leichte

My 1987 4.0 XJ seems to have moments where it is self-propelled. Many times when I start it and back out the (flat) driveway, all I need to do is take my foot off the brake and it will zoom backwards at 10-15mph. I've driven up pretty good inclines without putting my foot on the gas. One time I drove about 2 miles (some up hill) and never hit the gas. I was going about 20-25 mph.

Why is this happening? What do I need to adjust?

Thanks for your help!

ThatAintStock00

Check for a vacuum leak.

and out of curiousity, is your AC on when this happens??

jakbob

What is more probable is your TPS needs to be adjusted. It is a renix after all.

jakbob

Actually, do you have a c101 connector over the brake booster?

cruiser54

It's an 87. I imagine he does.



Renix Jeep C101 Connector Refreshing
　
The C101 connector on 1987 and 1988 Renix Jeeps was a source of electrical resistance when the vehicles were new. So much so that the factory eliminated this connector in the 1989 and 1990 models. The factory recommended cleaning this connector to insure the proper voltage and ground signals between the ECU and the fuel injection sensors. We can only imagine how this connector has become a larger source of voltage loss and increased resistance over a period of almost 25 years. The C101 connector needs to be cleaned at least once in the lifetime of your vehicle. Chances are it’s never been done before.

Almost every critical signal between the engine sensors, injectors, ECU, and some to the TCU, travel this path through the C101 connector.
The C101 connector is located on the driver’s side firewall above and behind the brake booster. It is held together with a single bolt in it’s center. To get the connectors apart, simply remove the bolt and pull the halves apart. You will find the connector is packed with a black tar like substance which has hardened over time.
Take a pocket screwdriver or the like and scrape out all the tar crap you can. Follow up by spraying out both connector halves with brake cleaner and then swabbing out the remainder of the tar. Repeat this procedure until the tar is totally removed. This may require 3 or more repetitions. Wipe out the connectors after spraying with a soft cloth.
If you have a small pick or dental tool, tweak the female connectors on the one side so they grab the pins on the opposite side a bit tighter before bolting both halves back together.
　
Revised 07-17-2012

XJ.89.Limited

Mine did that because the TPS was messed up. It had been sitting for years before I started driving it, so it did that from time to time. Now, it rarely does it. Sometimes when I start it up, it revs high and then goes down after a second or two. I just need to do a little cleaning in there. No big deal unless it gets to the point where the brakes won't hold the jeep at a stop. Just get to it before that happens.

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: 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 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.
FOR AUTOMATIC TRANSMISSION RELATED 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.
Revised 05-28-2012

XJ.89.Limited

Haha, thanks cruiser. I gotta see if my dad has a meter to read that with. He has a bunch of meters, I just can't keep track anymore.

cruiser54

A simple multimeter will be fine. Ohms scale to checksensor ground circuit and DC volt scale to check the TPS. Be sure to use the flat 3 wire connector for the test.

XJ.89.Limited

Will do. Thanks!