charlie2sods

Fellows I have an issue with my 1990 Chevy S 10 tahoe pickup, it is 4.3, auto transmission it was idling at 1200 rpm, the tps voltages are where they should be, I have checked for vacuum leaks, found none, when i check the aic, two terminals show infinity straight across upper and lower but diagonally or vertically no reading at all. the aic is closed and when running and idling the throttle plate appears to be closed after finding the idle adjustment screw i got the rpm down too 1000 and when you put it in gear it is about 650, isnt 1000 still too high, and what could be the cause of the high idle with the throttle place shut when it is idling, i even unpluged the aic while idling to see if that made any difference. if anyone can help this old guy with this it would be appreciated.

charlie

Irish_alley

unplug the map sensor, unplug the vacuum line see if you hear any difference in the motor then unplug the sensor itself

charlie2sods

i did all that and no change, but for some reason the idiling went down to 900, and in gear about 650 what do you think, and thanks for responding,

kevinkpk

Is the vacuum hose removed from the MAP have a good vacuum, I am asking as to determine if the vacuum line has a hole in it somewhere.
If not, I'd suggest take the MAP out of the scenerio, in other words, plug the vacuum line, unplug the electrical connector, drive it, see if that makes any difference.

Irish_alley

did the idle go down before, during or after messing with the map sensor?

charlie2sods

in the beginning it was idling about 1200, the next day i went out and was going over it and i noticed it had dropped to where it is now,have no idea why, today the rpm didnt change after unhooking the map sensor, i checked the vaccum after unhooking the map sensor and it was 20 , i was mistaken about the rpm, it is between900-1000 and 725 when in gear. i have sprayed brake cleaner around and dont see any change as of yet, i did see that the vaccum line was unhooked on the egr valve. i have tried it with the aic unpluged, the tps unpluged, and the map sensor, i cant find a vaccum leak if there is one. the throttle plate looks like its closing good, and i dont see any gap around it, i lowered the idle screw until it didnt touch, it appears to run good but just seems to be idling a little too fast,

Terminal Frost

It's idling to fast because the IAC is either working correctly or stuck in a partially open position. Live ECU data will be able to let you in on dirty little secrets. Find someone at a parts store that knows how to use the scanner and access live data. All you need to do is read it. Look at it or take a picture of it then you can share
TPS voltage should be from .60 idle to 5.0 volts WOT - .7 and 4.9 aren't going to kill you. as long as its close this isn't your issue. You can resistance test the TPS doing a sweep watching for any anomalies.These are adjustable and we like them as close to .50-.60 volts as we can get them. This IS base idle setting. I think in percentage it's like 10% ??? 1.0 volts at idle is a maladjusted TPS and it will cause issues. I do not believe it'll get below .40 volts but because of the idle screw you touched... lol
Check the readings on the coolant temp sensor. Reading cold causes the ECU to enrich the mixture causing high idle. This will also change with outside weather temps. On warmer days it wont idle as high. This IS THE BIGGEST PROBLEM CAUSING ISSUE I witnessed in my automotive days with Chevy TBI's and the engine management systems. Yes, a simple coolant temp sensor mis-reading. Please this is NOT the temp sending unit.
Another thing to look at - remove the air cleaner and the spacer so you can see the injector spray pattern. with engine running shine a flashlight through the side of the spray pattern, is it a cone (v-shape cone) shape or very spotty? to much fuel, even a minute amount will cause the O2 sensor to read rich causing the IAC to open up further causing a high idle...
You should have single wire O2 sensors- if able to view live data these should move from lean to rich at idle - if not suspect a bad one.
You can also wiggle the throttle shaft and watch for ANY movement up and down. This can cause a high idle as well. Chevy TBI's were bad about this.
Map sensor - take a hand vacuum pump, hook it up, start the vehicle and apply vacuum to the MAP- engine RPM's should change.
Any exhaust leak in front of the O2 sensor needs to be fixed

These items I've mentioned WILL NOT ALWAYS cause a DTC (diagnostic trouble code aka check engine light)
The "idle" screw you touched. Does nothing but set throttle blade position. It'll need to be reset :-)

Charlie, I've worked with and owned many of these Chevy TBI's during this era.
My 2C

charlie2sods

thanks bud, it will take me a little time to digest all this, but i will work on it, going to be cold here for a bit but i will get back after working with this , thanks again

charlie2sods

one thing i forgot to mention, and i dont know if it is important or not but thought i would mention it, the idle was a 1200 in the beginning, when i changed the idle screw it went down to 900-1000, thanks for the time you took to write all that information it will help me a lot.

Irish_alley

did your cell come on after unplugging the MAP?
ive wrote up how to test the sensors on the TBI trucks. also check your grounds/ do you notice black smoke out of the tail pipe?
Troubleshooting sensors and how to test