For my '89 MAF car, the specs call for 0.540 volts +/-.75 at idle and aprox 4.0 volts at WOT. I have my car set for .570 at idle. Rotating the throttle gets it to about 4.2 volts at WOT. I've already cleaned out the IAC and set the min idle to maintain 25-30 IAC counts in PARK with the car at around 200°F coolant temp.

But what actually happens when the TPS is adjusted to different levels? It seems some of the burnt chip people have it set for about 0.64 at idle. Others aim for .540 exactly. I'm guessing there are 2 different ways adjusting this could work.

1) Very little is changed. Just the gas pedal position where PE is engaged. Otherwise as long as you're within reasonable idle spec range, nothing else happens.

2) More things changing. Idle AFR, fuel economy, off idle hesitation vs more acceleration when taking off, decel pulse width of injectors.

Driving on the highway, I get an INT of 128 and BLM of 135. Around the city, I get 128 INT/140-145 BLM. At a stop light, it goes to 128-135 INT/155-160 BLM. I'm not sure if this has anything at all to do with the TPS setting.

I've been getting lousy gas mileage lately and can't figure out why. I've replaced or adjusted the following: Plugs, plug wires, fuel pressure set at 48 psi (vac hose off), base timing at 6°, cap, rotor, PCV valve, fuel filter, O2 sensor, no codes set, vacuum holds steady at 16.5 inches at idle. I shift at about 2000 RPM between gears (auto), maybe 2500 RPM while merging onto the highway. And despite changing everything listed above still only manage to get about 14 mpg per fill up. I'm stumped.

The car feels a little dull at idle. I'm not sure if it's the warm weather, but "cold" starting it (at 80°F outside), it will hang at about 500 RPM for awhile before picking up to 700 RPM or so. Warmed up it's at 600 RPM. Stepping on the gas pedal it's a bit hesitant. Voltage meter drops at long stop lights or heavy traffic. Nothing new there as it's just too low for the alternator to really charge. This is common in the summer here. Not an issue during winter. Once I'm moving, the volt gauge acts like it should. Essentially it feels lean at idle.

Getting back on track, what is the purpose changing the TPS setting?

 

Your basically right. Essentially, the ECM needs to know at what position the throttle is at for features like power enrichment, decel mode, EGR/carbon canister controls, and also to set trouble codes. Earlier models the the .56 is what the ECM understands to be "closed". Later years the ECM just measures the voltage while starting and assumes that is the closed voltage. If it was off a little bit it's not a huge deal as long as its working properly.

 

There is a min and max idle tps voltage or it will set of the CEL