I have a 79'. Replaced all vacuum switches, egr, cat, carb, cap, rotor, wires, plugs, oil and I passed California smog with flying colors. She was lacking tlc from the previous owner. So that major obstacle is done. The small problem now is the idle, she won't maintain a steady smooth idle. It is not that bad at all but I want it better. The smog mechanic told me it's a vacuum leak from the headlights. I told him one will not pop up so I know there is probably a leak there. Question I have is do the vacuum lines for the lights have an impact on the engine performance such as idle? Just want some opinions on the matter? I am going to replace all the hoses because they are really old but curious about this impacting the idle. I love my baby and want her to purr like a cat and roar like a Lion!

 

I'm new to the forum...

I would get the vacuum diagram-

Find the main vacuum hose that connects to the carb/intake that supplies vacuum to the large canister in the engine bay--

Start the car/let it warm up-

When it is idling rough--remove the main vacuum hose from the carb port--

Cap off that open port with a rubber cap temporarily--

If the idle smooths out...you've narrowed the problem down to old vacuum lines/vacuum switches/cannisters/check valves/etc--

If the idle doesn't smooth out...you've got another problem.

 

The short answer to your question of "can the headlights cause an idle issue" is YES. The headlights can provide a huge vacuum leak.

I agree with Doorgunners approach, cap off the vacuum source to the headlights (large vacuum source from the back of the carb, or from the manifold port behind the carb, I dont remember which it is right now) and see if you can get a good idle.

 

Should the check valve to the vacuum system stop vacuum leaks in the headlights/wiper door from affecting the engine? So even if you do have a vacuum leak in the headlight system, as long as the check valve works, it shouldn't affect engine performance? And if it is affecting intake vacuum, you should just have to replace the check valve rather than pull the whole vacuum system apart?

(those are questions, not answers, I assumed that was the purpose of the check valve, but I'm new to this!)

 

the check valve holds vacuum in the reserve tank. that is, air is pulled from the tank into the manifold, so that at wot vacuum in the tank will operate the vacuum system. if there is a leak in the headlight(or other) vacuum lines, the air will pass thru the check valve into the manifold.

 

Just for the record, I had a bad vacuum relay in my 79 corvette light system. Its effect on idle was so bad that I disconnected the light system from the mainfold and drove with the headlights up until I replaced it. If you have a headlight that won't go up, you probably have a bad relay. In a 79 vette there is a large hose that runs from the manifold to a check valve and then to the vacuum ballast bar in the front. If you pinch that hose between the ballast and check valve with a pliers and the idle smooths out, you have a leak in either the actuators or the relays. You can determine which by isolating the leak through further pinch tests. However, the relays are controlled by a vacuum source from the headlight switch, and if there is a leak there, the pinch test on the larger line won't affect the idle. There is a small vacuum line attached to the check valve that provdes vacuum to the actuator relay through the dashboard switch. If there was no effect from pinching the large line, pinch the smaller line off of the check valve and see how that affects the idle. A smooth idle would indicate a leak in that portion. Finally, it's possible in older cars that there are leaks all over the system. So if the first and second pinch tests didn't change the idle, a third pinch test between the manifold and check valve would be next. It that smooths the idle, then there are leaks in the front and rear portions of your vacuum system...not uncommon in these older cars with dried out lines. There are vacuum troubleshooting diagrams that show the vacuum line pathways so you know where to pinch and what to replace.

 

Ah ok, my mistake. In that case, the best bet would be, as doorgunner says, work your way from the carb through the vacuum system until you figure out where the leak is (if you have one at all). If you have a vacuum gauge, that would make things easier and you can figure out right from the get-go if there's a leak or not by checking the vacuum at the carb is the same with or without everything else connected.

 

Thank you all for the tips. I am going to have a little fun working on my baby next week

 

Keep us pic'd & updated......the same things will prpbably be broken on my project...LOL.