Hi Scotty....brand now to this kind of stuff so please be patient. Working on a newly acquired 2000 Saturn SL2 automatic. It has 111,000 miles, 205 lbs compression on every hole. The car runs fine…

cobbleup

Hi Scotty....brand now to this kind of stuff so please be patient. Working on a newly acquired 2000 Saturn SL2 automatic. It has 111,000 miles, 205 lbs compression on every hole. The car runs fine MOST of the time, however if climbing a grade and dropping to 3rd gear and revving to about 4000 rpm the engine will start to buck, usually as I start to decelerate ( if i keep my foot in it, it will keep revving higher with no bucking, it's when I start to back off the throttle that the bucking starts and the light flashes). As the car slows and load is lessened the bucking stops and the car seems to run fine again. A P0300 is always set as the "Service Engine Soon" light flashes a few times and stops flashing when the engine stops bucking. I've put new Autolite plugs in, ohmed the wires all at 7 ohms. cleaned the coil connections and ignition module connections (they all looked good but cleaned them anyway. I've cleaned the throttle body and iac, cleaned and tried the MAP sensor then swapped the MAP sensor, cleaned all the grounds and the ECM connector. On level ground the car will get up to at least 105 mph and if I keep the throttle wide open it seems fine, it's when I begin to let off the throttle that I get the bucking. It idles beautifully at 850 rpm. while running I disconnected the IAC and then sprayed carb cleaner all around the intake manifold and couldn't notice any difference in idle speed. My next step is to swap the coils out for known good ones and then the the ignition module if the coil swap doesn't help. also the throttle position sensor reads out a nice smooth increase up to 90% where it maxes out. Do you have some wisdom to share please??? And thanks too. Del H