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rojpon74 10-16-2010 02:45 AM

4.7 HO Engine Rough Idle Problem
Hello everyone. I'm new to this forum and this is my first post. Unfortunately, my first post has to do with a problem on my '02 WJ Overland.

I recently had the heads rebuilt and milled by a local machine shop (vehicle has 199k miles). The heads were reinstalled today and the mechanic working on it took it for a test drive, which I tagged along on.

While the Jeep was stopped and idling, for example at the stoplight, we both noticed the engine idling rough, like it wanted to stall out, with the RPM fluctuating between 400-500 RPM (it did dip to as low as 350 on a couple of occassions). No trouble codes were displayed, however. When the vehicle took off, the engine ran fine, except for a slight hesitation on some occassions when the vehicle started to accelerate from a dead stop. The overall test drive was less than 5 miles and less than 30 minutes long.

Upon returning to the shop, the mechanic thought the cause of the rough idle might either be bad spark plugs or bad iginition coils. I find both hard to believe since the spark plugs are brand new (OEM-spec Champion Platinum plugs) which I've always used and I can't see the ignition coil(s) going bad suddenly. He also mentioned that bad O2 sensors can cause a rough idle, but he said bad O2 sensors would not cause the engine to idle as roughly as it did. He also noted that he had to clean the injectors while the top end was apart since there were gummy deposits on them. FYI, all the injectors and both fuels rails were replaced last December. The Jeep is still in the shop needless to say until the cause is idenetified and corrected.

So, if none of the above is the cause, what else might be causing the rough idle? I was thinking it could be a vacuum leak (where might it most likely be after the heads have been removed and installed), a bad TPS or MAP sensor, or a bad IAC motor (though the mechanic said he did not touch the throttle body at all). I also thought it might be that the PCM has to relearn everything since the battery had been disconnected for nearly two weeks while the heads were removed, rebuilt, and reinstalled. It also sat outside of the shop for several days while the heads were at the machine shop. Of course, the worst case scenario, improperly set valve/cam timing when the heads were reinstalled, crept into my mind as well.

Thanks everyone in advance for any advice or help you might be able to provide.

StoneCold 10-16-2010 03:42 AM

Re: 4.7 HO Engine Rough Idle Problem

When I thought my IAC had gone bad, it tended to idle quite high, about 1000rpms, then dip up and down. On a few occasions it dipped to the point of almost stalling. Took the throttle body apart and removed the IAC and cleaned the whole lot out. Been fine ever since.

Could be worth giving that a shot, as its an easy DIY fix.

Milous 10-16-2010 04:25 AM

Re: 4.7 HO Engine Rough Idle Problem
Was the idle issue present before you had the heads rebuilt? What was the reason for having them rebuilt?

rojpon74 10-16-2010 10:03 AM

Re: 4.7 HO Engine Rough Idle Problem
No, the idle issue was not this bad before the heads were rebuilt.

The heads were apparently warped after they tore them down. Prior to them being pulled, I suspected a cracked head after they had failed a leakdown compression test. The machine shop however didn't find any cracks in the heads.


Originally Posted by Milous (Post 285605)
Was the idle issue present before you had the heads rebuilt? What was the reason for having them rebuilt?

Frango100 10-16-2010 11:05 AM

Re: 4.7 HO Engine Rough Idle Problem
Regarding the relearning of the IAC positions, the Jeep manual mentiones the following:

In the IAC motor system, the PCM will count
every step that the motor is moved. This allows the
PCM to determine the motor pintle position. If the
memory is cleared, the PCM no longer knows the
position of the pintle. So at the first key ON, the
PCM drives the IAC motor closed, regardless of
where it was before. This zeros the counter. From
this point the PCM will back out the IAC motor and
keep track of its position again.

So in fact the PCM should alrerady know at the first powerup the position of the IAC stepper motor pintle.
So if your mechanic didn't do anything to the IAC motor yet, it would be a good idea to remove the stepper motor to see if there is any dirt on the pintle and/or air passage.

rojpon74 10-18-2010 06:20 PM

Re: 4.7 HO Engine Rough Idle Problem
Thanks everyone for the responses. I'll let you know what my mechanic finds!

NuttyJeeps 10-18-2010 07:56 PM

Re: 4.7 HO Engine Rough Idle Problem
I am having the same problem with my 99 limited, when stopping at a red light with no ac or temperature contol on at all my jeep will idle under 500 but if i turn the ac on it seems to idle fine and like it is supossed to and when driving everything seems fine.
So far i have replaced the TPS and IAC
Would love to know what the problem is when you find it.

Frango100 10-18-2010 09:40 PM

Re: 4.7 HO Engine Rough Idle Problem
Hi Nuttyjeep, when you changed the IAC motor, did you also clean the throttle body and especially the idle air passage? The last one gets clogged up with oil and other debris and can cause the rough idle.

NuttyJeeps 10-18-2010 09:49 PM

Re: 4.7 HO Engine Rough Idle Problem
I have cleaned the throttle body but not the other thing you mentioned, I'm not really a very good technical car guy haha...Thanks for the advice

Frango100 10-18-2010 10:16 PM

Re: 4.7 HO Engine Rough Idle Problem
Hi Nuttyjeep, when you remove the resonator box from the throttle body, you can see the idle air passage on the lower side of the throttle body just below the throttle blade. When you remove the IAC motor (again), you can clean the air passage on both sides from the control opening. Or you just spray a special throttle body cleaner into it with the engine running.
I have no idea why they ever have placed the crankcase breather line into the resonator box, causing the oil deposits into the idle air passage. An oil catch can, already installed by several members overhere, would take most of this problem away.

NuttyJeeps 10-19-2010 12:31 AM

Re: 4.7 HO Engine Rough Idle Problem
Cool thanks for the advice I'll have to try this tomorrow.

Milous 10-19-2010 02:17 AM

Re: 4.7 HO Engine Rough Idle Problem

Originally Posted by NuttyJeeps (Post 287071)
Cool thanks for the advice I'll have to try this tomorrow.

Before you start, see this:

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