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Bannerman53 03-04-2012 08:41 PM

Very rough idle
I have a 1996 jeep grand cherokee laredo V8 5.2 liter, all stock. Its been sitting in the driveway for awhile, and when I charged the battery up and started it, it wouldnt idle unless you kept your foot on the gas. I then replaced the TPS(Throttle Position Sensor) and cleaned out thte throttle body with carb-cleaner and an old toothbrush. This seemed to do the trick and the idle problem was fixed(mostly). This was last weekend. Every day since then, when I start it, the RPM's jump from 7550 to 900(back and forth). Im 99% positive that this is a leak in a vacuum hose(as I had the same problem on my 2004 jeep). But today when I went out to take it for a spin, It started up, but idled really rough. It would go down to 200 rpm, then try and surge back up to about 650-750, but then it died. So I immediatley tried to start it again, but this time I held my foot on the gas so It would get to about 1500 rpm's, and after about 1-2 minutes of revving it, when I took my foot off the gas, it settled back into th 750-900 rpm range(but it was still dancing back and forth). I think this could either be the IAC or a bad fuel filter(or both). Any suggestions or insight?

P.S. My drivebelt(or fanbelt or serpentine belt or whatever you want to call it) is squeaky and needs to be replaced, but this probably doesn't have anything to do with the above problems, right?

jnowak 03-04-2012 08:47 PM

Iac. Classic if u have to put ur foot on pedal to contol idle its iac. No, the belt wont affect it unless ur voltmeter is dropping below 10v. Than its not turning alternator enough.

DeejayZeejay 03-04-2012 11:27 PM

May just need to adjust idle...always start with simple stuff.

PolkaPower 03-04-2012 11:36 PM

You don't adjust the idle. It's computer controlled. Pull the IAC and clean it off with a tooth brush and some carb cleaner.

DeejayZeejay 03-04-2012 11:47 PM

The 5.2 has throttle adjustment. If too low, idle will be erradic or stall out.

PolkaPower 03-04-2012 11:48 PM


Originally Posted by DeejayZeejay (Post 13177103)
The 5.2 has throttle adjustment. If too low, idle will be erradic or stall out.

Are you talking about the screw on the TB?

DeejayZeejay 03-04-2012 11:55 PM


PolkaPower 03-04-2012 11:58 PM

You shouldn't have to touch that at all and that definitely would not cause what's happening to OP's rig.

DeejayZeejay 03-05-2012 12:00 AM

Well my idle was acting up and I adjusted that. Now my idle sits perfect.

hdrocknroll 03-05-2012 12:10 AM

The IAC motor is mounted to the back of the throttle
body (Fig. 9) and is controlled by the powertrain
control module (PCM).
The throttle body has an air control passage that
provides air for the engine at idle (the throttle plate
is closed). The IAC motor pintle protrudes into the
air control passage (Fig. 12) and regulates air flow
through it. Based on various sensor inputs, the powertrain
control module (PCM) adjusts engine idle
speed by moving the IAC motor pintle in and out of
the air control passage. The IAC motor is positioned
A (factory adjusted) set screw is used to mechanically
limit the position of the throttle body throttle
Never attempt to adjust the engine idle
speed using this screw.
All idle speed functions are
controlled by the PCM.

PolkaPower 03-05-2012 12:17 AM

:agree:Yea, it may have adjusted the idle but it didn't fix what was causing the problem.

Bannerman53 03-05-2012 09:20 PM

Im just going to clean out the IACV with some carb-cleaner and tooth-brush, be back in a little to tell if i solves the problem.

E-rok 03-05-2012 09:28 PM

IACV is always the first thing suggested, but it can be a number of things, the magnum motors are notorious for rough idle.

Bannerman53 04-15-2012 10:40 AM

Took the IACV out to clean it and there was nothing on it at all. It looks brand new. What else could this be?

JasonStebbins 04-15-2012 10:47 AM

Wiggle your fuel injectors while it is running. My first ZJ did this occasionally until I replaced the fuel rail. How about the CPS? It controls the spark and fuel injectors also. Maybe you have a fouled out spark plug or bad wire or even a cracked distributor/cap? I would start by checking the basic tune-up stuff for an erratic idle.

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