Zenroth

So having a new issue with the jeep, and I'm guessing its from the IAC or possibly the TPS.

A few different things seem to be happening...

1. Sometimes the Jeep will suddenly start idling much higher after being driven basically now up in the 1.5-2k range when in drive with no gas.

2. Or the engine will suddenly get super rough, and with an unstable idle, or hold an idle but start cutting out bad if you try an accelerate.

In both of these cases, turning the car off, pushing in on the IAC/TPS electrical connections and generally messing with those has fixed the issue and the jeep is back to running fine.

I haven't unfortunately gotten the time to try an troubleshoot the issue much, as it usually seems to happen at night as well and of course when I'm away from the house.

My general gut is telling me the IAC is probably shot, thoughts, or any tips on how to actually test what could be causing the issue?

Thanks

cruiser54

Do this first, during the day. LOL.

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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

jnbhc

if it's an older cherokee with the fuel regulator at the fuel rail, I'd be monitoring fuel psi. Regulator goes bad can cause the very problem you described also.

cruiser54

Down the road that might be a good idea, but basic maintenance like throttle body and IAC cleaning has to be addressed first.

Thexjfiles

I highly recommend taking the entire TB off the manifold to clean it!

When I take apart TBs, (about 165 so far) the carbon is so hard in the housing, it usually doesnt come out until I hit it with the glass beads. It will restrict the movement of the pintle.

Spraying TB cleaner will never get it as clean as taking it apart and using a blaster or a few hrs in a parts washer.

cruiser54

That's probably better for sure. And get a new throttle body gasket for about $3 and replace it, too.

cleenrob

check you o2 sensor I'm having the same issue AFTER i replaced my IAC and i keep getting o2 sensor codes

cruiser54

He has a Renix Jeep which does not throw codes.
Maintenance needs to be performed first.

Zenroth

Thanks guys, certainly some food for thought.

I actually did clean the IAC pintle awhile back (before this problem existed) and the IAC chamber. (Didn't take out the whole throttle body and clean it though)

I need to double check the fuel pressure, I know it was good, but I do think there was also a time I checked it and it was like 41 PSI with the vacum attached to the regulator, vs the 30 something. Not sure if that would cause my above problems or not.

Is the TPS not a likely suspect in the above issues?

Finally on a slightly different question, how long should it take the 4.0 to get up to its 210 temperature or so?

I'm wondering if someone removed the thermostate, as my jeep rarely ever gets up that high (usually the needle doesn't even like to barely move) and if I'm able to travel at say 40+ mph with airflow it'll just immediately drop back down to like 140 or something, possibly lower.

The electric fan is currently on a toggle switch ( I assume temp fan switch is bad), so I wasn't to concerned with the engine staying cool (minus heavy traffic) but I've also noticed the MPG being a bit low, and I suppose the computer might be staying in Open Loop mode due to the temps. Honestly I never even need to turn on the electric fan unless stuck in traffic.

cruiser54

The only thermostat I use anymore is from the dealer 83501426. Warms up in about 4 blocks.

Here's how to test your TPS. Be sure to use the flat connector/engine side.

