For 'Knowlegable' 4.0 Jeep Engine Techs Only!

[RVTECH]

I posted this on a Jeep forum I am on but thought there might be someone her who might have an idea:

OK there is no Cherokee category but the issue is the engine so I though I'd start here. OK engine throws a code a while back (PO 171) and idles rough and gas mileage goes to about 1/2. Ok so it was starting fine and idling OK but then after it warmed up it would start to idle rough. Still with poor mileage it just yesterday started something new so please read carefully - the engine suddenly started to have an odd sound - almost like there is a massive airflow in the area of the air intake - like a large fan is blowing (and its not the secondary fan) OK so I 'work' the throttle on the throttle body and I can get this airflow noise to stop. Now it is randomly starting to stumble and nearly die. ANY ideas on this or where to start? The airflow noise is REALLY weird and is very loud when it is happening - almost like something is 'shunting' and causing a massive airflow through the cold air intake BUT with messing with the throttle cable at the throttle body I can get it to stop. FWIW I am a pretty good 'wrench' with engines & repair so you are NOT responding to a 'neophyte' but I'll admit I am a bit 'green' with throttle body and EFI issues so I need a place to start. Please advise! Thanks!!

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[robertg]

I'm guessing a vacuum leak, the sensors, or plugged up fuel line.
Don't know about the fan noise though, that's kinda weird.

 

[v0lcom13sn0w]

vacuum leak, skewed MAF or fuel pump usually causes a p0171. whats happening is your pre cat 02 sensor is seeing a lean condition and is richening up the air fuel mixture which explains the gas mileage. If you hear a hissing noise i would suspect a vacuum leak.if you can, have it smoke tested thru the booster hose to find the leak or if thats not an option u can use propane or brake clean and spray it around until your engine stumbles or stalls. i personally have access to a smoke tester and thats my prefered way to find vacuum leaks but in a pinch the propane or brake clean will usually work. i would be checking for any cracked or unplugged vacuum hoses, booster line, intake manifold gasket leak. if all checks out i would check fuel pressure next.

 

[jbett98]

My neighbor had a similar issue with his Jeep when it started idling rough, then die unless you feathered the throttle and increased the rpm.
Come to find out he had a bad battery and it wasn't putting out enough voltage to run the computer.
If you revved the motor it would run, but once you backed off the throttle it would die again.
He replaced the battery, then he had to start it up, run a distance, the shut down and repeat to clear the computer of the past sensor readings.
He also had to take the factory ignition key and go around the door locks and open close the locks to reprogram the alarm system.
So far, it seems to be fixed.

 

[Jamie6.5]

Your Jeep likely uses a MAP sensor, as Chrysler uses a speed/density system for fuel calculation. MAP stands for Manifold Absolute Pressure. When you develop a vacuum leak, the MAP sensor can't compensate enough, and your O2 sensor sends a lean signal to the computer, causing it to throw a lean mixture code, PO171.
It sounds like you have a bad Throttle Body mounting gasket, or the Throttle Body bolts/nuts have backed off (loosened). (this happened to my son's 4.0 Cherokee)
Check the studs/nuts on the TB mounting, and get a can of carb cleaner (the kind that says EXTREMELY FLAMMABLE on the cautions). If the throttle body mount nuts are snug-tight, then with the engine running, lightly spray the carb cleaner around the base of the TB and listen for a change in the idle speed and/or smoothness.
If the engine speed increases or the idle smoothes out when sprayed, that indicates the TB mounting gasket is FUBARed and needs replacing.

Good luck, and keep up posted.

 

[RVTECH]

Come to find out he had a bad battery

Well I do know my battery is weak and I have a new one ready to install. I'll do that first before proceeding on. Never considered a weak battery IE figuring if it would start it it would run the computer BUT it has gone dead a few times and I had to put a charger on it to start it.

 

[v0lcom13sn0w]

i think its safe to say look for a vacuum leak. BTW why year cherokee and is it a 4.0l straight 6?

 

[RVTECH]

BTW why year cherokee and is it a 4.0l straight 6?

99 4.0

 

[v0lcom13sn0w]

99 4.0

thanks.i just wanted to know if we were dealing with 95 or newer. i guess by the thread title i knew it was a 4.0. have you made any progress yet?

