C3 vacuum problem

[Douglas B.]

Is there a tried and true method of finding a vacuum problem causing headlights not to open? I have a 1978 L48 coupe that the headlights will not open on their own or with the valve under the dash. I am relatively sure that it is a vacuum problem because my A/C blows on the floor all the time also. I am unsure how to test the actuator relay valves near the headlights. I have already done a visual inspection of the hoses and replaced the headlight check valve near the intake manifold. It seems to have adequate vacuum at the manifold, but if I connect a manual gauge to the manifold port, how much should I have at idle? Any help would be appreciated.

[Don Y.]

Re: C3 vacuum problem

We have a smoke machine that is specifically used to find small leaks in pneumatic systems. There may be a repair facility near you that does emission control work on late model vehicles. The machine generates a vapor that is forced from the leak point and can be detected with a flashlight or a UV lamp for very small leaks. These work very well for systems that have a lot of tubing and would be otherwise difficult to troubleshoot with any other method. You could also use the old method of starting at the source with a vacuum generator, like a hand pump, and pinching off the tubing down stream until the leaking stops. This will get you to where you can pinpoint test from there.

[Jack H.]

Re: C3 vacuum problem

Get one of the books on troubleshooting Shark vacuum systems. The first step is to understand how your system is supposed to work, and from there it's pretty easy to troubleshoot.

You simply start from the vac source (the intake manifold) and work outward measuring observed vac pressure along the path(s) looking for interconnected components that drop more than 1-2 psi as you go.

Last, don't forget there's an in-line vac filter (keeps airborne particulates from being sucked through the system and into the intake). These filters were supposed to have been changed roughly once a year and few novice owners understand this maintenance responsibility... A plugged filter will give you decent pressure in the system but limit air movement with components moving slowly if at all...

[Douglas B.]

Re: C3 vacuum problem

Thanks for the input! The vacuum filter may well be the problem. It appears to have been there a long time. Probably never been changed.

[Tom R.]

Re: C3 vacuum problem

Doug:

The last time I ran the vacuum gaunlaunt, my 78 L48 ran I believe 15-18 pounds of pressure. Just hook hose to manifold with a gauge and you'll have a start point. Don and Jack describe it from there.