weird idle stall issue 1990 suburban 350 FI TBI

so my dad has a 1990 TBI 350 burban.....and he's having this issue that the idle is dropping too low at lights and stalling but if he keeps his foot on the pedal while stopped and boosts the idle its fine...it has a little hesitation at around 75mph but it goes without a problem, we've replaced the IAC, TPS and no change it still stalls out, also replaced all the vacuum lines and such and it doesn't seem to be that...was wondering what else there is we could I believe he told me he changed the throttle body gasket but can't be sure

The pickup coil in the dist can become corroded or the magnet can be weak causing a loss of signal at lower RPMs. Upper shaft bushing can also be worn.

Probably cheaper to replace the dist than to replace pickup coil and/or shaft (and magnet).

//2000CAYukon

would a bad pickup coil really cause low idle ? I figured i'd be looking at vacuum issues somewhere or bad injectors I mean wouldn't a bad pickup cause a misfire condition

I want to make that coil as your head....

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would a bad pickup coil really cause low idle ? I figured i'd be looking at vacuum issues somewhere or bad injectors I mean wouldn't a bad pickup cause a misfire condition

It can cause a low idle but yes you would probably feel a misfire.

If you have access to a scan tool, you should see if the Engine Coolant Temp sensor is correct. Also see if the MAP sensor is holding a vacuum.

Fuel pressure should be 9 - 13 PSI; however, you need a special tool to test it.

//2000CAYukon

I have a 1989 Chevrolet with the same motor and I was having the same problem except with alot of hesitation and I replaced the Coolant Temp. Sensor in the front of the manifold and it fixed the problem. That may be the problem with ur dads Sub