I just brought home a '98 Sport with a 3.9 5-speed, 193k miles. The body and interior is in quite good shape and it runs ok but idles a bit rough--probably needs plugs, wires, cap and rotor. Anyway, it has a few issues, (not surprising considering the mileage), including a consistent problem of having a delayed response to letting off the throttle. When you take your foot off the gas pedal it takes 2-3 seconds before the RPM's even begin to drop. As a result, when shifting normally the tach never drops at all between shifts. If the truck is at idle and you simply tap the gas pedal and release it, the RPM's rev normally, hang up there for a couple or 3 seconds before dropping back to normal. I've attempted to search for a solution but have not seen anyone else with this exact problem. Any ideas?

My truck does the same thing 2000 3.9 5 spd.

Sounds like it might be a bad Throttle Position Sensor. Check the center wire on the TPS with a voltmeter. With the key in the on/run position but engine not running, the center wire on the TPS should have about half a volt. As you slowly open the throttle plate by hand the voltage should slowly and smoothly increase to about 4.5 to 5.0 volts with the throttle held wide open. Any different voltage or erratic voltage readings indicates a bad TPS.

You might also check the Idle Air Control Valve. Remove it from the back side of the Throttle Body and clean the black crusty crud off the pintle end of it with a shop rag soaked in TB cleaner. Don't drop the sensor or handle it roughly or you will damage it and need a new one.

The tune-up items you mentioned may help fix the problem too.

Jimmy

Thank you, Silver. Does it have to be TB cleaner or is standard carb cleaner compatible? Do I need to check/clean other sensors (MAP, or anything else) while I'm at it?

The parts store should have Throttle Body Cleaner for sale. I use plain old carb/choke cleaner in a spray can. I have never used any of the TB specific cleaners but have never had any problems from using regular carb cleaner on my TB or IAC either. I keep all the linkages and pivot points on my TB clean and lubed with spray silicone.

If the MAP sensor was bad you would probably have a significant loss in power and gas mileage. Yours might be OK but it's easy to test it and find out for sure. With the ignition on but engine not running, the center wire on the MAP sensor should have about 4 to 5 volts. Start the truck and let it idle, as the truck warms up the voltage should slowly drop down to 2.0 to a minimum of 1.5 volts. If it goes less than 1.5 volts, the sensor should be replaced.

Unless your throttle cable is sticking, these are the things you might check first. Others here may have more info for you too.

Jimmy

Yes, the first thing I checked was to be sure the throttle cable wasn't binding or hanging, but if you open the throttle manually on the TB and release it, it snaps back to position normally but the RPM's have a delayed response.
The mileage doesn't seem to be suffering a whole lot, but I just got the truck and haven't yet filled the tank after the initial fillup to calculate it. Will probably do so after work today. Like I mentioned, throttle response seems to be pathetic, but the rear axle is a 3.21 ratio compared to my buddies 3.55. It seems that would make SOME difference, but should it that make THAT much of a difference between the two trucks? His is obviously very much quicker-responding. Mine is a '98 3.9 5-speed with 193,000 miles, his is a '99 3.9 5-speed with about 120,000 miles.

I think I have read on here other guys with the 3.21 rear end gears were complaining about them too. Some have swapped out for 3.55's. I have read that is the better gear to have. To be honest I really don't know a whole lot about the rear end gear ratio and which is better for which engine/tranny. My truck has the 3.92 rear end gears and it does OK. It's not the fastest truck on the street but it's good enough for me. I just hope I never have to replace the axle. I change out the differential gear lube every 12,000 miles.

You might want to check the MAP, TPS and IAC. Even if your throttle plate opens and closes OK by hand, those 3 sensors are the ones that tell the PCM what to do with your air-fuel mix and that can have a direct effect the RPM. With a poor throttle response problem like that I think the first thing to check would be the TPS. Then remove the IAC and clean it up. If the TPS has a bad spot in it that can cause idle problems like that.

Jimmy

how about if you idle is up and down between 4-800 rpm while clutch is in and rolling. replaced every sensor. IAC,MAP,TPS, plugs wires cap rotor, pcv valve.... Maybe its the computer....