I apologize if this has been talked about before. But, what is happening is that about once a month or so, after long trips, my 3.9 idles high (about 1,000 Rpm) then slowly drops down to an unusually low idle (about 500 rpm). Then the next day i went out and started it up again and it revved itself up to 1,500 RPM and held it there for about a minute. It then settled back down to a normal idle (800 rpm i think) after which i tapped the gas to see what it would do, it revved up to 1,500 and held itself there again for another minute.

What i'm wondering is, that, could it be a bad throttle position sensor causing this ? or is it something else ?

 

Oh. Almost forgot. My truck is a 1999 regular cab 3.9 5 speed manual.

 

Sounds more like it might be a bad or dirty Idle Air Control valve. Have you cleaned it and the throttle body itself? It could also be the TPS, with the ignition switch in the run position but engine not running, probe the center wire at the TPS wire connector plug with a voltmeter. With the throttle completely closed you should have about .5 to .8 or .9 volts. With the meter still connected, slowly open the throttle by hand. The voltage should increase steadily to between 4.5 to 5 volts when you get the throttle wide open. Anything less or if the voltage is erratic as you open the throttle means the TPS may be bad. Usually though with a bad TPS you have trouble getting up to speed and keeping the truck running at a steady highway speed.

Jimmy

 

Thanks for the reply, I will check the tps tomorrow when i get up. As far as cleaning the IAC or the throttle body, I don't think they have been touched since it left the factory in '99 haha. This may sound a bit newbish but where exactly is the IAC located ? and what would I clean the IAC and throttle body with ? Thanks again.

 

IAC is bolted to the back side of the throttle body with two T20 Torx head screws. Best way to clean it is unplug the electrical connectors from the IAC, TPS and the MAP sensor, disconnect your throttle cable and cruise cable so you can take the throttle body off the manifold. Remove all 3 sensors and clean the TB with TB cleaner and a shop rag. You can buy throttle body cleaner spray at the parts store it comes in a spray can just like WD 40 or brake cleaner. Do not use carb cleaner spray on your TB. They are two different chemical compounds and carb cleaner is too harsh for use on a TB. Clean the TB very well especially the throttle bores. Dirt and carbon collect in there and the plates don't make a good seal which causes problems with the idle. Clean the black crusty junk off the pintle end of the IAC and lube the shaft with a little 3 in 1 oil. Do not move the shaft in and out though. It will damage the stepper motor. Spray the TB linkages with silicone spray lube. Reinstall the TB with a new gasket which only costs about $2.00 at the parts store. Do not drop your IAC or handle it roughly or you could damage it. While the TB is off the manifold it is a good idea to have a look inside the manifold with a flashlight to be sure you don't have a leaking belly pan gasket.

Jimmy

 

Thank you. The only thing I can do right now is check the TPS with my dmm, cleaning the throttle body and sensors will have to wait until i get enough money for it (broke college student here :/ ). As far as the belly pan gasket.....its been leaking for a while, don't have the money or time to fix it right now unfortunately. Thanks again for the help.

Edit: If I had enough money i'd just throw a V8 TB on it with new sensors :P

 

FWIW... I also had idle issues with the engine RPM surging or dropping too low when I came to stop. I cleaned the IAC and it improved for a while, then it started up again a month later. I checked TPS with multimeter and it was within correct range but I had a few people here tell me to change it anyway. I listened and got a Mopar unit on Ebay for roughly 60 bucks. Not only did it resolve the problem but immediately, the truck seemed to pull smoother from a stop with a lighter pedal feel.

 

Getting a new IAC or TPS will have to wait, i only have enough (had to ask my parents, which i absolutely hate having to do) to buy the throttle body cleaner, and the new throttle body and IAC gaskets. Hopefully it fixes the problem long enough for me to get a job and just get a V8 throttle body and new IAC, TPS, etc.

 

Understood. Hopefully the IAC clean will do the trick for you. Good luck and keep us posted.

 

Well, it seems to have worked. When i started it up after i got it put back together. It revved up like it normally does and dropped right back down to its normal idle, instead of holding it at 1500 RPM. Took it for a drive around campus and it didn't rev itself up at all. I'm glad all i had to do was clean it. Thanks again for your guy's help !