BigHarv

hey guys, I need some help. My 96 4.0 is idling rough. The CEL light came on, so I went into Autozone. They said it needed Plugs and wires and I also needed to clean the Mass Airflow Sensor. I did what they said....New Plugs, new wires and cleaned the MAF. It still is Idling rough. When I get on it, she gets up and go, but when I slow down to 30ish mph she runs rough and jerks. What else could it be? I am new to the 4.0 so please help me out.

OH......I did put a Rusty's off road cold air intake on it. It started doing this after I put it on. I have since gone BACK to the Stock intake.


Thanks Guys!

BigHarv

Oh it also took me 1/2 a tank of gas to go 70 miles!!!

jeepfan90

sounds like u have a loose rocker arm do u hear a light tapping noose or feel a puls in ur break pedal.or it could be ur throttle positiong sencer or both hope this helps

freegdr

did you clean the iac and the hole it mounts in

BigHarv

no tapping.....whats the iac???

Spoonman

IAC = Idle Air Control Valve.
I am new to jeeps and unsure of what models have what but if yours has an IAC Valve it would be mounted on the throttle body and will have an electrical connector going to it.

You of course will not see the pintle end of the installed IAC valve which is the goldish colored part of the IAC valve.

tjwalker

It is best to post the EXACT code numbers here so as to get the most accurate replies.
With OBD, the devil is in the details.

Keep in mind that new plugs and wires is NOT a complete tuneup. You need a distributor cap and a rotor to complete the job. Don't want to be chasing your tail for $20 worth of parts.

the_big_h

??? Mass Air Sensor ???
No mass air sensor on the 4.0.
And the Idle Air Control valve only controls idle speed - it won't make it run rough(except by idling too slow), and it can't affect fuel mileage.
You need to know the fault code, to pinpoint the problem.
How did the old plugs look? Possibly you had a bad/fouled plug, but got the wires crossed when replacing them?
Guessing is probably counterproductive without knowing the fault code, but you may want to check the fuel injectors.

Spoonman

I believe that is the exact situation quoted from op below.


I am new to jeeps so I cant comment on this much other to say this seems to be true. There is however a MAP (Manifold Absolute Presssure) sensor which will cause poor running if it is not operating properly as well as a throttle position sensor which could cause poor running for idle and off idle. The firts thing I would check if it were me is the IAC.

The TP sensor will be mounted on the throttle body on the opposite side of the throttle linkage.It will have I believe 3 wires going to it.
The MAP sensor can be located any where really but is usually close to the throttle body and will have an electrical connector with I believe 3 wires going to it as well as a vacuum hose going to it.

MickDime

Check your fuel injector wires

Mine were fused together and the idle it shook so much that exhaust was coming out of the wheelwells

mdtobe

Hi Harv and everybody else,

I've been having the exact same symptoms! It starts with the rough idle and stumbling at lower speeds, then progresses to really bad mileage and the Check Engine light coming on. I get the code read and it is 0171. I was told this means the oxygen sensor has failed with possible other causes being misfire, MAP, etc. etc.

I have had my mechanic replace the oxygen sensor THREE TIMES in the past few months and he claims he checked everything else each time. Every time, it runs okay for a few weeks (though the rough idle and stumbling at 30mph can sometimes be felt), than after a few weeks, the Check Engine light comes on, and the mileage drops to just 9mpg.

Then I take it to him and he puts another O2 sensor in (and on one trip, replaced the catalyst), and claims everything is fine. But it's not fine. I can feel the stumbling at 30mph and rough idle coming on within days of picking it up. Said symptoms come and go, but eventually, the Check Engine light comes on and the mileage drops back down to about 9mpg (these two things happen simultaneously). Basically, I think something is CAUSING the rich condition, which then eventually fouls the O2 sensor(s).


This is driving me crazy!

Were you ever able to solve your problem? Because I think I have the same problem.

Further symptoms:

Definitely running rich (blackened plugs and big-time sulpher smell coming from the Catalyist, which had to be replaced at the time the 2nd Oxygen sensor was installed)

Thanks for your time,

John Mohan

mdtobe

Why do you think a loose rocker arm would cause these problems and how do you check to see if it is loose?

Thanks in advance,

John

HershMan

You might also check for cracks/holes in the exhaust manifold. I know mine is cracked pretty bad causing my idle to be rough and mpg are down. Haven't gotten CEL light yet, but if the read is lean enough it might start.

mdtobe

Thanks for the suggestion. I'm going to do a vacuum check tomorrow - I figure that's the cheapest, easiest step to take. I'm not holding out much hope though because I watched my mechanic do a vacuum check last time it was in and he didn't find one.

I'm still hoping to hear from the OP because I'm pretty sure my problem is the exact same one his was - the symptoms are identical. Hopefully he solved his problem and likes to share.

Thanks again,

John

mdtobe

Update:

Since my last post, I've:

Performed a thorough vacuum check - there was none.

Replaced the MAP and reset the computer - problem is still there: as soon as the vehicle reaches operating temperature, the misfiring starts and there's lots of black soot coming out of the tailpipe. And the check engine light comes back on with the P0171 code.

Checked the fuel pressure: It runs at about 49 lbs. but as soon as I turn off the engine it drops to less than 20 lbs. in less than a minute. If I pinch off the supply line, the fuel pressure stays constant, so it is not a leaking injector(s). Must be either the fuel pump or the fuel pressure regulator/filter (attached to the top of the fuel pump) allowing it to leak back. This also explains why sometimes it takes a few seconds of cranking before it catches when starting while warm.

Here’s a funny thing: At idle, the fuel pressure gauge needle shakes back and forth so fast it’s blurry. It is bouncing between about 40 and 60 lbs. (just flipping back and for in an arch with the 50 lbs. mark at the middle of the arc). As soon as I increase the revs to 1200 or more, the bouncing back and forth ceases and the needle stays steady at about 49lbs. of pressure.

Regarding the fast pressure drop after turning off the engine, there are no fuel leaks in the system - it is definitely just going back through the fuel regulator and pump.

Could this fuel system back-draining or the way the needle bounces at idle be causing my computer to make the car run too rich? I wouldn’t think so, because the problem surfaces when I’m driving, often at freeway speed – not just at idle. Any opinions are welcome!

I'm also wondering if maybe I fixed the cause of the problem when I replaced the MAP, but the symptom is still there because the symptom is that my O2 sensor got destroyed by carbon fouling from the engine running too rich. Do I need to spring for ANOTHER O2 sensor?!?!

Lastly, is the fast drop in fuel pressure after turning off the engine caused by a problem with the Pressure Regulator allowing back-flow or the Fuel Pump allowing back-flow?

All help appreciated,

John