1990 XJ-Texas Edition

Hey all, I'm new to CF but not to Jeeps. I'd like to say that I'm coming here to ask ya'll what tires I should upgrade to, or what lights are the best, but I have a more serious problem. I have a high idle speed, but only when the engine is warm and has been driven for a while (5-10 minutes~). I have done some research here, and found another who had the same problem, and his turned out to be his IAC and TPS were out of whack.
https://www.cherokeeforum.com/f2/ren...-still-106800/
I'd really like some honest advice into how I can fix this.

I can read schematics (and understand them) for a $1.5 million wind turbine, but schematics for the RENIX engines baffle me.

cruiser54

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: 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.
If you see more than 1 ohm of resistance 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 a three-wire TPS mounted on the throttle body.
This manual transmission vehicle TPS provides data input to the ECU. The manual transmission
TPS 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. Adjust the TPS until you have achieved this percentage. If
you can't achieve the correct output voltage replace the TPS and start over.
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--FOR 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.
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. If you have TRANSMISSION issues check the four-wire
connector side of the TPS. If you have ENGINE issues check the 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.


1990 XJ-Texas Edition

Thanks for the reply, I really do need to pull the throttle body off and do a thorough deep clean of it. I kinda did that the other day, and I ruled out that it is a vacuum leak causing it to happen by using the "plug-the-air-tube-and-see-if-it-runs" method and it died. And I am having tranny problems, but they are most likely from it being old and neglected, the main one being that it takes about 10-20 seconds to shift into reverse on a cold start.

JohnWongFat

One thing about Cruiser's instructions is that it doesn't explain exactly how to adjust it. They are extremely helpful though, I just used them and once I figured out what the heck I was doing, it fixed my TPS problem. On the TPS, there is a lever that needs to go under a bar on the throttle body for the thing to work. Once you have that part right, you just kind of shimmy the TPS to get the proper voltage and cinch it down. There's not a lot of room to work in there, I found a 90 degree screwdriver helped a lot. Good luck, I'm still chasing my vacuum leak!

cruiser54

I know you used the "plug the air tube" method but while you've got the TB off, check to see that the butterfly is completely closed by holding it up to the light. Someone may have messed with the the butterfly adjusting screw in the past not understanding how this system works. The screw is there just to provide a stop for the butterfly so it doesn't wear into the bore of the body.

cruiser54

I never even thought about the instructions not addressing HOW to adjust the TPS.
I guess they're more directed to what the correct values need to be and assume the user knows to rotate the TPS for adjustment.

Pinch off your brake booster line with plieers and see if your idle goes down if you've alrady sprayed everywhere. Wouldn't hurt to check the butterfly stop screw as I explained to Texas ^^ in the above post while you're at it.

1990 XJ-Texas Edition

Would a vacuum leak inside the brake booster cause it? Or is that even possible?

cruiser54

Sure is, hence my previous suggestion to pinch off the line with pliers and see if the idle changes.

1990 XJ-Texas Edition

Hmm..see I thought you were saying that the fitting that goes into the brake booster might be leaking, but now it makes a little more sense. Would you be able to hear a vacuum leak in the booster from the inside as a slight hissing when the pedal is barely depressed?

JohnWongFat

I pinched it shut, but I didn't notice a change in RPMs. I imagine I could always pull it completely and stop it that way... I tried spraying both sides with throttle body cleaner and the RPMs stayed the same. I do hear a slight hiss when I press the brakes, particularly when cold. Otherwise, there are vacuum tubes on the underside that I can check too, correct?

cruiser54

Well, your booster is not causing a vacuum leak.