Engine runs rough when the headlights are on

I am pretty sure it is a vaccuum leak on my 81 C3.  Any suggestions on where to start looking?

Joe, could be from the headlight switch, can you hear a leak with the knob pulled out and the engine running? There should be a diagram of the complete system online so you can trace it out as well.

If an '81 still has the manual headlight control switch under the steering column, you might try that and see if the vacuum leak is still present. That would rule out the headlight switch, maybe.

Thanks, I will try that!

Thanks!

You can also try using an unlit propane torch and play the nozzle around areas that you think the leak might be and see if the engine idle speed changes. If it does, you have found at least one of the leaks.

A shot of ether/starting fluid spray will do the same thing, have the same effect.

If the engine runs fine with the lights out, I would also suspect the HL switch or the up/down control valves up front. +1 on using the override to possibly eliminate a bad switch. If the lights go up with the override switch pulled and the engine runs fine, it’s probably the switch. If the engine stumbles when the override switch is pulled then I would go to the control valves. They are a very common leak and may need to be replaced. Good luck and let us know what you find.

Propane is a lot less messy especially when you're dealing under the dash or on plastic parts.

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|UPDATED|1/7/2018 1:21:11 PM (AZT)|/UPDATED|