Hey guys,

My truck's actin' up: erratic rpm's, revving up and down, as if it was between gears , but all the time. I thought it may be the Throttle Position Sensor, or.... Let me know what you think.

It's been doin' this for quite some time now, I switched fuel stations where I fill up, I changed the fuel filter and pre-filter. I'm not sure what else to do.

Let me know

 

The first question I have is, Are the rpm's actually changing or is the tachometer drifting?

Tach drift is common, and can be corrected by cleaning and adjusting the sensor on the front of the crankshaft (right above the vibration damper). If that is the problem, clean it and then use a matchbook cover folded over as a feeler guage, reposition the thing and tighten it with a bit of blue loctite on the mounting threads.

Now, if it is your engine actually speeding up and slowing down as you are driving in gear, but it runs just fine when in neutral or park, I would suspect the main drive clutch is going. However, it could be the converter lock-up, or mis-adjusted bands in the transmission. If the problem is worse in cold weather than hot weather, check the band adjustment.

If the engine is speeding up and slowing down when in neutral or park, check your overflow valve (located on the inside of the injection pump next to the engine block, and is where the fuel return line is connected).

If the engine is speeding up and slowing down under load, and it is hard to start after setting overnight, then check your fuel lines and the heater electrical plug for air leaks.

The throttle position sensor is only used to signal the transmission, so unless you are getting a connect and disconnect from your torque converter lock-up clutch or constant shifting from the overdrive, it is unlikely to be the throttle position sensor. This is pretty easy to tell, as it is like you are shifting the transmission up and down constantly, and the engine will run just fine in neutral or park.

If the truck is down on power, and the idle speed has dropped below the setting listed on the CPL (found down on the front edge of the timing gearcase, below the injection pump), and the rpm's drift when it is just sitting there idling, look for severely dropped timing. If that has happened you will need to retime the injection pump.