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Weird Idle/Startup: Known Issues

Dammerung
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Dammerung Comanche Aficionado

Dammerung

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I’ve made multiple posts about the idle on my 1988 4.0 liter, and now I want to list all my symptoms that I believe may be from a single overlying issue.

Firstly, on startup my truck will sometimes hit 2000-2500 RPMs. Other times it will hit around 1500 and come back down. I have replaced the idle air control valve but I used a junkyard part. It’s unknown if it is also bad, but replacing it seemed to have lowered the frequency of high rpm startups.

Next, when idling, my RPMs move between ~550 and ~800 RPMs. I thought the idle air control valve would help in this case but it has not.

Also, when in reverse, my truck will sometimes drop RPMs further, bouncing between around 300 and 700 RPMs. I don’t know what may be the issue in this case, but I performed a cleaning on the oil dipstick stud as well as redid the eyelets, which did not help my idle.

My truck was also “modified” with a trailer brake unit that I mostly removed. It’s also unknown if that has affected anything.

 

I’m at my wits end about this, and I don’t know what to do anymore.

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WahooSteeler MJ Maniac

WahooSteeler

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Have you disconnected your IAC while running to see if idle changes at all? Also, check your exhaust manifold bolts for looseness. I had similar idle issues and on another "Idle problems" thread here saw someone mention this. Sure enough all of my bolts took a couple turns and at least 2 of them took several turns to get good and tight. Idle is now consistent 750-900. 

 

On another note, really helps if you put a signature on your account with year, engine & trans info at the least as it helps people make suggestions that may be model specific. :L:

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Dammerung Comanche Aficionado

Dammerung

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Have you disconnected your IAC while running to see if idle changes at all? Also, check your exhaust manifold bolts for looseness. I had similar idle issues and on another "Idle problems" thread here saw someone mention this. Sure enough all of my bolts took a couple turns and at least 2 of them took several turns to get good and tight. Idle is now consistent 750-900. 
 
On another note, really helps if you put a signature on your account with year, engine & trans info at the least as it helps people make suggestions that may be model specific. :L:

I checked the manifold bolts and they all seemed tight. I’ve removed the IAC and my idle will run very high, so I think it at least has an effect on idling performance. Also sorry for the signature, I’m using this on iPhone and still haven’t figured it all out. I have a 1988 4.0 liter with the AW4.


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Dammerung Comanche Aficionado

Dammerung

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As in removed the wiring plug with the truck running, or removed the entire component (leaving a large hold in the throttle body) with the truck running?

Unplugged. Should have specified.


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Dammerung Comanche Aficionado

Dammerung

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As in removed the wiring plug with the truck running, or removed the entire component (leaving a large hold in the throttle body) with the truck running?

Just retried the test and now my idle hasn’t changed. Plugged in or unplugged it still idles the same.


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Dammerung Comanche Aficionado

Dammerung

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Okay, I replaced the IAC and it had some affect. When I first started up, it hit idle RPMs and stayed there. But it still bounced around as usual during idle. I’ll try cruisers tip, and I’m gonna reclean the c101 connector on the firewall.


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Dammerung Comanche Aficionado

Dammerung

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Any improvement?

Probably oughta check the TPS adjustment now. 

Slightly, I think. Idle when parked is at the mark below 1000, so around 700. It still bounces (not as badly though), so it’s not entirely finished. Also, when I put it in gear it between 500 and back to the first mark. I don’t know what else there is to be honest. It’s still seems like a better improvement over what I had before.

 

 

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cruiser54 ComancheClub Legend

cruiser54

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RENIX TPS ADJUSTMENT

 

Before attempting to adjust your TPS, there are a few things that need to be done.

  1.  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.
  2.  With the KEY OFF, and using the positive (red) lead of your ohmmeter, set on the lowest scale, probe the B terminal of the flat 3 wire connector of the TPS. The letters are embossed on the connector itself.
  3. 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. It is covered in detail in Tip 6.

TPS ADJUSTMENT FOR ENGINE ISSUES

Both Renix manual and automatic transmission equipped XJs and MJs have a flat three-wire connector to the TPS which provides data input to the ECU. The three wires in the connector are clearly embossed with the letters A, B, and C. Wire “A” is positive. Wire “B” is ground. DO NOT UNPLUG THE CONNECTORS!Backprobe 2

  1. KEY ON, measure voltage from “A” positive to “B” ground by back-probing the connectors. Note the voltage reading–this is your REFERENCE voltage.
  2. KEY ON, back-probe the connector at wires “B” and “C”. Measure the voltage. This is your OUTPUT voltage.
  3. Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For example: 4.82 volts X .17=.82 volts.
  4. 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.
  5. 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.

TPS ADJUSTMENT FOR AUTOMATIC TRANSMISSION ISSUES

Renix automatic transmission-equipped XJs and MJs 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 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.

First off, DO NOT UNPLUG THE CONNECTORS!

  1. KEY ON, measure voltage between “A” positive and “D” ground by back-probing the connector. Note the voltage. This is your REFERENCE voltage.
  2. Back-probe the connector at wires “B” and “D”. Measure the voltage. This is your OUTPUT voltage.
  3. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage. For example 4.8 volts X .83=3.98 volts.
  4. 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 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. The square 4 wire connector is just not used.

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