1999 IsuzuTrooper is idling too high while driving.

ECM might be sending the wrong signal to the Idle control Motor. Verify the signal into the motor. A vacuum leak can cause high idle also, listen for air leaks. Just thought of this, make sure something is not keeping the throttle plate stuck open. Debri or some other obstruction.

The most common cause of high idle is an intake air leak. It will cause a lean condition to the O2 sensors and the PCM's solution is to increase the fuel mix resulting in faster idle. It can also cause a Check Engine Light if the idle remains too high for too long. Do a smoke test or soapy water test around the intake manifold and throttle body. If you detect any leaks, seal them and try again.

try clearing the pick up in the fuel tank and check the fuel lines for block ups and even small pin holes on the pickup fuel line can draw air and cause this problem try running it from a makeshift tank see if this solves your problem

Check the Idle accelerator valve located on front side of throttle body, it is either stuck or needs cleaning real bad, this is what controls your idle speed, by letting small amount of air in. It has a plunger on it that moves back and forth slowly to adjust amount of air mixture, it is controlled by computer. Unplug it and just turn on key and check for voltage.This valve gets real gummed up, probably need to take apart throttle body and clean. HUNT

There is a sensor in your car telling your computer to light up your check engine light. When your car drives okay it is usally associated with a faulty switch or sensor. Probably oxygen sensor

check your maf sensor could be dirty or needs replacement

 An electronic throttle control (ETC) system to control an idle speed of an engine includes an accessory that increases a load on the engine and a controller that generates an idle request signal based on the increased load.
Your going to have to get your Isuzu scaned. the code is P1511 which is a defective ECT. Sorry I can't help you with more but it sounds like this may be the problem. 
Good luck and hope this helps

If its the 3.0 diesel it could be the rail oil pressure sensor starting to pack up .

I had the same issue on my 99 Trooper.

It would usually start when completely cold, but idle around 2k RPM.
It would drive fine. But if I shut it down, ran into a store, and came back out, it would start but immediately idle.
Had to ride the gas a little to get it into gear.
The Check Engine light has been going on and off for 6 months or so.

I took this to my local trusted repair shop (no Isuzu dealers within 200 miles).
The diagnosed the light, and found that the Intake Gaskets were bad or leaking.
The idling problem was related to the car not being able to control the amount of air coming in.

They replaced the Intake gaskets by the next day, and I haven't had a problem since. And no more check engine light, which has been on half a year.

Here's some details from my bill:
(1) Intake Gasket set $107.95
Misc materials $21.00
2.1 hours of Labor

The total cost to me was $345.00, and I definately don't feel gipped.

If it is automatic you put fluid in through the dip stick tube for the tranny.If it is standard you put oil in through the fill plug on the upper side of the tranny.Hope this helps