Welcome to the Corvette Forums at the Corvette Action Center!

ECM problem?


Well-known member
May 21, 2003
'81 Black
Well this is a long one!
I just rebuild my E4ME almost to the letter put it on the motor and warmed everything up. After that removed the aircleaner and tried to get the dwell as close to 30 as I could but there is the problem. I also have my dual A/F meter in the exhaust and the A/F meter has some simple output that it is sending to the ECM so it can control the lean/rich situation.
Now the problem. Dwell is always at 25! The needle is searching a little (maybe 1 or 2 degrees total). But my A/F meter is saying i'm running rich at 11.5:1. So I turned the idle screws out until the mixture was around 13:1 to 15:1. The vacuummeter is around 20Hg (sometimes a little more) and I checked for vacuumleaks (when it was running lean) no problem there. When I'm driving slowly acc. to 30mph the mixture is at 12.5:1 (not 14.5 as it should be) so it is running rich again. When going back to idle the mixture is leaning out to around 14:1.

When I was back at the garage I looking inside the carb from the top with a bright flashlight and could see the plunger isn't moving! And that would explain the rich condition when driving. So I took solenoid out of the carb and replaced it with another one (I tested before putting it in!).
On this one the dwell meter still said 25 degrees well it was a horrible idle with a 18:1 mixture. When I "flash" the carb with my hand (to decrease intake air) the idle was going up. This means the A/F meter is right! Checked again with my light and the plunger was moving up and down (could also hear the sound. Again set idle to around 14:1 and dwell still at 25. Driving around and again the same thing in the beginning I see the mixture around 14 or 15:1 but after a few minutes I'm back at 12.5:1. And yes after checking the plunger again dead in the water (at full rich)

First I was thinking the dwell meter was broken, now I'm hoping the dwell meter is broken... When I set the key to start the engine the ECM (check engine) light is working and after starting it goes out. The ECM light is never on (even when I disconnect the CO sensor in the exhaust the light doesn't come on). When I remove the connector from the solenoid at the carb then the light goes on (but the mixture stays the same...)

So if somebody can point me into a direction please do as I think another ECM or bad wiring are my only options right now.

Greetings Peter
Small update. Took the A/F ratio stuff out yesterday (after that I still had no changes). Today I put some 300 miles on the vette so when I was driving it back into the garage I was thinking, well hotter then this isn't possible. So I connected the dwell meter and now it was at 40 (still hovering around 1 to 2 degrees). When opening the throttle I can see the meter going to 30 so something is working (same thing the day before). So I put the idle screws 2x 1/2 turn IN, no change in dwell. Put a cloth on the intake to rich up the mixture. Still 40 dwell.... Hmmm. Well is it the ECM that is the problem or the PROM... or something else?

Greetings Peter
I'll begin with reading the closed / open loop chapter in the service manual :)

Greetings Peter
sure hope you find your problem. Remember you are the specialist in ECM carb and stuff.
Unfortunately, I can't be much help (since I'm still learning from you) but don't worry I have my own problems right now with my solenoids not kicking in. Aren't these cars fun.
Best of luck....and I'll keep reading. :beer
I'd like to better understand how you are measuring some of the characteristics you're talking about.

Describe your "dual A/F meter". Is this a gas analyzer or a display device which uses oxygen sensors?

Also, you mention you have a "CO (carbon monoxide) sensor in the exhaust". Can you explain that?

Lastly if your "A/F meter" shows 11.5:1 air:fuel, you may be seeing black smoke out the exhaust. Are you?
Put the EMC into diagnostic mode, does it flash code 12? If not go through flowchart #5 or #6 in the Service Manual and perhaps this will tell you something.
Hello Hib,

I'm working with a Fast A/F meter. As my '81 also had a oxygen sensor (sorry not CO!), I can setup the Fast A/F meter to send out a normal oxygen sensor output to the ECM so it can correct the A/F combination.

At 11.5:1 there is no black smoke (there is on the wide band sensors. But if the mixture goes to 9:1 - 10:1 I get smoke and fuel vapor irritating my eyes.

Thanks Lanny I can still to that and see if I can get a code 12 out of it. What I did in the mean time was using index 2 (dwell fixed between 10 and 50). I simply had to do two things. Jumper the coolant sensor at the sensor and at the ECM and do the same with the oxygen sensor. Dwell didn't change so the service manual says or my connections between ecm and connector aren't right or the ECM is faulty. I'm still thinking the last so maybe it's time to change to the 1228079 ECM :)

Greetings Peter
Today I put the ECM in diagnostic mode. I only get a code 12 over and over. As I disconnected the O2 and coolant sensor one of the two gave a code but it wasn't stored or could that be because I reconnected everything?

So it looks like the ECM isn't dead just wouldn't go into closed loop.

Greetings Peter

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors


MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Advertise with the Corvette Action Center!

Double Your Chances!

Our Partners

Top Bottom