1988 s-15 4.3 v-6 tbi My truck makes the idle air sensor open up all the way everytime I put the shifter into reverse or drive ,and it won't idle down any less in nutral or park. I have already replaced throttle positioning sensor, and the idle air sensor,also temperature sending unit.

K1500 PICKUP. ? 5.7 TBI. GAS
i will assume eng. compression good and spark good and timed by the book.
it's staving for fuel.!
fuel pressure not at spec,
fuel pressure regulator bad.
injector clogged (injectors screens inside packed)
bad fuel (was bad now bad fuel is in all that path)
MAP sensor bad or clogged up. EGR stuck open
meter mAp pin out,1.5vdc out,keyon to 4.5V WOT (wide open throttle)

we test fuel pressure 3 ways
key pressure
idle
and WOT
the manual covers that, buy the service manual?
so the pressure specs for TBI are at hand./?
there is no fuel rail like MPI so has very odd connections to do pressure tests TBI. (below)
you do know most injectors have 10micron screens inside,and only ultra sonic reverse cleaning cures them right? (or new injectors) I send mine out for pro cleaning, (Usonic)

the hard part of 1995 TBI is getting the TBI specs.
the manuals were 500 pages paper and nobody digitized all page or skipped most the Injection pages.
it is a problem but the used books are sold at EBAY
from GM (real)

rumor has it
"In stock form a tbi operates 11 to 13 psi running a bit higher makes them run better to a point of about 14.5"
the TBI if you look close has a FPR, seen here
it must be good.(diaphragm)
some TBI have 2 injectors, start+idle (and fast run)
as seen here

https://www.rockauto.com/en/catalog/chevrolet,1995,c1500+pickup,5.7l+v8,1031443,fuel+&+air,fuel+injection+pressure+regulator,6124


next up how to read the pressure (tap ports)
I think this is a returnless system (early)
so they make up filter tap point. fuel. or TEE fitting.


https://www.reddit.com/r/MechanicAdvice/comments/10nz5rd/best_way_to_check_fuel_pressure_gm_tbi/

The transmission is bad.

there is no idle position sensor, there is an iac,idle air control stepper motor that is located on the tbi,throttle body injection unit,there is a tps on the tbi as well,throttle positioning sensor, so if you are looking for an idle adjustment it is not possible to just turn a screw, first you should check for vacuum leaks,a vacuum leak will cause the idle to go up,the block learn will also be up past 128,use a smoke machine and smoke the engine to help detect a vacuum leak first,no leaks found then reset the throttle body set screw to zero,clean the tbi with a good carb cleaner and a little brush,no biding,check tps for correct value, from 0.6 to 0.8 volts at close throttle and up to 4.8 volts at wide open with no voltage drop or glitches,make sure the pcv valve is working properly,if you have idle quality problems make sure all is good,hoses,cap rotor wires plugs,leaking valve seals can contaminate the air intake and play a role in extreme cases. check plugs for a build up of carbon which indicates this. good luck

If it stalls in drive but not reverse, it could be a transmission problem.
Is the check engine light on ?

tune up,fuel filter,air filter.

Ign module might help.

If you disconnected the battery,the computer needs to relearn the idle profile. This may take a bit of driving.Also,make sure the air intake piping is properly secured to the throttle body and Mass airflow sensor at the filter housing and not allowing the engine to **** in any false air past the MAF

You either have a bad Idle air control valve or your injectors are clogged. Try turning the steering wheel and turning on the AC and fan. If the idle goes down the IAC is bad. A normal IAC should raise the idle a little then settle when a load is put on th engine.

Air in the cooling system is most likely cause. But could be computer, air leak, injectors. (TBI system or FI?) idle air motor.

have them replace the TPS (Throttle Position Sensor)