2001 Jeep Grand Cherokee starting rough and idling rough


#1

Hi,
pretty soon this thing’s gonna have ALL new parts and STILL running rough! The Jeep has about 40K on a Jasper motor, 175K on everything else. i was experiencing starting issues (crank for 20-30 seconds), and noticed a P0171 code on my odometer (O2 sensor?) so off to my mechanic. He replaced two O2 sensors, no change. New code read MAP sensor, replaced, no change. Finally, changed Fuel Pump in tank (Fuel filter was changed 2 months ago), still no change. i tried starting it tonight by turning the key 5-10 sec. to prime gas, started but still rough at idle. Runs strong above idle 1500-2000RPM. Any suggestions…mechanic can’t check fuel pressure at rails? for some reason. My next thoughts were spark plugs and vacuum lines…anything else? Thanks much.
Dan


#2

Sorry, it’s a 4.0 staight 6, fuel injected. That would help!


#3

idle air control dirty? I just had that prob on my Cherokee. 4.0 L str6 cyl. it was really easy to clean it and the hole where it seats and I did clean the throttle body too


#4

mine is MPI multi port fuel injected they also came as TBI throttle body injected


#5

I hope your mechanic cleaned the throttle body and aic before he changed the fuel pump, it almost sounds like he is guessing with your money. if that s the case I d go elswhere


#6

Thanks much WESW! I’ll have him check it. He’s been pretty reliable over the years (emphasis on years), maybe he needs an extra hand or two to help (and diagnose) these days!


#7

@‌Dannyboy442

“a P0171 code on my odometer (O2 sensor?) so off to my mechanic. He replaced two O2 sensors, no change”

Please don’t take offense, but you may need to find a new mechanic

The oxygen sensor is not the cause of the P0171 lean code. The oxygen sensor measures oxygen content

By replacing the oxygen sensor, your mechanic shot the messenger

P0171 means the pcm saw a lean condition. You need to determine if you do indeed have a lean condition

Intake leaks
exhaust leaks in front of the upstream oxygen sensor
low fuel pressure
etc.

20-30 second cranking time is quite excessive. Has anybody hooked up a fuel pressure gauge?
Sounds like fuel pressure may not be holding once the engine is shut off. Could be a bad fuel pressure regulator, leaking injector, etc.

By the way, if you have vacuum leaks, I wouldn’t count on the MAP sensor working correctly

It sounds like your fuel rail doesn’t have a test port. Tell the mechanic to bust out his fuel pressure test kit, use the appropriate adapters and tee in.

Just throwing some ideas out there . . .


#8

my problem was almost identicle to yours. hard starting and rough low idle , with low RPMs at idle.

check really well for vacuum leaks too. I had a couple that helped a little when I fixed them( just a matter of cutting the bad end off and re attaching or just attacing where they fell off)

the main improvement came when I cleaned aic and throttle body tho. it really easy too.

the internet and you tube have great jeep videos and mechanics too. eric the car guy and bleepin jeep are my two favorites. there is a guy in Canada who is really obscene who is ok too, but he edits out his mistakes and makes himself look more skilled than he really is. plus I don t like his filthy mouth


#9

Thanks to all…I know he’s seen better days…after I suggest some of your ideas, and hopefully get things going…i’ll look around for a new mechanic.


#10

Danny

To make it easier . . . ask your mechanic to look for those leaks I mentioned, using the evap/smoke machine

Assuming he has one. It’s a lot easier than blowing cigarette smoke into the intake, or spraying carb cleaner, or what have you, and looking for an idle improvement

Wes had excellent advice, about cleaning the throttle and the idle air control valve. I second his advice on that


#11

I learned most of that here…


#12

If you sift through all the ranting and raving, you might come up with something useful


#13

Idle air control valve is probably the culprit for the rough idle.
If the fuel pressure checks ok,then I would have the knock sensor checked for the hard start problem. if the knock sensor is malfunctioning the computer is probably retarding the timing too much making for hard starting.