Wide Open Throttle Conditon

[Randy in Maine]

I am a1990 MJ 4WD 4.0L longbed 5speed all pretty much stock.

 

It has been running pretty well lately. Last week I changed the plugs, cap and rotor, changed the oil  and have about 60 mles on it since.  Has been running a bit smoother and pushing about 22 mpg.

Today I was doing some errands and all of a sudden it just died on me,  Tach went to zero like I had shut off the key.  I pulled over and had a quick look under the hood and nothing seemed to be amiss.   

 

I restarted it and the engine went to 2500 RPMs and just stayed there.  I shut if off and repeated it about 3 or 4 times and the same 2500 RPMs.  I looked to see if the acceperator pedal was hung up or something and it was not.  I had it towed to my "fix it guy".

 

I am sort of leaning to a problem with the throttle positoning switch or something similar but not sure if I have one or where it might be located.  I would buess it would be on the front side of the throttle body where there are 2 electrical connections. 

 

Can someone point me in a direction to fix the problem?

[Randy in Maine]

Well I have downloaded this and willhead over to the fix it guy's place with my VOM and do a little diagnostics on my own.

 

http://api.viglink.com/api/click?format=go&key=c3c53278e61f21f919d3e084737fceaf&loc=http%3A%2F%2Fwww.jeepforum.com%2Fforum%2Ff11%2Ftech-faq-links-tsbs-89273%2F&v=1&libId=2fe001e0-b2a6-487f-b47a-59a065bfac50&out=http%3A%2F%2Fwww.lunghd.com%2FTech_Articles%2FEngine%2FBasic_Sensors_Diagnostics.htm&ref=http%3A%2F%2Fwww.jeepforum.com%2Fforum%2Ff11%2F&title=Tech%2FFAQ%20links%20and%20TSBs.%20-%20JeepForum.com&txt=http%3A%2F%2Fwww.lunghd.com%2FTech_Articles%2F...iagnostics.htm&jsonp=vglnk_jsonp_137306200286510

[jeephack]

I had a problem similar to this some years back.  Jeep idled at 2500-3000 rpm every time it

was started and would go no lower.  After much dead end testing, I discovered that a section

of the wiring harness under the dash had come loose and fallen across the accelerator cable

next to the firewall, not letting it fully retract.  This is probably not your problem, but it can

happen and only takes a couple of seconds to check

[Randy in Maine]

Well I will certainly have a look at that to see if that is the problem.  Thank you!

[ComancheKid45]

Most likely a failed/failing IAC but also could be a TPS issue. Have you ever cleaned the throttle body or atleast had a look inside? 

[Randy in Maine]

I have not looked at them yet, but will be soon.

 

I will post up "the fix" in a couple of days.

[cruiser54]

You really should do a ground refreshing, sensor ground test, and connector/relay refreshing before this below. click on the link in my signature for the info you need.

 

^{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 body and 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}

[Randy in Maine]

Well the testing confirmed that the IAC was indeed  faulty.  Replaced it for $65 and now it is running well again.  Not a lot of carbon in there.

 

With 212K miles I don't think the warranty will cover it.

 

Thanks for the advice.  It was spot on.