Re: Tuning parameters PART II
Throttle Position Sensor Adjustment
The TPS sensor is used to tell the ECM how far open the throttle blade is open in order to assist in determining how much fuel should be injected at any given moment. This is not the only tool used. Air flow, engine load (LV8), etc. are also calculated and input for fuel control.
TPS adjustment is another area where folk lore has long persisted with no basis in fact. The sensor may be adjusted for idle voltage and wide open throttle voltage.
Before we start, there is still a lot of misinformation floating around from the the past regarding settings for the TPS. It comes from before we understood how it worked. Don't be mislead by someone that tells you that there is some single magic number which is required for the car to run properly.
Note that some programmable chip makers specify a certain window for both the idle range and the wide open throttle range when using their programmable chips. These guidelines must be followed in order to correctly program their chips. Read the instructions that come with the chip and follow them in order to ensure proper function with these chips.
Let's start with wide open throttle settings.
The factory programming shuts down the AC compressor at 4.0 volts so the compressor is not working when you go wide open throttle. Therefore, you should exceed 4.0v for this setting.
I normally set mine somewhere in the range of 4.2-4.8 v if I have no instructions from the chip maker.
Performance does not change whether the setting is 4.2 or 4.8v.
If you have too high a wot tps voltage, then you will throw a code 21 (see trouble codes under troubleshooting section so it is best not to set the wot tps over 4.8 even if it will go higher.
Rather than worrying about why you only have 4.48 volts at wide open throttle when your buddy told you it should be 4.65v, it is more beneficial to check that the throttle blade has moved as far as it can when you put the pedal to the floor. Look at the throttle blade lever when the pedal is pressed to the floor and see if it has gone all the way to the stop signifying that the blade is wide open.
If it is not all the way open, there are generally three possible reasons as to why it has not.
a) The floor mat is restricting the pedal from traveling as far as it should
b) The throttle valve cable from the transmission has been adjusted too tightly and is not letting the the lever move as far as it should
c) The throttle cable has stretched and is not moving until the pedal has moved a short distance. If you get down in the floor board and look at the pedal bell crank where it connects to the cable, you can see the slack, if it exists, between the ball on the end of the cable and the bell crank. An easy fix for this problem is to put a small tie wrap around the cable between the ball and the bell crank to take up the slack. This will make the throttle blade begin to move as soon as the pedal moves and should restore full blade opening.
To determine the source of the problem, grab the throttle lever and move it by hand...
If it goes all the way and the blade is open, or nearly so...then it would appear to be a cable stretch problem.
If it goes all the way, but, the blade is substantially a distance from being wide open...removing some off the lever for more throw should help. I have never seen this problem but, I guess that anything is possible.
If it does not go all the way and make contact with the throttle body housing, then be sure there is nothing obstructing the pedal such as the mat. If there is not, then disconnect the tv cable and see if the lever will now move the rest of the way. If it does, then you know the problem is with the tv cable.
Now, let's look at the idle TPS setting.
It does not matter with regard to idle quality whether the idle TPS voltage is 0.38 volts or 0.46 volts.(The factory spec appears to be 0.40 +/- .05v.....in other words, 0.35-0.45 v) The ECM looks at the initial voltage when the engine is started and creates a reference mark for fueling so there is no difference in fueling whether the voltage is 0.38 or 0.46. Now, if the voltage is above 0.46, the ECM will think the engine is not in the idle mode and will change the fueling, thereby messing up the idle. Normally, when you turn the key to "Run" (engine not running) and check the TPS voltage, you will note the setting such as 0.42 volts. Now if you start the engine, you will see the voltage now reads 0.44 volts. For this reason, we usually try to set the voltage no greater than 0.44 in order to be sure it does not slip out of the idle range and confuse the ECM when the engine is started.
So how do we adjust TPS voltage?
Turn the key to "Run" but, do NOT start the engine. Loosen the two bolts that hold the TPS in place enough that the TPS can be moved. Note that the TPS is slotted so that the sensor may be rotated as well as moved fore and aft. (The top slot affects the idle setting and the the lower slot affects the wide open throttle voltage) Push the entire sensor as far forward as possible so that the two bolts are up against the back ends of the slots. Then, rotate the top of the sensor CCW a bit until your scan tool indicates an idle TPS voltage of say 0.38-0.44 volts and tighten the retaining bolts so that the sensor cannot move. Note that the idle may change as you move the sensor. Don't be deceived that you have changed or improved the idle. When you shut the engine off and restart, the idle will go back to where it was before you moved the sensor unless the sensor was out of range to begin with. As stated prior, the ecm rezeros the idle at each start.
The ecm does not look at tps voltage when adjusting fueling. Rather, it looks at the tps percentage of full scale. Whatever the idle voltage, it is auto-zero'd to 0 percent when the car is started. Therefore fueling is not changed whether the idle voltage is 0.38 or 0.46 as the computer is feeding zero percent and commanding the fueling specified for idle fueling. Once the engine is started, and the tps set to zero percent, any movement of the tps is reflected as percent change even if the change is within the idle window checked when the engine was first started.
Having set the idle voltage, now recheck the wide open throttle voltage. It will probably be around 4.5 v. If someone tells you that you should file the slots so you can get more voltage out of it, ignore them. Do not set the high end voltage to more than approximately 4.8 v or you may get a tps error code.
After finishing, slowly open the throttle while watching the TPS voltage on the scan tool. It should increment smoothly and steadily with no dead spots or jumps which would indicate a problem within the sensor.
If you do not have a scan tool yet, you can use a digital voltmeter. Put the positive probe on the dark blue wire coming out of the TPS sensor and the negative probe on a good ground point such as the intake. With the key on, you will read the TPS voltage.
One further note on the TPS voltages. If you adjust the IAC, turning the screw CW will increase your TPS voltages and the opposite will reduce them. Be sure that you don't let the TPS get out of the idle range.
