Truck runs great, but recently when the engine comes off the enriching circuit. At idle it slowly drops idle speed and stalls. It is not plugs, cap, rotor, TBI is new last year. No codes are coming up. I have
turned up the idle screw in the base plate of the TBI, which does not seem to help, it just goes back to low idle and stalling again, is the a way to get that back to the original factory setting? I am presuming
that screws up the main computer. I open to suggestions, put will start
with replacing Throttle Position Sensor, tighten the 3 screws on the TBI, check for vacuum leaks. Should I be looking at the IAC sensor, like I said I am not getting any codes. I only have this problem when sitting at idle for a couple of minutes, then it starts to drop slowly then stall. Have checked for vacuum leaks and TBI gasket. Can you have a bad Idle Sensor will out have a code. Is there any other sensors that could cause this problem?
I'm no expert, but from what I understand, the computer changes its behavior based on the feedback from sensors including the ones you mentioned, and if one of the sensors is giving out bad information to the computer, the computer isn't going to necessarily throw a code for that sensor but rather will change its behavior based on the erroneous data from the failing sensor, and if it thinks it's idling too high, it'll try to idle lower until it ends up just sputtering out.
Like I said, I'm not an expert and I might just be blowing smoke, so take my advice with a grain of salt. I have a Beretta that had to have an idle relearn procedure after battery disconnects, and it would stall out whenever I would drop to neutral after driving a bit if I failed to do it. Maybe there's a similar procedure for your vehicle? My Beretta is also a '90.