DaddyO

I hear a loud hiss (vacuum leak?) coming from under the driver's side dash. I poked my head under there and did not see any loose hoses.

Any thoughts?

PS - my car is stumbling and hesitating when I drive it. Could these be related? (Just got the car back from the body shop where it has been sitting most of 6 months other than being moved around their lot.)

:banghead:

Twin_Turbo

Headlight vac override switch (pull down **** under steering column) leking or one of the hoses is loose. Could also be the vacuum hose on the cruise control disengage switch (behind brake pedal, there are 2 switches, one is for the brake lights and doesn't have a vac. hose on it)

79VetteMike

:iagree:

BSeery

Yeah - vac problem somewhere, and that is why your car runs badly.

I had a backfire on mine blow some hoses off and the car went to really lean and running like garbage until I could pull it over and fix the mess.

KenSny

:iagree: That is unless it's a snake up there getting mad..... :D

DaddyO

Oh great......at least I have a one of those mity vac pump and gauges to help me along.

What is the first step in vac system diagnosis?

BSeery

I would look at each vacuum line going through the firewall from the engine side. I would clamp each one off one at a time and find out where the source is . Once you know the source, then go under the dash on the same line and then move down the line, clamping off as you go and releasing until you find out where the leak is.

DaddyO

Do they call come in on the driver's side?

BSeery

well, my 1980 is different.
I have on that comes in on the drivers side that is used for the cruise control and another for the headlight system.
I think there is a small on on the passenger side that goes to the a/c system to control the vents.

SAC

Maybe the first thing I would do is plug the vacuum line running off the manifold. This way you could tell if the engine is stumbling from a vacuum leak from this line to begin with. I would check with a vacuum gauge with this line plugged and unplugged. You could have a different vacuum leak and a leak under the dash. Then go to isolating it from there.

DaddyO

I'm a novice mechanic. (but a quick learner and very handy!)
Which hose is the manifold vacuum?

Fevre

It is the one coming of a port right behind the carb. You can use a squeeze clamp to make it easier. Try starting your car and letting settle into a warm idle then use the clamp to pinch the hose right after it exit the manifold port, if the car smooths out and you loose your hiss noise then you have found a leak. Unfortunetly you then have to figure which hose is leaking under the dash. :(

Sigforty

A good cigar and a spotter work well for tracing vacuum leaks. I just blew smoke into the lines and had someone look for the smoke. I eventually found out I had some bad relays. I still need to trouble shoot the inside of the car. I have a feeling the heater door relay valve has a leak, or maybe the vent controls inside.

DaddyO

Great idea!

mbeeman350

:iagree:

jdunne

The hoses can split where they connect to the headlight switch and where they connect to the override switch,they look ok and seem to be in position but still leak,look at the dip switch connection as well......john :chevy

DaddyO

Thanks to everyone for the advice. It's definitely a vacuum leak. When I clamp the vac line that comes from the manifold and goes to the vac system, the car runs much smoother.

Can I drive the car with that line closed off? Does it only go the the lights, wipers and heating system?

DaddyO

So, is it ok to drive the car with the vac lines clamped off?

PRNDL

Yes, should be no problem driving like that (unless it rains or gets dark :lol: ). On my car the manifold port feeds only the wipers and headlights. (not the nonfunctional heating system). Power brake vacuum would typically come from the backside of the carb if you have p/b. MJ

The Wanderer

You might want to check this link out ... http://vetteworks.tripod.com/techdocuments/Vaccum.html . Also I have a very nice PDF drawing of the whole system and all of its componets. Let me know and I can email it to you.