2000 Jeep Cherokee Sport idling funny

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My vehicle's stats: 2000 Jeep Cherokee, 6-cyl 4.0, automatic transmission. 54k miles, had a tune-up at 40k miles at the Chrysler dealership. I haven't had any major engine work done to it. I get an oil/fluid change every 5k miles.

Recently, I've noticed it idling funny. At stop lights (and other times where I come to a stop) the idle RPMs drop down to the 300-500 range. This happens both in drive and neutral. I've noticed this happens more often in the evenings. (not sure if that matters) Last night I pulled into the garage and let it idle for a few minutes so I could see what was happening. I noticed the headlights were flickering and the battery meter was showing the voltage dropping slightly. After a couple minutes of idling around 400 RPM and light flickering, the engine revved up to ~1000 RPM and stayed there until I turned off the engine.

Funny thing is, over the last couple weeks, when I would come to a stop it would idle high, around 1500 RPM. Now it's idling low. And it's sporadic, sometimes it does it, sometimes it's fine.

I took the battery in to be tested (it's the original battery) which tested fine. We also tested the charging system and it was also working normally.

My mechanical ability is novice at best. I can check the oil level and change a battery and that's about it. I do have a knowledgeable mechanic but he's about a forty minute drive and I would like to have at least an idea of what might be wrong before I take it out to him and possibly fix this myself. Thanks in advance for any help/advice.

posted by  I aint yer Pa

Try cleaning your Idle Air Control Valve. Carbon build-up may be the suspect to your issue. There are many threads on the internet regarding carbon/idling problems.

posted by  theinfamious

Some people would, but I don't think it's a good idea to try to clean the IAC.
IAC Valve.Here is a flowby test for the IAC. I'm not the author. :wink2:

First
Remove four hex screws holding the plastic cover

Check the cable harness carrying voltage to your IAC-

Disconnect lAC valve,

Key on, engine off.
Measure Voltage PWR circuit voltage at the lAC valve harness connector.

Test Connectors supply of power to your IAC


Is voltage greater than 10,5 volts?
Yes;
KEY OFF, GO to KE3 (My valve measured 11.2 volts)

No;
REPAIR open circuit.

(My hope is that this is not your problem……As repair of an open circuit may be more difficult to trouble shoot than the
IAC alone)
************************************************** **********
KE3 CHECK lAC VALVE RESISTANCE

lAC valve disconnected,
Measure lAC valve resistance between the two pins,


Is resistance between 6.0 and 13.0 ohms?
Yes
GO to KE4
No
REPLACE lAC valve,

(Mine measured 0.022 therefore was very suspect at this point)

******************************************

KE4 CHECK lAC VALVE FOR AN INTERNAL
SHORT TO lAC CASE
Measure the resistance from either lAC valve pin to lAC valve case,

Is resistance greater than 10,000 ohms?
Yes

GOto KE5

No
REPLACE. lAC valve,

(With this my valve measured OL and therefore it became clear that a repair cleaning was of little use )

Anyway - Further checks go on as follows if yours passes the above tests
KE5 CHECK AIR INLET FOR PLUGGING

Inspect the entire intake air system for debris, blockage and other damage.

Remove and inspect lAC air tubes (if equipped) for blockage and other
damage.

Remove and inspect the air cleaner element for excessive dirt.

Is the intake air system OK?
Yes
RESTORE inlet air system. GO toKE6 CHECK FOR INLET AIR LEAKS
Key on, engine running.

With engine running at idle, listen for vacuum leaks.

Inspect the entire intake air system from the mass air flow (MAF) sensor
to the intake manifold for leaks such as:
Cracked or punctured intake air tube.
Damaged or loose lAC air tubes.
Loose intake air tube at air cleaner housing or throttle body.


lAC valve or gasket seal.
EGR valve gasket seal.
Vacuum supply connector and hose.

Many on the net have written about cleaning the IAC then re-installiung it- Well I think that is fine but- Only when it passes the electrical tests above – and cleaning also carries the added cash savings of about $65 or, in my case $88.00 dealer cost- I have only two warnings about it though and that is this;-

There may be a chance that while cleaning your untested valve, you perhaps may just rattle an intermitant internal short connection enough to make it work for a week or two – then when you least expect it- got ya!

posted by  lectroid

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