Engine Stalls when I let Go of Gas Pedal- '92Deville

[acarranza89]

I have a 1992 Deville 4.9L. I have been having problems with the ISC motor and TPS. I have recently installed a new ISC and a used TPS and used MAP sensor. The TPS has been adjusted as it says to do so in the factory service manual. I have also performed the idle re-learn procedure. The car starts and drives fine but sometimes when I take my foot off the gas pedal the engine shuts off. I pull over, put it in park, turn the key back on and it immediately starts with no problem. I currently have no codes on my onboard diagnostics. My check engine light does NOT come on either. In addition, my gas mileage has gotten extremely lower than usual. I use to get about 15-16 mpg city driving and I am currently only getting about 10 mpg! I have already cleaned the throttle body and only fill it with Chevron supreme gas. This is my first and only car so it is becoming really hard to afford to drive. Any help/suggestions on how to fix the stall on slow down and gas mileage problems would be greatly appreciated.

[adallak]

Welcome to the board!

Have you touched the minimum air screw located on the throttle body? After replacing ISC motor you should have adjusted it. Have you done it? Idle learn procedure alone is not enough (and is not that important because the car will learn itself after you drive for a while).

Low fuel economy does not have anything to do with ISC motor. How're the oxygen sensors? You can test them through your driver info center. Check out the FSM.

[Ranger]

Have you checked the FPR for a leak? That will not set a code, but WILL drop your mileage.