hypedonmonster

ok so i have posted more on this topic but i want to branch out and get more people to help because im screwed if i cant get my ONLY car running this weekend. my 1988 cherokee 4.0 automatic runs great at idle like it is supposed to with no starting issues at all but once it gets up to running temp it sputters, and backfires through both the intake AND exhaust, and wont go above 15 mph. and when i pin the throttle it doesnt help at all. i tested my cps (crank position sensor) and it didnt read any a/c volts at all when unplugged from the harness and cranking the motor. but it reads .8 volts when i back probed the harness/cps when the motor is idling and the voltage goes up with the rpm's. is this a bad cps? i could really use all the help i can get!

F1Addict

I would say your symptoms are not consistent with a failing CPS. Not sure why you were reading zero volts but if it starts and idles fine then maybe it was just an issue with how it was probed.

Given that you say it idles fine, starts fine, and that everything seems normal up to about 15 MPH I'd say you've got an issue with fuel delivery. That could be the ECU, vacuum line, injectors, fuel pump, filter, TPS, pressure regulator, etc. Wait for others to chime in with their opinion before you go throwing parts at it.

Radi

.8 at idle is good, normal to increase with rpm's.

Is your EGR valve operating correctly? ie: not stuck partway open?

Honestly I'm completely lost on what you've checked/replaced based on the other thread... if you haven't already, check each of your sensors for proper operation:
http://www.lunghd.com/Tech_Articles/...iagnostics.htm

F1Addict

That test doesn't work with the CPS plugged into the harness. It's performed with the CPS disconnected so that any voltage present is generated by the CPS and only the CPS, not from somewhere in the harness.

tjwalker

The two engine mgmt. sensors that come to mind include the TPS and coolant temp sensor. But there are certainly other things on the suspect list.

1. Verify fuel pressure. Must be done with a gauge. For an 88, you should have 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approximately 39 psi. Be sure that the vacuum line is pulling good vacuum and is not wet with fuel (ruptured regulator).

2. Test and adjust your "throttle position sensor". Very important sensor to ensure proper fuel at throttle. How to do that shown below.

3. Start testing other engine management sensors starting with the coolant temp sensor that provides input to the computer (not the one for the gauge). The link Radi provided is a good one for that.
----------------------------------------------------
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. 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 has 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.

hypedonmonster

Ok, you guys are being helpful but I probably should have put more info in haha.
1) fuel pressur at idle=38psi pres with vac removed=42psi and that's on the replaced fpr, and there is good vac and no fuel in vac line(s)
2) I checked my tps and adj it and it's fine
3) I tested my CPS while it was connected, AND while it was disconnected.
4) and i can drive it all the way up to 115 mph when cold, it's only when it gets warm that it spotters and won't go above 15mph
5) replaced my cts, iac, fpr, injectors, no clogged lines, new fuel filter, good fuel pump, good pump b-resistor, new distributer, cap, wires, plugs.
6) can I swap ecu's from my bros 88 renix 5 speed 4.0 to my 88 renix aw4 4.0?

Radi

Good catch. I missed that.

hypedonmonster

Thanks for the help guys I fixed it, it was my crank position sensor.