RENIX TPS ADJUSTMENT
Before attempting to adjust your TPS be sure the throttle body has been recently cleaned. It's especially important that the edges of the throttle butterfly are free of any carbon build-up.
IMPORTANT NOTE: With the Key OFF, and using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS. The letters are embossed on the connector itself. Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it parallels the valve cover and also over near the MAP sensor on the firewall. If you see more than 1 ohm of resistance, or fluctuation in your ohms reading, some modifications to the sensor ground harness will be necessary. The harness repair must be performed before proceeding. I can provide an instruction sheet for that if needed.
MANUAL TRANSMISSION:
RENIX manual transmission equipped XJs have only a flat three-wire TPS mounted on the throttle bodyand it provides data input to the ECU. It has three wires in the connector and they're clearly embossed with the letters A, B, and C. Wire "A" is positive. Wire "B" is ground. Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors. Note the voltage reading--this is your REFERENCE voltage. Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For example: 4.82 volts X .17=.82 volts. Loosen both T-20 Torx screws attaching the TPS to the throttle body and rotate the TPS until you
have achieved your desired output voltage. Tighten the screws carefully while watching to see that your output voltage remains where it is supposed to be. If you can't achieve the correct output voltage, replace the TPS and start over.
Sometimes, after adjusting your TPS the way outlined above, you may experience a high idle upon starting. If that happens, shut the engine off and reconnect your probes to B and C. Start the engine and while watching your meter, turn the TPS clockwise until the idle drops to normal and then rotate it back counterclockwise to your desired output voltage.
AUTOMATIC TRANSMISSION: RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a flat three-wire connector, same as the manual transmission vehicles have, and it is tested the same as the manual transmission equipped vehicles, as outlined above—FOR ALL ENGINE MANAGEMENT RELATED ISSUES.
However, the automatic TPS also has a square four-wire connector, clearly embossed with the letters A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module. THIS SQUARE FOUR WIRE CONNECTOR IS USED FOR TRANSMISSION/SHIFTING RELATED ISSUES ONLY. Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your REFERENCE voltage. Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage. For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage. If you can't, replace the TPS and start over. So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and the other side feeds the TCU.
FOR AUTOMATIC TRANSMISSION RELATED ISSUES: Check the square four-wire connector side of the TPS.
If you have ENGINE ISSUES check the flat three-wire connector side of the TPS.
For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.
Revised 09-22-2012

Zenroth

So I put a thermostat in today, it was indeed running without one, and that seems to have fixed temp issues as would be expected, still need to replace the fan coolant switch though.

I also took out the throttle body, and completely cleaned it, along with the IAC again as well. Replaced the throttle body with a new gasket, and adjusted my TPS voltage to spec, and did a sweep, the TPS seems fine.

Unfortunately since doing all the cleaning, I seem to be stuck with an idle at 1500-2k RPM's that shifts about in park. In drive its idling right at around 1k. I've double checked vacuum lines, as well as sprayed around the throttle body to see if could possibly have a vacuum leak but also found nothing.

So I'm guessing that either my idle screw needs adjusted (can't seem to find an allen that fits it, and it has blue lock tight on it, which might mean its still factory set) or that my IAC motor is indeed really shot now.

Any possibilities that I might of missed here?

I've tried the general disconnect battery, toggle key on, did do about a 30 minute drive as well, but no real change in the idle condition here.

I also checked my fuel pressure and its 31psi with engine running, and hops up to 39 with the regulator vacuum line disconnected, which seems to be in spec.

cruiser54

Check this out:


Cruiser’s Renix Throttle Body Butterfly Adjustment
Okay. Let's start from scratch. First off, that's not an idle adjustment screw. It's a throttle butterfly stop screw. It's purpose is to allow the butterfly to be as close to completely closed as it can be without binding or wearing into the throttle body. It was never intended to be adjusted in the field. But, Uncle bob didn’t know that, did he?

Engine off. Back off the butterfly stop screw with a 3/32" allen wrench until the butterfly is completely closed. Now. turn the screw in until the FAINTEST movement of the butterfly opening is detected. This can be done more easily with the throttle body removed. If you remove the throttle body, be sure to replace the gasket underneath it after thoroughly cleaning the old one off.
Revised 07/07/2012

Zenroth

Thanks for all the help cruiser54. I ripped back down the throttle body, adjusted the butterfly stop which was indeed quite a lot more out than I ended up with it when adjusted, and also re-cleaned surfaces and I have a low and beautiful idle =)

cruiser54

Cool. Someone's Uncle Bob must have messed with that screw at one time. Were the instructions clear enough? Easy to understand?

Now you need to adjust your TPS, since the butterfly has moved, so it's right on.

Zenroth

Yeah they seemed clear enough to me, certainly a bit easier than having to rip the throttle bodies off of a corvette and drill out parts of them to get access to basically this same type of screw.

Thanks for the reminder about the TPS, forgot about that when pushing it back together at midnight.

Next up is the fan coolant sensor, which from a brief look, looks quite painful to reach and deal with.