XJ loses power at 50mph

I have an 88 XJ 4.0, 5 speed that is stumbling around 45-50 mph. It only does this when it is 65 degrees and warmer. When I down shift and floor the throttle it hesitates then smooths out, but once you level off on the throttle it will start to stumble. I have tested and changed the TPS and the AIC, next is the MAP. Does anyone have a suggestion or solution?

My 88 would stumble and buck right around 2200 RPM, when I tried to cruise on the interstate. I fixed two issues at the same time, so I'm not really sure which it was, my TPS ground or my O2 sensor. I replaced the O2 sensor and fixed the ground (at the same time) and the problem went away. Best guess is it was the O2 sensor and the TPS ground was my idle issues. May have been a combination of both.

EcoMike says you can unplug the O2 sensor and make a test drive, run it on the defaults (limp home mode) and see if the problem goes away. I've never tried it myself, but Mike says it works.

In addition to what 8Mud said, too little fuel volume (pump, filter or pressure regulator), exhaust leak blowing on the CPS, dust from the clutch on the CPS, BAD TPS (was it a new one or used one?), CLOGGED CAT, weak spark coil are things I would check.

Have you looked at the rotor, cap and plugs yet?

This does lean toward a CTS or MAT (IAT) temp sensor issue

It only does this when it is 65 degrees and warmer.

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Funny part is the limp more on a jeep works better than normal mode until the Cat burns up.:wow:

8Mud said:

My 88 would stumble and buck right around 2200 RPM, when I tried to cruise on the interstate. I fixed two issues at the same time, so I'm not really sure which it was, my TPS ground or my O2 sensor. I replaced the O2 sensor and fixed the ground (at the same time) and the problem went away. Best guess is it was the O2 sensor and the TPS ground was my idle issues. May have been a combination of both.

EcoMike says you can unplug the O2 sensor and make a test drive, run it on the defaults (limp home mode) and see if the problem goes away. I've never tried it myself, but Mike says it works.

Click to expand...

Mike mentioned the CPS, I've had temperature sensitive CPS's before. The ohms would go up as the CPS heated up and proper function would get iffy.

Best guess is the O2 sensor, unplug it, take a test drive and see what happens. Half a dozen times a stumble or buck (actually feels like a torque converter lock up issue or maybe fuel supply issue) right around 2200 RPM while cruising, turned out to be a O2 sensor failing on multiple Renix XJ's. Not always the problem but often enough to make it a prime suspect.

I've also had a TPS with a dead spot at right around 1/3 throttle. Every time I'd hit that spot and try to hold it (cruising up the highway) the motor would fall flat on it's face.