OK, there's been a number of help needed threads lately so I thought I'd add some more tech information that no one will ever read.
Some of the questions relate to the base idle set screw (BISS) and the throttle position sensor (TPS) so I grabbed a throttle body (TB), a multimeter (DVOM) and a camera and wasted my evening being a nerd.
This picture shows a TB with the throttle plate shut and what the resistance reading through the TPS sensor is for that given position:
The resistance is 4.68K ohms.
This next picture shows a partially open TB with a resistance of 4.31 K ohms:
Notice how the cell phone magically appears now. This is caused by an increase of airflow which is sucking more items in to the TB.
The next picture shows the resistance to be even lower (3.475 K Ohms) as the throttle plate is around 50% +/- open.
The last picture shows the TB plate fully open. Resistance has dropped to it's lowest point of 1.583 K Ohms.
What does all this mean? Well, the computer sends to the TPS sensor a voltage. Usually 5-8 volts. The resistance inside the sensor dictates how much of that voltage flows through the sensor and returns to the ECU/PCM. So at rest resistance is greatest and the TPS signal is going to be low. As you open the throttle plate the resistance decreases and the signal voltage will increase.
What does all this mean? Well, if you read many of the posts I had before this one, in various other threads, then you'll now be able to understand the value of a multimeter when you're trying to see if your TPS (or any sensor) is bad. Additionally, you'll be able to TUNE your BISS and TPS sensors with the information above and below.
Understanding the adjustments on your Throttle body:
Many people see something they can manipulate and they want to do so without understanding the effects. The BISS and TPS are two of those items and messing the them without knowing what you're doing can cause poor idle, poor part throttle performance, poor fuel economy, a hunting idle or other strange problems.
On the turbo and 2.4L models (according to the 99FSM) the resistance between pins 1 and 4 should be between 3.5 to 6.5 K Ohms. To check the sensor's range of operation like the pictures I just posted above you move your test lead to pins 2 and 4. As you increase throttle position the resistance should change proportionally and smoothly. Open the plate very slowly as you will increase your chances of finding a glitch if there is one. "If the resistance is outside the standard value, or if it doesn't change smoothly, replace the throttle position sensor. NOTE: Always adjust the TPS after replacement."
There is a second portion of the throttle position sensor called the "Closed throttle position switch". To check this put your leads on pin 3 and pin 4. With the throttle plate open you should have no continuity (Should show up a 0.L meaning open loop or infinite resistance). With the throttle plate shut you should show 0.00 ohms of resistance.
It's important to check this circuit when you're adjusting the BISS and TPS. The TPS sensor has a wide range of resistance BUT the computer typically only has 4 different modes of throttle range each with their own purpose.
Cruise/idle mode for little to no throttle input, acceleration mode for light to moderate acceleration, Deceleration which the ECU/PCM reduces or shuts off injectors, and WOT for heavy or all out acceleration.
If you have an improperly adjusted TPS sensor then you may be allowing in enough air for cruising but the signal from the TPS puts the ECU just beyond cruise and in to the acceleration mode. Likewise if you're sitting at idle and the TPS is adjusted too far then it may think you're trying to accelerate when you're really not thereby wasting gas. Also, if it adds fuel for an acceleration but you're just idling it's going to keep fighting o2 sensor feedback and TPS input.
There's several problems with having so many adjustments and they frequently show up when the car is in the hands of someone not fully versed in the nature of the beast. Things like vacuum leaks, improperly adjusted throttle cables and dirty throttle bodies typically get overlooked because they are never mentioned by the guy with a crystal ball helping you "Fix" your car.
A dirty throttle body like this one:
Is basically starving your engine for air. The carbon, oil, and dirty build up is blocking airflow required to maintain your idle quality. This is a big reason why people start messing with the BISS in the first place. They don't clean the throttle plate, it starts lowering the idle as it gets dirtier and because some Ass Hat on the web said "Twist this cool screw" first people create a bigger problem when they finally start learning how to do things right.
A cleaner throttle body like this one:
Will improve air flow thus improving idle quality.
This is a Base Idle Set Screw on a 420A TB:
While the TPS on the 420A is fixed and non-adjustable it's still possible to create a problem with it. When you have a dirty throttle body and you twist on the BISS not only do you increase airflow to bring idle back up but you change the throttle position sensor signal too. Now, every time you crank the car, you're starting out at a higher TPS signal then you should.
With an adjustable TPS sensor it's possible to increase airflow while changing the TPS sensor to show that it's even more closed than it was before you messed with the BISS. What this does is create a condition where the manifold pressure sensor sees more airflow than what the TPS is reporting it should have. It really gets crazy when you have so many adjustments and you don't know which is further out of adjustment.
