1989 Buick LeSabre 3.8lt...free car but in good shape (sat for 2 yrs,but was started every cpl months)
She drives awesome, just had CV axles replaced....
Since we got car, she goes really good, cruise works etc.
However...when we are going down a hill, let off the gas to "coast", the idles "stumble". They'll be at 2500rpm (for eg) in OD, let off to coast & idles drop to 900 Rpm, back up to 2500 rpm, then back down, repeatedly...until we apply gas & she'* fine again..almost like she thinks she has a jake break It seems to always do it, whether we touch the brake pedal or not...more so if we've touched brake...but does it either way
It does this with or without cruise on...have tried getting codes read, as SES light intermittently comes on, but doesnt change how she runs. Its not holding any codes to be read
She idles perfectly, no stumble, drives great & has awesome fuel mileage.
Have been told to try spraying carb cleaner into TBI..but kinda leery on doing that, dont want 2 cause another problem..any thoughts?

The IAC controls the idle speed. (intake air control) try removing it and clean it with TB cleaner.
Have u done a tune up new plugs, wires etc?
You could look towards a vacuum leak broken cracked rubber line or even spray throttle body cleaner around the intake body to see if there is any rpm loss / leaks while running
Other suspects are throttle position sensor or mass airflow sensor.
Could a stuck EGR related issue cause this? look for testing it on that car.

one other is the pcv valve....cheap to replace it. Could be plugged.

One more..have the battery load tested and ensure your charging system is putting out good voltage at idle under load. Computerized car dont like weak batteries and lower voltages.
I would check the voltage while driving by using a connection inside the car when it stumbles.

i would suspect something with the egr, thats when its programed to activate

Ya I hear ya on the EGR Justin.( Exhaust gas re-circulation )
If the car sat for 2 years what did you do about the fuel and fuel filter..change it.
Try getting some water remover in the fuel as well as a serious dose of Sta-Bil ethanol fuel stabilizer or marine Sta-Bil into the fuel too. Run some super high octane in there initially or even a non ethanol fuel with the Sta-Bil and see if it runs better.
I also wonder if the SES light was on what kind of scanner is being used. A pro shop with a good scanner may be able to find the stored codes if Angie was using home scanner.

Thx for thoughts The car was ran every cpl months while it was parked..when we got it, filled tank with new gas,filter new, plugs cap/rotor/wires new/battery new
The SES was read at a shop with code reader, & we were told the car isnt holding or sending any codes to their reader...dunno..will be checking PVC,air lines EGR....just dont wanna be digging to far into it, as we have winter upon us..may just drive her as is till spring.....hate cold & snow for mechanicing in driveway

try unplugging the egr and checking the tubes for cracks, you will get a code for that but if its not sealing all the time hopefully the symptoms will change and you will know its that

I just want emphasize to anyone reading this thread how important it is to use an ethanol STA-BIL fuel additive when storing any of your gasoline engines or vehicles. Sta-Bil makes the fuel good for only one year, without the fuel souring or phase separating. The other problem is that even a 10% ethanol based gasoline will attract and hold moisture, so also using a water remover additive would be greatly beneficial. In the case where a car has sat for 2 years without any Sta-Bil treatment, I can only image what gummy, sticky crap is sitting on the bottom of the tank and none of it is good news. Really that tank should have been drained and refilled before starting and I would have also added Sta-Bil to the new fuel before starting.
In a recent post topic re ethanol fuel see post # 18 This video shows examples of the fuel in jars after it has been sitting untreated.
https://www.gmforum.com/lounge-99/re...-305919/page2/
Adding fresh fuel to the old gummy fuel is not the best idea. If that was my car I would add a full bottle of Sta-BIL MARINE or ethanol treatment, 2 bottled of STP water remover, and two bottles Chevron Techon Concentrate with the highest octane fuel and drive that car on a serious 300 mile highway boogie to get the fuel injectors/ intake clean etc.

Treating then Cleaning =