Engine Stumbles At Around 2500-3500 Rpm, New Comanche Owner

[Chknjdn]

I recently purchased a 1988 Comanche for my daughter to drive and as a project for the two of us.  It has been interesting figuring stuff out together. 

 

I have been trying to get it to run well enough to teach her how to drive manual transmission.  So far, I have replaced the TPS, fuel pressure regulator, fuel filter, and the fuel pump.  It has gotten better, but it still doesn't want to run smooth.  The idle seems to be at a point that it almost stalls when I don't have my foot on the accelerator, then when I do get it on the road it accelerates normally up to aroudn 2500-3000 rpm area then starts to act as if it isn't getting fuel.  It runs rough, bucks and surges, even occasionaly backfires a little.  When I am able to get it to go past around 4000 rpm, it smooths out and seems to run like a champ with lots of power.  It's just that area inbetween that I can't figure out.

 

Any Suggestions on what could be causing this?

 

Thanks,

Chuck

[FrankTheDog]

Check your fuel pressure before you do anything else.

[Egm89]

Or the vaccum hoses, all of em! Try to check for any loose/crack/rotted vaccum lines, seems to me that thats the problem.

[cruiser54]

Chuck, did you adjust the new TPS?

 

Click on the link in my signature and do the ground refreshing, TPS adjusting if it hasn't been done, the C101 cleaning and the connector/relay refreshing. Known issues and a good dirty job for the daughter. Win-win.

[Chknjdn]

I did adjust the TPS when I installed it in accordance with the Haynes manual that I have.  I looked at the grounds and they are on top of the fresh paint job, so I will take those off and make sure it is a better connection than that.

 

I checked the fuel pressure again.  It starts at about 32 psi when the engine is first started, but then slowly falls to around 25 psi.  I pulled the vacuum line off the regulator, but it had no effect.

 

As far as the vacuum lines go, they appear to be good, but I have a question.  On the back of the carb there is one line that goes to the MAP sensor, but there is another hole there that has no connection nor is is capped.  Is this correct?

 

Thanks for the suggestions, sorry for the delayed reply.

 

Chuck

[cruiser54]

Fuel pump is dying. You need to set the TPS with my instructions using the flat 3 wire connector.

 

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 only a flat three-wire TPS mounted on the throttle body and it provides data input to the ECU. It 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. Loosen both T-20 Torx screws attaching the TPS to the throttle body and rotate the TPS until you
have achieved your desired output voltage. Tighten the screws carefully while watching to see that your output voltage remains where it is supposed to be. If you can't achieve the correct output voltage, replace the TPS and start over.

Sometimes, after adjusting your TPS the way outlined above, you may experience a high idle upon starting. If that happens, shut the engine off and reconnect your probes to B and C. Start the engine and while watching your meter, turn the TPS clockwise until the idle drops to normal and then rotate it back counterclockwise to your desired output voltage.

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, as outlined above—FOR ALL 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 square four-wire connector side of the TPS.
If you have ENGINE ISSUES check the flat 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 09-22-2012