Thermal switches
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Joined: Sep 2001
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Thermal switches
Hey guys,
A few months back i installed an edelbrock non egr intake and a 1406 carb on my 77 400. I assume that the things on my thermostat housing with all the vac lines on them are thermal switches. Since i have a non egr intake to i still need the switches or is there a simpler way to hook it up?
THanks
David
PS how do i check if the vac advance is working?
A few months back i installed an edelbrock non egr intake and a 1406 carb on my 77 400. I assume that the things on my thermostat housing with all the vac lines on them are thermal switches. Since i have a non egr intake to i still need the switches or is there a simpler way to hook it up?
THanks
David
PS how do i check if the vac advance is working?
Last edited by dbarracuda; Aug 15, 2003 at 07:11 PM.
Posting Guru
Joined: Aug 2000
Posts: 1,127
Likes: 3
From: (near) Denver USA
Thermal switches
Most late model 351M/400s used two thermal PVS (ported vacuum switch) on the thermostat housing. One controlled EGR (kept it off until coolant reached normal operating temp), and one switched the vacuum advance signal between the carb spark port and manifold vacuum for HIC (hot idle compensation).
The HIC system provided a way to cool the engine down during extended periods at idle to prevent overheating. When the vacuum advance switches to manifold vacuum, timing is advanced, increasing the idle speed and turning the water pump and fan faster to cool down the engine.
Check your vacuum diagram to see how your system was set up.
To test the vacuum advance, put a timing light on the engine and disconnect and plug off the hose to the vacuum advance diaphragm. Check the timing with no vacuum. Then attach another hose to the advance diaphragm and apply a vacuum. You should see the ignition timing advance when vacuum is applied. Also, make sure the diaphragm holds a vacuum. It shouldn't bleed down.
The HIC system provided a way to cool the engine down during extended periods at idle to prevent overheating. When the vacuum advance switches to manifold vacuum, timing is advanced, increasing the idle speed and turning the water pump and fan faster to cool down the engine.
Check your vacuum diagram to see how your system was set up.
To test the vacuum advance, put a timing light on the engine and disconnect and plug off the hose to the vacuum advance diaphragm. Check the timing with no vacuum. Then attach another hose to the advance diaphragm and apply a vacuum. You should see the ignition timing advance when vacuum is applied. Also, make sure the diaphragm holds a vacuum. It shouldn't bleed down.
Posting Guru
Joined: Aug 2002
Posts: 1,214
Likes: 2
From: Minneapolis, MN
Thermal switches
I would suppose they could go bad but I haven't seen it. However, with no EGR you can eliminate one and if you watch the temp and kick it in neutral and increase the RPMs at a light (if it's getting hot) you shouldn't need the other one, unless you leave it idling unattended for long periods. Should clean things up a bit.
