changed when the problem started. Been reading my Haynes here at work, i'm gonna look more into the EGR and my fuel pressure in the A.M. I'm want to hold off on bringing it to the stealership....but it is frustrating. It will be definately worth it if i find out its the EGR. Does anybody think the fuel gauge issue is related a fuel pump going out? Thanks for the help so far. I will post the solution...if i find it. Thanks for the help. I feel a membership comming on. blaise
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Old June 11th, 2006, 23:59
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Re: Low RPM stumble/poor idle

fuel gauge - u might want to check your ground on your gas tank.

and check your ISC.
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  #8  
Old June 12th, 2006, 05:52
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Re: Low RPM stumble/poor idle

Update: did EGR solenoid and valve check according to Haynes this morning. The diaphragm on the valve itself moves fine. Did the vacuum checks on the solenoid itself and got all the correct readings. On the last step the Haynes manual states that the EGR valve can still be bad if the solenoid checks ok. WTH?? How is it checked then? Roll the dice and drop $123.99 on a new one? Anyway I noticed also that the stumble only started after she warmed up/not on the initial start up. Jeepdude what is the ISC? Anyone know if the good compression numbers clear me of a valve problem? blaise
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Old June 12th, 2006, 09:28
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Re: Low RPM stumble/poor idle

ISC - is idle air valve near TPS(throttle posision sensor).
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  #10  
Old June 12th, 2006, 09:31
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Re: Low RPM stumble/poor idle

I removed it and cleaned it with contact cleaner and a wire brush. How do you check it?
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  #11  
Old June 12th, 2006, 09:45
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Re: Low RPM stumble/poor idle

i checked mine by unscrewing it and pull it out slowly when jeep ruuning, it should pulse out to compensate for u pulling it out and keep it running. You only need to pull it out 1/2" other wise it could come apart. u could even leave the screws in so it does not fall apart. and to put back in just put back in slowly. this ISC runs slow.
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  #12  
Old June 22nd, 2006, 05:42
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Re: Low RPM stumble/poor idle

Update:
Haven't had much time to troubleshoot lately. Waiting for a fuel pressure tester comming in the mail. Checked the IAC, good. Can a bad break booster be causing this? I noticed my my break pedal went to the floor when i was moving cars around the other day..... blaise
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  #13  
Old June 22nd, 2006, 07:55
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Re: Low RPM stumble/poor idle

Do u have ABS brakes, and did u due a vacuum check?
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  #14  
Old June 22nd, 2006, 08:49
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Re: Low RPM stumble/poor idle

Quote:
Originally Posted by blaisetd
Update:
Haven't had much time to troubleshoot lately. Waiting for a fuel pressure tester comming in the mail. Checked the IAC, good. Can a bad break booster be causing this? I noticed my my break pedal went to the floor when i was moving cars around the other day..... blaise
a leaking brake booster can cause the equivalent of a large vaccum leak. Pull the hose going to the brake booster and plug it to see if it makes a difference.
A failed brake booster should not have any impact on the pedal going to the floor. If anything a failed booster would make the pedal a lot harder to push. The brake booster fails "safe" so if it fails on the road you don't loose your brakes.
Of course if you have the Bendix ABS system everything I just typed is not relevant
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  #15  
Old June 22nd, 2006, 15:54
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Re: Low RPM stumble/poor idle

Non ABS truck. I have been looking/listening for vacuum leaks. Not found any...yet. Got my fuel pressure tester today, going to check that and the break booster.... thanks for the help so far. blaise
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