 

[RVTECH]

Not yet - need to move it to the 'drydock' side of the shop and move my Corolla over to the 'use' side of the shop and hopefully be able to get it out of the driveway - I have 2+' of snow here!

 

[Joe Link]

Moving this to Off Topic for better exposure

 

[Gunwheeler]

Idle Control Valve. Search on Youtube. Takes about an hour.

 

[RVTECH]

Idle Control Valve.

Yep that's next on the list - seems this is an often overlooked issue and all it needs is cleaning

 

[Mikej]

Not yet - need to move it to the 'drydock' side of the shop and move my Corolla over to the 'use' side of the shop and hopefully be able to get it out of the driveway - I have 2+' of snow here!

Right on!

 

[Scvette]

Almost sounds like what my daughters wrangler started to do,might be a long shot but on hers the cats were clogged.

 

[RVTECH]

Idle Control Valve.

Well cleaned and reinstalled and I'll admit it ran better and the idle stayed nearly normal for much longer but the check engine light came back on so on to the next item on the list.

 

[DuneHopper]

One thing that keeps popping up in my head is throttle body spacer?
The fact it started to create the problem slowly could be a gasket there or the spacer its
self is loose as well as any horses attached. This comes to mind because if it started ok, you would even notice a leak there until it warmed up a bit. Hard to say without seeing it run and hearing it but that was my thought. As others have suggest it sounds air leak related but I was thinking the spacer. Heck might even be just loose.

 

[v0lcom13sn0w]

still thinking its a vacuum leak.

 

[Member 5605]

Posted this before I finished reading all of the thread:

#################

I'm on the train of a vacuum leak..

As we know the throttle plate closes at an idle and the air goes through a bypass to regulate air speed/volume to modify the idle speed.

Right?

So when you open up the throttle manually, by foot or hand under the hood, the whirlwind goes away. Now that it's not needing such air to keep engine going since its sucking elsewhere.

Check that gasket under the throttle body, get some carb cleaner and spray around to see where the leak is. You will notice that difference soon as the cleaner gets sucked into the intake.

OR you have a failing pre-cat sensor... Unlikely but it could be causing the faulting readings. Have you tested it?

Wondering if the fault in the O2 is causing the IACV to wack out. Hondas are notorious for getting sticky and causing the revving up and down at stop lights.



I am not a jeep guy. Just an ignorant fool. Take the advice with a shot of whiskey.

 

[v0lcom13sn0w]

Posted this before I finished reading all of the thread:

#################

I'm on the train of a vacuum leak..

As we know the throttle plate closes at an idle and the air goes through a bypass to regulate air speed/volume to modify the idle speed.

Right?

So when you open up the throttle manually, by foot or hand under the hood, the whirlwind goes away. Now that it's not needing such air to keep engine going since its sucking elsewhere.

Check that gasket under the throttle body, get some carb cleaner and spray around to see where the leak is. You will notice that difference soon as the cleaner gets sucked into the intake.

OR you have a failing pre-cat sensor... Unlikely but it could be causing the faulting readings. Have you tested it?

Wondering if the fault in the O2 is causing the IACV to wack out. Hondas are notorious for getting sticky and causing the revving up and down at stop lights.



I am not a jeep guy. Just an ignorant fool. Take the advice with a shot of whiskey.

just to put it out there, i dont know everything, i never will but, i fix p0171 and p0174 almost weekly on GM vehicles. usually its a bad manifold gasket, line, or fuel pump. i have had it be a bad MAF once but the cherokee uses a map sensor instead so that can be ruled out.

the code itself is a generic lean code so it doesnt matter which rig its on,it means it has a lean condition.


keep in mind,manifold vacuum is greatest when the throttle is BARELY OPENED. BARELY. once the throttle blade is opened more then your manifold vacuum decreases (assuming your cat isnt plugged, but a clogged cat wont cause a p0171) that is why the noise goes away and engine smoothes out from the way the OP explains.

i like the idea of a throttle body gasket or intake gasket leak but dont throw out the possibility of a collapsed or cracked rubber vacuum hose. hoses and connectors on a 16 year old rig usually starts getting about now.