This is my first post to hopefully find a solution to my idle problem. What I have is a 1227747 ECM based TBI mounted vertically with an adapter to a 1960 era Rochester fuel injection manifold on a 336 CID Chevy small block in a 1960 Corvette. It runs real sweet except when using the engine to decelerate to a stop the idle drops to 500 and I get a near stall (sometimes it does) before it “catches” and goes back to its IAC controlled 900 RPM setting. This happens only after deceleration of the car with the engine. If I’m cruising at say 2500 RPM and just push in the clutch and let off the gas at the same time it drops to perfect idle. I’ve been using a CATS $42 tuner program along with 27C512 chips to try different program changes without much success.
I did not like the CATS replacement chip socket. While time consuming to replace the original socket it provided too many chances to break static control during installation and (I presume) I consequently fried the ECM and had to get another. I bought some cheap 28 pin chip sockets with the same style output pins as the 27C512 chip, soldered output pins 1,2,26,27 and 28 together and using some plastic spacers epoxy glued it into the existing ECM socket. This allowed me to do rapid chip changes (without the GM chip holder) also without having to make up another ECM access cover because everything fit under the stock plate. Since removing and replacing the cover screws so frequently eventually strips the screw holes I also installed rivnuts which have more thread support.
What I’ve tried so far. Open Loop decel AFR enabled, on and off. Open loop idle AFR enable below 1100 RPM. Up and down idle AFR. Max throttle follower IAC steps up and down. Idle IAC set at 900. I might add that the cam is, what I would consider, mild because it idles reasonably smooth particularly when warmed up at about 16” of vacuum. I noticed, in the data stream, that even though it was supposed to be doing open loop idle (as best as I could interpret the setting) I was still getting a closed loop flag. I changed the closed loop enable temperature to 230 deg F to force it open loop all the time. Ran the same with a slight improvement in the post decel idle. Been going up and down on the Max AFR at open loop idle and Lean idle AFR at open loop and as I said, a bit better but not great.
Needless to say I’m new to this electronic tuning and am looking for advice. What other information do you folks need and any suggestions to try?
I also might add that it only has TPS, coolant temp, MAP, and spark count (RPM), and O2 sense, no VSS, knock sensor, etc.
Reply With Quote
Bookmarks