Hi there,

I have a mechanic working on my 1981 C3. I had him install Dougs side pipe headers. They look and sound great. While underneath the hood we discovered a litany of issues including a harmonic balancer that was cutting into the timing cover. Anyway, I had asked him to see if he could make sure the ECU was operating as is should. I just received the following email from him:

"I need to know (positively) if the ECM in this vehicle will function properly without the air pump electronics and the EGR valve electronics. I have read several web sites giving all kinds of different views. The wiring that was disconnected (cut) at the engine was attached to the air pump electronics and the EGR valve electronics. I do not know if the ECM has the ability to know if these things are there, if it can not sense their operation then their absence would make no difference and the oxygen sensor would have to compensate for the absence of what they do. Please check with your Corvette group and try to get an answer.
Have not heard back from the person with the scan tool yet. Mixture control solenoid tests good but the ECM will not make it pulse, should have OEM style oxygen sensor tomorrow afternoon, will install it and see if that lights up the ECM to control the mixture."

If anyone here can help we are at a dead end apparently.
Thanks
Mike Fesi

 

I believe the consensus is that it will run like garbage and throw codes if you make changes like that. And that's just from the headers. That's why lots of folks revert back to the 1980 spec, or upgrade to EFI and/or LS swap.

You can read codes with a paperclip, no need for a code reader.

 

81 ECU and a modified engine just don't match up.
80 and earlier HEI properly curved to the engine/gearing you have now, and a earlier carb or a EFI conversion.
Otherwise your just chasing your tail.

 

The ECM has no way of knowing if the EGR or AIR is there.....but it does know they are plugged in.....so you can remove the pieces but the actual A.I.R. diverter valve and the EGR vacuum solenoid have to be plugged in........
Just lengthen the wires and install them plugged in out of the way somewhere unseen......


Jebby

 

you really need a Carburetor and a 1980 or older distributor, a 1980 carb and distributor would be a direct bolt in or aftermarket system , it will be hard to calibrate your carb with the headers to get the mixture solenoid dwell right

 

This is true.....the ECM has a pretty tight "window" of adjustment and all that can be done is get a PROM chip to move that window....that is what the aftermarket PROM's did...that and some timing parameter tweaks.....but even then.....the timing will never be optimized for performance............
Converting these cars to Pre-1981 spec is pretty straight forward......but whoever does it needs to have a strong knowledge of how pre-computer cars work.....
You have cut wires and the mixture solenoid will not cycle so your system is pretty much dead, if the ECM is not functioning, it is not controlling any timing, the car must feel way down in power......it would be a huge PITA to make the CCC work properly again.......

A nice HEI distributor, a good Q-Jet done by a builder like our member Lars here and a little tuning will reap huge benefits here......and major jumps in power and fuel economy......

Jebby