My 96 Cherokee is having issues and the repair shop can't figure it out. Every time it dies, I have been on a highway driving at least 30 miles with the AC on. I go through a drive thru and at the first or second stop light I hit after; it dies while idling. The only symptom before it happens is it starts to sound like a diesel engine when giving it gas. No warning lights go off and it will start back up 20-30 minutes after with no problems. At this point I have replaced the thermostat, battery, PCM and a couple of parts on the AC. The shop said all the systems are checking out..........all I know is it runs perfect without the AC and if I use the AC it may die. Any ideas?

 

What "systems" has the shop checked? Did they hook up a diagnostic scanner and look for codes?

 

Yes, they did. The code it gave was for the IAC (505) which was already replaced. The PCM was 17 years old so they replaced it. They checked the engine cooling, electrical, AC and power systems. They can not recreate the dying during idle

 

They replaced the IAC and PCM and can't recreated the dying at idle.... but it stills dies at idle when you're driving?

 

Yes. It's weird. I would assume its overheating, but its not. The shop's next "idea" is the fuel pump.

 

...the next idea? They didn't check the fuel pressure yet? I've had two failed fuel pumps from two different XJ's in the past.

One was in the pre-97 XJ model and it gave no indication that it was failing. It just died in mid-drive while I was in Lucerne Valley on my way back from Big Bear.

The other time was in my 98. It was hard to start for a few weeks because the check-valve in the pump wasn't holding fuel pressure over night. It shut off in the middle of a 4 way intersection on me eventually.

If it IS the fuel pump, do NOT buy an Airtex. After mine failed after a month of use I scoured the internet to find out they always fail in XJ's.
Only buy Bosch fuel pumps (NAPA has them).

 

Oh yeah, the Airtex failed me mid-drive 50 miles from my house. I had to get it towed for $80 after sitting on the side of the road for a few hours. In the end buying the airtex and paying for the tow was just as expensive and inconvenient (I also had to install the new bosch pump, and if you're paying for maintenance that just costs more money).
Also, Airtex's are NOT refundable. You can only swap it for another POS which will do the same thing.
If you don't mind paying for a new fuel pump every other month and being stranded on the side of the road for a tow truck (and I don't know anyone who does), the extra money for the Bosch is the only way to go.

 

They did check the fuel pressure, said it was fine. One of the guys there told me of his issues with a Jeep fuel pump and told me to drive around with a gauge taped to the windshield so I could watch the pressure. He said his didn't drop until after 150 miles. The only other suggestion I've had is to replace the battery ground cable, which came from another Jeep lover.

 

Now that I think about it there's no reason for it to die at idle. The electronics involved with the idle have nothing to do with the fuel pump maintaining fuel pressure. If it had anything to do with the pump it would be worse at wide open throttle due to a failing pump, or hard to start in the mornings due to a failing check valve.

The IAC, TPS (throttle position sensor), IATS (intake air temperature sensor) and MAP (manifold absolute pressure) sensor are all located within half a foot of each other and used in determining fuel trims.

If there was anything wrong with the amount of vacuum in the manifold you would have an OBD Code concerning the MAP.

If there was anything wrong with the TPS you would have hesitation on acceleration and possibly some engine stumbling on deceleration.

The IATS I'm not too familiar with. Never had any problems and to be honest it only sends the current temperature of the manifold to the PCM for finer tuning of the fuel curve. The MAP, TPS and 02 sensors are much more important to proper operation.

You've already replaced the IAC, but just because you replaced it with a "new" one doesn't mean it's a good one. The shop should have tested the IAC with the procedures listed in the manual before swapping. Parts don't always work right out of the box and the IAC could still very well be the problem. Until the part is tested and verified to be within the operating parameters it is still suspect (to me anyways).

Have they hooked up a scan tool and monitored the fuel trims while in operation?

 

.. and it's also possible they popped in a good IAC that worked and due to an electrical short in that particular circuit, or a bad ground, it is no longer working properly.

 

They did monitor the fuel trims, said they weren't the best but they were good. Funny you say that about the IAC, the one that is in there is the 4th part in about 1 year. On the third IAC we replaced all the wiring too.

I totally agree with you about the fuel pump. I'll look into the TPS, it could be that. The times it sounds like a diesel when you give it gas; it doesn't have the get up and go it normally does and sometimes is shaky when decelerating.

I greatly appreciate your suggestions!

 

There's no reason to go through so many IAC's in a year. Definitely check to TPS to make sure it is in spec, and check the IAC as well (voltmeter required).

Pull out a wiring diagram and find the ground for the IAC. It will be common to other electrical items as well. Visually inspect it and if it looks corroded/dirty/oxidized then disconnect it and clean it off.

If you haven't replaced 02 sensors in a long time I would do that too. I think the turn around for replacement is 60 or 80 thousand miles. From my understanding all Renix style engines use Bosch 02 sensors. Everything else is NTK. Since yours is a 96 you need NTK 02 sensors.

 

Also, what were the fuel trims doing? How much were they fluctuating?
If they were fluctuating at idle for no good reason (more than a few percent back and forth) then you might have a small vacuum leak.

Vacuum leaks wreak havoc on the IAC's proper functioning because it tries to maintain a certain level of vacuum to keep the idle at 750 RPMs. If you had an actual vacuum leak, depending on how large, the idle would constantly be rising and lowering while the IAC was trying to maintain equilibrium.

 

I don't know why it didn't occur to me initially but you said it dies only when the AC is on. That indicates a vacuum issue. Check all of your vacuum lines. You have a few coming out of your manifold.
One goes to the brake booster (fat black rubber line less than a foot long). You can pull that off to inspect it easily.

If you have cruise control another is made of hard pre-formed plastic that runs along the firewall to the passenger side fender over by the battery. It connects to a little cruise-control-contraption. I believe there is a T line there somewhere and the other end runs to a small black plastic ball that is enclosed and attaches behind your bumper.

I had a vacuum leak through the cruise control contraption (all of my vacuum lines were fine so I knew that was what was leaking) so I unplugged it from the vacuum line and put a rubber cap over the end of the vacuum nipple. It solved my problem.

Yours is a 96 and I don't know what kind of weather it's beenn subjected to but I've heard that dry rotting vacuum lines is a common problem.

There are also vacuum lines that run behind the AC evaporator (google the picture and location) through the firewall. Those actually run all the way to the ***** themselves that switch between where the air flows (defrost, top vents, floor vents, top and floor vents). If you disassemble your console and have a look you'll see multiple colored vacuum lines. My XJ was a 98 and it had red, blue, yellow and black. Black is the one that comes from the intake manifold and runs along the top of the firewall then goes through the firewall at the evaporator.

If you simply disconnect the black line from the **** and hook up a vacuum pump (after you close off the end that goes into the manifold of course) and pump away to see it hold vacuum then you know it's fine without having to actually trace any of the lines in the engine bay.

You should do that for all of your vacuum lines. Disconnect one end, hook the vacuum pump up, and close off the other end and see if it holds vacuum. If you're having a shop take a look at it for you then they will already know.

 

Not to sound like Captain Obvious here, but has anyone ever cleaned the throttle body as part of routine maintenance or in an attempt to address this problem?

Courtesy of TJWalker:
--------------------------------------------------------------------
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