'89 renix motor fuel issue while cold

1989 MJ, renix 4.0 approx 200k with a manual trans.

I'm not sure where to start the diagnostic procedure. I'm pretty positive it is a fuel issue.

I fire it up on a cold start, it will crank, fire and idle normal and I can take off and its fine.

I then drive for about 1/4 mile or less and I push in the clutch to come to a stop the idle will drop and catch itself. I go to take off again and any bit of throttle I give it whether in gear or neutral, it will bog the engine down and almost kill it, if in gear it is much worse and usually does kill it. If it dies, I can fire it right back up and the issue is completely gone

When I continue driving it (if the engine doesn't bog down and die) I will select each gear and it will do the same thing-bog down, but then catch back up after a slight back fire out of the intake. This issue goes away after about 5 mins and won't come back again until a cold start.

obviously this a fuel issue, but should I start by checking fuel pressure? and if I do and it indicates a lack of fuel pressure, where the hell would I go next?

thanks in advance everyone!

Check fuel pressure. 31 PSI at idle, 39 PSI when you pull the vacuum hose off the fuel pressure regulator. When you pull the hose off, check for gas in there. Shouldn't be any.

Check to see if your intake manifold bolts are loose causing a vacuum leak.

Your Throttle Position Sensor could be bad or out of adjustment causing your problem.

I will check the fuel psi, thanks for the proper pressures for it. I had a separate idle issue before this and have already ruled out a vacuum leak and tps.

Which side of the TPS did you use for testing/adjusting? The flat connector or the square connector?

it was long enough ago not to remember. I will run a test on it again. which connector should I use? and it should range from .5v-5v?

If you're dealing with a driveability problem, use the flat connector. If it's a trans/shifting problem, use the square one. Unfortunately, most instructions have you using the trans connector to diagnose driveability problems. Use these:

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.

Joshooha said:

.....and I push in the clutch to come to a stop the idle will drop and catch itself.

Click to expand...

cruiser54 said:

Which side of the TPS did you use for testing/adjusting? The flat connector or the square connector?

Click to expand...

I'm gonna assume he adjusted the ECU side of the TPS (since it wouldn't have anything connected to the TCU side even if it was replaced with an automatic TPS)

throttle body gasket and intake gasket can "cure themselves" once the intake manifold warms up, making it tough to diagnose anything after a cold start-up.

a failing CTS can tell the ECU that the coolant is warmer than it is, and result in a lean misfire until warmed-up (or a restart at a temp. closer to the initial temperature reading from the CTS)

You got me on that one, Shorty. Been up for 19 hours so far today.

I like your idea on the intake gasket or throttle body gasket. Never a bad idea to snug up the intake fasteners before starting the engine some morning.

16.5hrs at work today, so far-- I'm still bright....

I do like your write-up on adjustment procedure, though. Definitely beats the standard "check this site" procedure for setting the TPS!!

Thanks Shorty. I'm working on an outline of instructions/tutorials aimed at the Renix Jeeps covering many of their quirks. I oughta send it to you.

cruiser54 said:

Thanks Shorty. I'm working on an outline of instructions/tutorials aimed at the Renix Jeeps covering many of their quirks. I oughta send it to you.

Click to expand...

I agree, you should send it to me:clap: In about another year and a half, the Chrysler era Jeep is going to the daughter and my Renix is going to return to daily driver duty. It's been in the reserves and is getting called back to active duty:wow:

I love the tps write up man! thanks! Now with a failing CTS...isn't there 2 on this motor? one in the block and one in the head?

There are two temp sensors--one in back is for temp gauge only; one in front is for ECM only. It's the front one you want to check.

Temp guage SENDER is at the back of the head. Coolant temp SENSOR is on the lower block, left side.

cruiser54 said:

Thanks Shorty. I'm working on an outline of instructions/tutorials aimed at the Renix Jeeps covering many of their quirks. I oughta send it to you.

Click to expand...

^^^^^ me too please :smootch:

I had a similar issue and it turned out to be the resistor for the fuel pump.

I've only done a few things that are in the outline. I'm working on getting them all done. I'll send it to ya though.

Renix has FOUR temp sensors. One is coolant, lower drivers side block under the manifolds, one in the cool side of the radiator, which is a thermal switch to turn on the E-fan, the rear drivers side top of the head is the sensor for the dash gauge idiot light or temp gauge, and the fourth is an air temp sensor (IAT), in the rear top of the intake manifold, that works with the MAP sensor to estimate mass of air flow into the combustion chamber.

The CTS or IAT could be the problem (mine was the IAT). The fuel pressure regulator leak is unlikely as it would not go away after warm up. TPS likewise, would not usually go away after a warm up.

The O2 sensor is a possibility, as it needs the 12 volt feed to the O2 internal heater to work right at idle on cold start ups!!!!! If there was a vaccum leak that was temperature sensative near the MAP sensor vacuum source or line it could be the cause, like a loose connection on the vac line at the throttle body?