CCABJ Racewerks

Hi all,

I'm new here and I just picked up a $500 '88 cherokee 4.0 that runs very very rich. It drives ok but it literally almost dies it is so rich. I am thinking it is either a throttle position sensor, O2 sensor, maybe a MAP sensor? At idle, the revs vary and it feels like it's going to die. Getting terrible gas mileage. It couldn't be an injector, could it?

On an unrelated note, the tranny (auto) must be shifted manually from 1-3 because it won't do it itself. I have been doing some research and it seems like it could be the TCU? Can anyone confirm? I have a feeling that's what it is but haven't had a chance to mess with it yet.

DFlintstone

Hey, A Renix! For a start you might want to check the vac. line to the map. VERY important that has no cracks. You can pull it off the map and check for vacuum.

You might want to check your TCU fuse below the glove-box.

CCABJ Racewerks

Thanks, I will check the vacuum line in the morning along with the TCU fuse.

cheers

ZINGGG

Very good ideas flinstone's got there. As far as running rich, I'd definitely start with the upstream o2 sensor

T-VIS

my 89 limited was idling like that also when i got it, turned out the FPG was REALLY shot, shot to the point when i removed the vacuum line and gas was spitting out like crazy. just a pointer to add to the rest.

cdf43

i have an 87 xj that was having fuel trim issues, it ended up being the O2 sensor.....id also test the TPS to make sure its reading correctly

cruiser54

First things I would do. Welcome to the world of Renix.

CCABJ Racewerks

Sorry I'm new to jeep lingo, what's the FPG?

thanks for all the other responses guys.

tjwalker

1. Test your fuel pressure. A ruptured fuel pressure regulator can leave you with a rich mixture. Gotta use a gauge; no shortcuts. At idle, you should have 31 psi. Now disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approximately 39 psi. Be sure that vacuum line is pulling good vacuum!

2. Test your map sensor. Directly involved with fuel/air ratio

3. Replace your oxygen sensor if it's old. They have a finite lifespan. I recommend NTK, the OEM supplier of oxygen sensors to Jeep.

Bustedback

Best advise yet. I've seen a fuel pressure regulator that had a ruptured diaphram and raw fuel was being sucked into the intake through the vacuum hose. It ran just like what the OP described.

wildcat

what about the knock sensor?

Bustedback

The knock sensor (if it has one) only retards timing, it does nothing for fuel.

CCABJ Racewerks

Thanks guys. i just picked up a TCU for free and got a MAP sensor and throttle position sensor free also. I will see what these do and report back later if they don't do anything. I can't work on it or check it til monday but after that i will have it at my house. Thanks for all the suggestions, I will definitely do these.

cruiser54

If you're gonna replace the TPS, use these instructions to adjust it:
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.
Revised 11-28-2011

T-VIS

my bad that was sent from my phone, should have been FPR (fuel pressure regulator)