Rough idle + 7MPG

[Chip682]

This problem has ben hagging me out for quite a while now. I have a 92 XJ cherokee with the 4.0 I have been getting about 6-7 MPG which is terrible even for a jeep. Also when idleing it runs slightly rough almost seems like its getting too much or not enough fuel. As soon as the gas is pushed it runs good exept for the mileage. There is no CEL on either. I was thinking maybe an O2 sensor or fuel pressure regulator? Any input will be appriciated

 

[SteelHeadz]

I would start with the Throttle body. Give it a good bench cleaning from all the carbon build up. The IAC particularly is notorious for idle fluctuations, so make sure that you clean and check it really good.. If the issue still exist , clean the injectors by adding fuel injector cleaner with your fuel when you fill up at the gas pump..With the lack of the SES lamp , i doubt that it would be an O2 sensor but i could be wrong. A rough idle may also be a misfire in the cylinders at idle but then again, that would set off a SES lamp for a P0300 DTC. A fuel pressure issue can be checked with a Fuel pressure tester if you have access to one. with this tool, you can check the fuel pressure at idle, WOT and test the fuel pressure regulator for internal leak down after the engine has been shut down.

 

[Chip682]

Thanks man i will try cleaning the IAC and throttle body and see if that works.

 

[mud4feet]

Yeah, I was thinking the O2 sensors might have something to do with it.....maybe not entirely, but contributing. However, if they're that bad, they should throw a code. Have you pulled your plugs and taken a look at them? Could yield some clues.

my $.02,
mud

 

[Chip682]

Yeah the plugs/wires/filters/cap/rotor and such is all new. maybe a fuel pressure issue? gonna try cleaning the IAC and see if that helps

 

[TerryMason]

Are you sure that you got the plug wires in the correct sequence when you replaced them?

 

[Chip682]

Yeah they are i checked it by the repair manual.

 

[Chip682]

I have cleaned the IAC and throttle bodyto no avail still. i also tried unplugging the O2 sensor and it ran even worse. Everything seems to check out with testing sensors and whatnot. The only thing i havent checked is fuel pressure cuz i cant afford to buy a gauge right now. no one close by will rent one to me. I am still lost

 

[SteelHeadz]

I have cleaned the IAC and throttle bodyto no avail still. i also tried unplugging the O2 sensor and it ran even worse. Everything seems to check out with testing sensors and whatnot. The only thing i havent checked is fuel pressure cuz i cant afford to buy a gauge right now. no one close by will rent one to me. I am still lost

Another cause for a rough idle is a dirty EGR valve and port. Also make sure that you have no vacuum leaks on the manifold and lines.

 

[Chip682]

I dont think i have an EGR on my Heep

 

[TerryMason]

Maybe you knocked the timing off when changing the rotor / cap?

 

[Chip682]

I broke down and got a fuel pressure gauge. Turns out its losing pressure after being turned off. It drops off all the way to zero. That means the fuel pump is bad i think.

 

[SteelHeadz]

I broke down and got a fuel pressure gauge. Turns out its losing pressure after being turned off. It drops off all the way to zero. That means the fuel pump is bad i think.

Yup! that would do it. What was the pressure @ idle?. 2 things that would cause a leakdown. Fuel pressure regulator or the o-rings on the fuel pump module.

 

[Chip682]

Pressure at idle was about 31 psi. I checked the regulator by unplugin the vaccum to it and it jumped up about 10 psi which is right by the book. So that is good so its probly the pump

 

[SteelHeadz]

Yes, it's gonna be the fuel pump module..Good Job..

 

[Chip682]

Finally some results!!! I replaced the fuel pump today and EUREKA!! it runs like a champ. Much praise to the guy who designed the tank so you dont have to remove it to get to the pump. Also thanks to you SteelHeadz for helpin me out you are a great resourse, thanks again.

 

[SteelHeadz]

Your Welcome.