1990 TBI 5.7 Idle surging,stalling

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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?


Re: 1990 TBI 5.7 Idle surging,stalling


cabopeter wrote:
Quoted text here. Click to load it

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.

Jody

Re: 1990 TBI 5.7 Idle surging,stalling


cabopeter wrote:
Quoted text here. Click to load it
Initial symptom might have been an EGR valve stuck open.

--PJ

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