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stumbling in overdrive

89eliminator

NAXJA Forum User
Location
Delphi, IN
89 4.0 renix. has normal power under acceleration and thru all gears and all RPMs. however, once it gets into overdrive, it starts stumbling really bad. is this a TPS issue?
 
Mine had a bad stumble or bucking at right around 2000-2200 RPM, only happened on a warm motor (above 140-160 F.). Turned out to be an O2 sensor issue. It sure acted like a torque converter issue, but wasn't.

Before you spring for a new O2 sensor check the harness really closely where it runs down the front of the motor to the O2 sensor, knock sensor and engine temp. sensor. When somebody changes out a water pump they tend to route the harness wrong during re-installation and it cooks on the exhaust manifold.

It could be somthing else, like ignition or the TPS, but the TPS is likely to act up in all gears and not just overdrive, most times the first noticeable issue with the TPS is odd shift times, either way to late or early. May even be a vacuum issue, the intake/exhaust bolts tend to loosen with age.

The first step I'd take is to adjust the TV cable just to eliminate it as a possibility. Does it go into passing gear (drop to a lower gear) when you stomp on the gas?
 
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yep, it will kick down when i accelerate at speed just like normal. i havent done any motor recently (months). it will do it when the motor is warm or cold. as soon as it goes into OD is happens.
 
Does your volt meter twitch or drop off when it shifts into fourth? I'm having a bit of brain lock here and really can't think of much that would cause issues, just in fourth gear. All I got is some maybe's.

Low vacuum on a cylinder or two that is causing a lean condition, that cases bucking. Tends to be more noticeable when you are cruising at a constant RPM and may be hardly noticeable as you accelerate through the lower gears.

Ignition usually causes a poor idle and/or breaks down at higher RPM's.

Try riding the brake just a little, just enough to turn the stop lights on. This will disconnect the torque converter lock up and take it out of the equation as a possible issue.

I guess it is possible a tranny shift solenoid has low resistance and is sucking the voltage down to other motor management components. And/or you have a voltage issue.

The TPS is two parts (two separate TPS made as one unit with the automatic transmission) on the Renix, the transmission half and the ECU (engine) half. The engine half has a lot to do with spark advance and may be your issue. I've seen a few failure modes in the TPS, first is poor ground, second is it is full of water, solvent or oil, the third is it has a dead spot, fourth is the connector pins for the TPS are corroded, covered in oil or coolant. In my experience if it is going to act up, it wouldn't be just in fourth gear, but maybe it is a very specific chain of events that makes it most noticeable in fourth gear?

I'm trying to get a sense of whether it is a tranny issue or a motor issue, this can sometimes fool you.
 
it seems to be the TPS. i had one in my parts pile that i put on. I went for an hour drive and the stumbling issue never happened.
 
Having the same issue in my 95 (AW4) just put a new TPS in her about two months ago. Didnt start happening again till i put a new (after market) muffler on.
 
So here is where I am, new dist, cap, rotor, plugs, plug wires, put stock exhaust back on minus cat, still getting the stumble, in all gears.
 
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