Werejackal

Hi im new here ive been looking over all the misfire threads and cant find what im looking for,

so heres my issue My 1990 4.0 jeep cherokee, will start up rough, smooth out and idle fine as soon as you go to give it gas in park or in gear it will backfire out the exhaust and run like crap no matter how much gas is given

the things ive replaced
Cps sensor,
Cap, rotor, plugs, and wires.
ive also checked all the vacuum lines and they r fine

im stumped

kjc

You might check the IAC and the TPS. The IAC pintle gets pretty carboned up and can be cleaned. (i read something about NOT twisting or pushing it in/out while cleaning) The TPS can be tested for smooth operation with a VOM. Pull off the connector and probe 2 of the 3 terminals (i forget which two) and move the throttle slowy from closed to wide open. You should see the needle move smoothly across the scale. Analog meter works best here, but a digital one will work too. If that checks out good, you can also hook it back up and back probe it with the key on (volts scale on the meter) This shows if it's getting the proper signal from wherever the signal comes from.. you might want to search for the 'how to test TPS' article I found on this site for better instructions. Been a while since I did it Hope this helps

Werejackal

ive checked the tps the referance voltage 4.82 and the output voltage is .62 the other thread says the output is suppose to be .81 and to adjust the tps till i get that but i dont know how to adjust the tps

tjwalker

Backfiring out of the exhaust can indicate a "rich" mixture. Two things that could contribute most to this would be something like a ruptured fuel pressure regulator or a faulty map sensor.

If this were my vehicle, this is how I would proceed. You must TEST your way to a solution here......don't throw parts at this.

1. Test fuel pressure with a gauge at the rail. You should have 31 psi. Now disconnect the vacuum line leading to the regulator and the pressure should jump to approximately 39 psi. You must use a fuel pressure gauge here; no shortcuts. They can often be rented from many big box parts stores.

2. Test your map sensor. Also remove the vacuum line leading to the map sensor and be darn sure that the vacuum line is pulling good vacuum

3. More on the RENIX throttle position sensor below.
---------------------------------------

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 one 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 other 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 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 four-wire connector clearly embossed with the letters
A,B,C, and D.. For the automatic transmission equipped vehicles the four-wire connector
provides data to the TCU.

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.

VTJeep

Possibly o2's! The IAC would more likely be tied to high idle issues, but is still possible. The TPS is a definite possibility. If backfiring from exhaust it's runnin rich and if it's from manifold area it's running lean. Check all air, fuel and spark delivery to ensure proper firing...