67 GMC

1987 Renix 4.0. TPS, plugs, wires, rotor, cap, oil, oil filter, air filter all good or less than 6 months old. Dipstick grounds and C101 refreshed. New fuel pump (2 weeks ago) and fuel filter. Engine runs good at idle, runs rough at 1100-1300 RPM then smoothes out above that. Feels like a timing issue. I replaced the CPS a month ago but it ran similarly before. Would re-indexing the distributor help?

Puzzled as it's running good otherwise. What's special about 1200 RPM????

Thanks

austinjoe13

Did you adjust the TPS according to Cruiser's write up?

If not:

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.

My 89 runs MUCH better, though still idles high. He told me to try adjusting the output circuit to 15% rather than 17%. Another thing to check is vacuum lines, make sure they're all snug. The intake/exhaust bolts like to loosen up too, so tighten them in an outward spiral pattern.