Yarbles

Hi,
I'm totally new to jeeps so I hope this isnt an obvious question. Here's some background:

I just got a 2000 cherokee se that's giving an engine light error code pointing (according to mechanic) to the idle control system.
The mechanic says the idle valve/servo is receiving erratic signals from the 'computer' and the problem is intermittent. He wants to replace the 'computer' and valve to trouble shoot, which I get to pay for if it's not the issue.

The guy said he'd 'checked the 'vacuum system' but I'm wondering what other inputs the ecu uses to control the valve. I'm guessing throttle position and RPM.. O2 even? I guess my question is what all should be checked before resorting to switching out the valve and ECU.

Any hints/insight into how the system works will be a great help to me. I'm trying to get a good understanding of it before letting the mechanic spend my money on trial and error trouble shooting.

Thanks in advance for any help.
Jon.

Milehighzj

Ummm... Does he mean the iac sensor?? Idle air control? That could be a issue.. Can u provide some details on the symptoms ur getting how its running or if its running?? I know off my head u got anything really can cause idle issues.. Battery, iac, tps, map, maf, for that matter timing, plugs.. Anything can uf u see where i am going.. I highly doubt the computer needs replaced if it running codes and only 1.

Yarbles

Actually no symptoms at all but it won't pass inspection. Idle is smooth and rpm seems steady.
He didn't mention an iac. He did say that the idle control valve had two pairs of wires coming from the 'computer' and said they were AC and shrugged like he didn't know why. These were the wires sending incorrect signals apparently and he said the 'driver in the computer" (that sends the signals) might be bad.
I guess it might but it seems like a longshot to me..

Bustedback

Time to find a new mechanic. This one is willing to just throw parts at the problem at your expense. The TPS, RPM signal, MAP sensor, and possibly the coolant temp sensor help the ECM decide what the desired engine speed at idle should be and "tells" the IAC what adjustment should be made.

Radi

What is the exact code it's spitting out?

FWIW, erratic IAC operation is often caused by a dirty, sticky IAC. It's a small servo valve attached to the throttle body that bypasses air around the throttle plate as necessary to control idle speed. Remove and carefully clean, see if that fixes the problem.

cruiser54

What Radi says:
Courtesy of TJWalker:
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The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

"Gently" wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Yarbles

Aah so the four wires were for a 4-wire stepper motor so that explains why he'd be thinking it was AC. It still doesn't sound likely that the stepper motor driver would fail intermittently. I guess I'll go find out if he has a price for the computer swap-out and decide how to gamble..

Omg all of this for a car that runs perfectly. I just need that fault code to go away and i can pass inspection and be done. (Oh I'll ask what the fault code was too)

Thanks for all the help so far.

Muaddib420

Your best bet is to go to autozone or any parts store and have them read your CEL code. Write it down then come back and report back here. Everything else is speculation at this point because you're not having any symptoms.

Yarbles

According to inspection station:

P0505
'idle control system'

Here's the back-story:

Went to buy jeep from private party, saw engine light was on.
Seller had been told by autozone that it was an O2 sensor error.
Seller took jeep to his mechanic who replaced O2 sensor then apparently found there was still a check engine light so he took it upon himself to remove the check engine bulb (!) (Seller didn't know about this. He's a decent guy)
Buyer (me) foolishly didnt notice check engine light wasnt coming on before engine start and bought the jeep.
Inspection station did notice engine light not coming on at all, found missing bulb and supplied fault code.

Muaddib420

Likely your idle air control motor has failed. It is a small sensor/motor attached to your throttle body. Pretty easy to change out. You will need a torx 20 bit.

http://www.oreillyauto.com/site/c/de...2169&ppt=C0018

You could try just carefully cleaning it with throttle body cleaner before buying it. It's $70.

Yarbles

The mechanic tried a new iac motor and said it idled high and the motor didnt seem to be moving. He reinstalled the old one.

I have the jeep back and now it's giving three codes. P0505 idle system, P0122 Throttle/pedall pos and p1694 'manufacturer controlled computer and aux outputs' whatever that means.

It seems like the TPS could be the cause of the IAC issue but wouldnt I see other symptoms from a bad TPS?

It only showed the one fault code 0505 (IAC) code before going to the mechanic and since leaving the mechanic the idle's no longer smooth.. Aaarrgh

Muaddib420

Might find some info here. Could be a faulty CPS.

http://www.naxja.org/forum/showthrea...ighlight=p1694