found a nice '90 chevy truck from an elderly man for 5k...but the problem hes had for many years is the high idle and troubles at basic road speeds...something i noticed is the mapp sensor isnt in use and the rear throttle body vacuum is capped off..when the vacuum is attached to the sensor the engine dies...it starts then dies...but not using the sensor it starts fine but with high idle...i would take it to a local shop but this man lives 60 miles from any town...simple fix or not?
you will need a scan tool that works with OBD1 gm and displays live data stream...i will go over this.. since you don't have a scan tool.... things that can make the system run at fast idle....missing, broken or incorrect temp thermostat... it has to be a 192F ...broken coolant temp sensor.. or broken pigtail connector.. (yellow and black twisted pair of wires) could also be broken insulation of the black and yellow twisted pair i have found this on some cars and truck.. back in the harness... its important to properly test this circuit... when the connector breaks or the harness the ECM thinks the coolant temp is -20F part # SU102 at AZ.. about 16 for the pair... if the coolant temp does not get to 185 in the first few minutes.. it will back the idle air control motor back to about 100 counts to warm the motor.. a worn throttle shaft bore... engine off.. open the throttle part way.. see if the throttle shaft wiggles in the bore.. if it does... it can cause the throttle position sensor at the other end come back to a different voltage every time... this fools the computer into thinking that your foot is still on the gas.. so it keeps the idle air control valve backed out to about 100 counts... sometimes more..her is a list of the sensor data... if you can get a scan tool... look at the engine coolant temp..the throttle position sensor voltage.. make sure it comes back to the same exact voltage each time..look at the map sensor voltage with the key on.. engine off.. should be close to full range.. when started it should drop to about 50 percent of the voltage... as these engines idle at about 15 inches of vacuum... half way from 30 inches ...i have an 87 S10 coming by next week.. a similar fast idle problems.... the only cheep scan tool that i know displays live data is the actron CP9110 other than the high end full professional models... and the older professional models.. the (OTC monitor 2000, 4000, 4000$, genesis .snap on MT2500....
Bet the throttle body to intake manifold gasket has a vacuum leak, very common problem with age and mileage. With the leak the intake vacuum is lowered and the MAP sensor measures vacuum, the lower the vacuum the richer the fuel mixture will be to the point it could be too rich at idle and cause stalling. With no vacuum at the sensor the sensor is out of "normal range" and goes to a default value which the engine will continue to run OK. Not saying this is the problem but have seen plenty of bad gaskets as 13 yrs as a GM mechanic. New TBI gasket, air cleaner base gasket, and fuel line O-rings for I guess $20-30 and an hour of time will wake up this truck. Also a truck of this age more than likely has a lazy O2 sensor but this would not cause a starting problem only when engine is warmed up enough (around 165 degrees) I'm assuming the check engine light is on with a code 34? As Wayne suggested the coolant temp sensor will cause similar problems as an open or unplugged sensor will read -40 degrees and run rich which the MAP sensor will compound the problem and make the idle speed high but more like a fast idle like a carburated engine, I'm assuming the idle is over 2000 rpms which leads to a vacuum leak. Even if it needs a new coolant sensor you're still looking at $50-60. This would set a code 14 or 15. Good luck
+1 bigblockbuick. throttle body gasket,very common. allowing unmetered air into intake,thus high idle.
my bad bigblockbuck.sorry
local repair shop says the engine/transmission is in rather good shape internally and mechanically... they tried to hook a OBD-I scan tool to it and it will not "connect" to the ecm... so they are going to try and chase out the problem to get the ecm to connect to the scan tool... but already the bills $300+ for shop time and labor...and friends recommended for the money iv put into repair shops i could of swapped the top end from EFI to carburated...sounded like a good idea... but is it? and wat kinda problems if any would i run into... or should just continue to let the shops continue...ADVISE?