I'm having some issues. I have a 2000 xlt 4x4 ranger. While driving I take my foot off the accelerator and the rpm's automatically go to idle. When I aply the accelerator pedal and get to the proper rpm's for the speed the transmission re engages. The transmission feel smooth when it shifts and has no issues with o/d. At times there's also a faint hum as well. Also on cold starts it starts normal. When warm the idle goes up then drops almost dies then goes up again. It's driving me insane. The last 3 years I have had a bent rod, cracked piston, cracked heads, and later 2 blown head gaskets. About 2 months ago I had the motor completely rebuilt.

 

Not sure if this matches your symptoms, but have you checked and cleaned the throttle body yet? For a few bucks, go and buy some throttle body cleaner (not carb or choke cleaner). Remove the throttle body, then spray it down with the cleaner, paying attention to the backside of the throttle body plate (butterfly). Carbon build up over the years builds up, and causes poor idle issues.

It's a cheap fix to many idling issues, and should be done every couple of years.

If this is transmission related (hard to identify in your description of the problem), perhaps an automatic transmission fluid change is in order. Sounds like you have an automatic transmission. If the fluid is old (when's the last time it's been replaced?), it may have lost it's additives in the oil making for smooth precise shifts.

There's all kinds of debates as to flushing automatic transmission fluid that have long service life between flushes, so if you don't know the transmissions service records, then try just dropping the transmission pan, replacing the filter, then adding the required quarts for a pan drop, and see if it improves.

I like total flushes, as they remove ALL of the old automatic transmission fluid. Ford states today that most automatic transmissions have "life time" fluid; this is simply silly, unless one looks at lifetime as the length of time an automatic transmission will last without a fluid change. I choose to flush mine every 30K. However, some have bought used vehicles with high mileage, then reported they got a fluid flush, and transmission problems arose from it, stating the crud within the automatic transmission was dislodged during a flush, and plugged something in the automatics valve housing.

All new flush machines used in the "better" transmission repair shops use a low pressure flushing machine, so this is not an issue. IMO, it's best to remove all the fluid and replace with fresh oil. A pan drop is only going to net you about 3 to 5 quarts of fresh oil, but it's better than nothing.

Other possibilities to an idle surge are vacumn leak s(when the engine was rebuilt, were all the rubber lines replaced?), and bad air intake sensor (MAF - mass air flow), the tube that runs from the air cleaner box to the throttle body having cracks, thus allowing "unmetered" air into the intake, causing the fuel measure to be incorrect.

 

I replaced the tps sensor and the iac valve and it fixed everything. Both parts cost less then $100.00. No more rpm drop when driving and no more irratic idle on warm starts. I had allready replaced the catalytic converter, and the mass air sensor. It's like having a new truck again.