[Chip682]

Hey guys i have a 92 XJ 4.0 I have been through everything on it. Fresh tuneup with everything filters/cap/rotor/wires/plugs/etc. But for some reason it runs rough at idle and gets terrible gas mileage. I thought maybe an O2 sensor but it checks out. There is no check engine light on either. I also cleaned the IAC and throttle body today. Also checked for vaccum leaks as well. Only other thing i can think of is maybe a fuel issue. Any ideas will be apriciated. Thanks

[drvn93]

cps? clogged fuel injectors?

[boredcherokee4l]

When you cleaned the IAC you didn't push it like a plunger did you? Cause that causes them to fail which could cause it to run rough. I learned the hard way on that.

[Lynch_6913]

are all 6 cylinders fireing? timeing could be off...

[Chip682]

No I didnt push on anything with the IAC because i read that you need to be careful with it. Could it still be bad even if it doesnt throw code? Also the timing is not adjustable with this Heep.

[Chip682]

Would fuel pressure be low if the pump was sucking air? Or the seal in the top of the tank where everything goes in May have air leak? just a thought i guess

[JoBo]

92's or anything else older then a 96 don't/can't throw codes That's a obdII thing that started in 96. California may be different

Do a compression test and let us know what you find

[tjwalker]

It sounds like your engine management system may not be dropping from open loop into closed loop. If it doesn't get into closed loop, your mileage is going to really suffer. Is that 7 mpg calculated; ie did you do the math (miles driven divided by gallons consumed) to come up with that number?

I'd start by testing the coolant temperature sensor and map sensor. They can both affect fuel/air ratio. Need a manual and a meter. Cleaning the IAC can be helpful and it's always a good place to start, but it doesn't fix all idle issues; they are mechanical and can go bad and therefore would need to be replaced. A quick test of the IAC would be to "load" the engine with the A.C, headlights, etc.... and see if the IAC quickly adjusts the idle to keep relatively constant rpms.

How many miles on the engine? A compression test is never a bad idea as a matter of course, especially for very high mileage engines. Don't throw parts at this problem; test your way to a solution. Good luck and let us know what you find....

BTW, yes a 92 can trigger a check engine light and set a code. A 1992 is an OBD1 system and while not as advanced as OBD2, it works basically the same way.

[Chip682]

I did calculate that mileage. Also i bought a fuel pressure gauge today and checked that. The pressure drops off to zero after it is turned off for about ten minutes. This means a fuel pump right? It pumps fine when the gas is pushed and has good pressure while running and the regulator is good.

[Chip682]

So anyways, i replaced the fuel pump this morning and to my surprise it was quite easy. It also fixed my issue by about 98%. Its running much better. I think there may still be some air in the fuel lines from replaceing the pump so i am going to purge it out later today. Otherwise its pretty much mint. Thanks to all who helped in resolving this issue.