I've been driving the 84 often lately. When on the move it runs awesome. However sometimes when sitting still idling, it will throw the check engine light on with a code 34. idle will get rough but won't stall or die. I've been searching for the code on here, but all the ones I'm seeing apply to TPI cars with mass air flow sensors. I found the code in the service manual. Says VAC sensor is the issue. Problem I've having is, it says there should be a vac line going to the sensor, along with a three pin connector. The only vac line is the one that I have mocked up but not hooked up that goes to manifold vacuum, and the other end of the vac line switch goes to the hood scoop. I thought it was only a function of the scoop. I guess I was wrong. The other thing I can think of is the cap for the vac port in the manifold worked loose.

According to what I've found the VAC sensor is the last block on the far driver's side on the firewall. Am I on track so far? It only throws the light when idling. So I think I'm on the right track here but because of no vac line going to that far last block I'm confused as to how the hood scoop vac switch controls the vacuum at idle, because the line for the scoop doesn't seem to have an impact on how the motor runs.

I have just about all my vac line ports capped off due to either not remembering where they go or not finding locations for their connection. AIR lines are capped off, the small line at the back of the carb is capped off, the port in the intake manifold is capped off, and the port by the TPS is capped off. 1984 L69 carb, 1987 LG4 motor.

 

I found the vac line at the vac sensor. It was connected at the sensor but not at the carb where it should have went. I'm guessing once that is hooked up I should have a better idle. I doubt the vacuum sensor is bad. I think after a while the computer decided to give me grief over not having it hooked up and started acting up.

 

There may have been a MAP sensor on the firewall connected by hose to the "N" port on the rear of the carb. This would be a separate device from teh vacuum switch which operated the hood/cowl louver.

 

Idle was better after I fixed the VAC sensor line. But while it ran as good has it has before, when I let it sit in the driveway for a few minutes idling the light came back on. I tried to get the code, but decided to turn off the car. When I turned the car off, and tried to pull the code all I got was a continual 1-2 flash, system ok. So I don't know why the light came back on. I do know that light really only comes on when a sensor is out of its range. So maybe I'll just let it go for now and try again when I feel like it.

 

I took it out right before dinner on a close to 10 mile loop. Put the car back in the garage as it was supposed to rain through Monday, light never came on. So I don't know what is going on. But it does look like I need to have the car running in order to get the codes. Also once the light came on when I was sitting still, then as I pulled away the light went off. Don't you just hate check engine lights when they come on for things that have nothing to do with the overall performance of the car?