HEAD SCRATCHER

88 MJ, 4.0, B/A10. I have a new short block installed and have developed a most irritating problem.

Initially the truck starts and runs strong , then when the temp comes up and it is in the transition phase from open to closed loop, it dies when I come to a stop. Once it dies, it does not want to restart with the key; but if I am in a place where I can roll start it, it fires up immediately.

I have swapped out the MAP and IAC from my stash, cleaned the throttle body, checked for vacuum leaks,disconnected the O2 sensor, and re-seated all the relays and connectors with no change in symptoms. My C101 has been removed.

If I feather the throttle when coming up on a stop, and keep the RPM up around 1200-1500, it will stay running.

the last time this happened, I checked the spark at the coil and had a strong spark. I have 4 hole injectors in it and the plugs are running a light gray color. looks a little lean to me.

The strange thing is, once this happens and I get it re-started, it runs fine the rest of the day. I am leaning toward a CPS going bad. Any ideas or suggestions?

Have you tested every sensor ground with an ohm meter, and tested and calibrated the TPS, and tested the CTS and MAT temp sensors for correct values? Any one of those is typical of your problem.

A bad Renix CPS is usually a no start problem.

Also check the vacuum line to the FPR, fuel pressure regulator, if you see or smell gas, the FPR is bad. It will cause your problem.

Yes on testing the grounds, I do that first, every time I have a problem.....

I haven't tested MAT yet but I will tomorrow. the CTS was moved up to the thermostat housing when the short block went in and it is new.

I am beginning to think that maybe one of my splices where I removed the C101 might have gotten stressed when the short block went in, even tho i used aircraft grade environmental splices.

thanks for the suggestions, keep em coming.

I have an 01 mustang and while its obviously a different engine my IAC valve on the TB got gunky. I cleaned it to perfection to save 15 bucks, but still same exact symptoms you have. Got a new one as recommended by mustang forums, never had a problem. What if it were that simple, Haha.

Test your egr system try pluging the vaccum line to the egr vaccum solenoid.

Can hurt to check the EGR too.

Great suggestions guys, keep em coming.....

IAC replaced about 6 months ago, and swapping it out with one from my stash made no difference.

All EGR lines are capped off, but the valve itself is still mounted on the manifold.

One more piece of the puzzle, My knock sensor got broken during the engine install, I haven't replaced it yet....

Several of us have been running Renix rigs with out knock sensors for years, so that is not the problem. It is likely temperature related, which is why I gave you the list below.

Be sure to test the grounds, hot, cold temps, and live to see if there is any standing voltage on the ground wires while running, and wiggle the wires in the harness during the tests too. Some ground anomalies can be temperature and vibration sensitive.

Try removing the IAT and cleaning the tip of it with throttle body cleaner.

You did get your grounds attached tightly to a clean and shiny dipstick tube stud, right?

unplug the IAC and check the connector and make sure the pins are not spread open causing a contact issue, it is a very overlooked common problem

closed the pins on the IAC and re-seated. THOROUGHLY cleaned the throttle body. Drove it almost 100 miles today and it did just fine. Idle is up around 1100 but I can live with that....

Thanks for the suggestions....

Now try readjusting the TPS to get that idle right.


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. THIS SQUARE FOUR WIRE CONNECTOR IS USED FOR TRANSMISSION/SHIFTING RELATED ISSUES ONLY. 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 08-27-2012

I recently had a similar problem. As it turns out it was a coil wire. Try swapping the coil wire with one of the other wires and see if the problem improves.