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On a 75 the purpose of the exhaust heat riser valve is to close the flap in the exhaust when cold and open when hot but is it constantly opening and closing as revs rise and close as revs drop? It's my understanding it's either open or closed and doesn't operate like the butterflies on a carb. This is what the heat riser valve in doing on my 75 and it sounds like there is an exhaust leak at the valve. If I disconnect the air line to the valve the sound of the exhaust leak disappears.
The default for vacuum loss on a heat riser valve is for it to be stuck open but this happens about half the time.......
I urge you and anyone else to rid you life of that terrible thing and replace it with an open spacer........available from the Vette vendors.....
Even if I was restoring one of these cars to original condition......this would not be on the vehicle......
Notice the finger stops on the heat riser...
When you pull the vacuum line off the actuator, the rod extends opening the heat riser valve...
When Vacuum is applied through the TVS switch, vacuum to the actuator will pull the rod up closing the heat riser valve.
The default for vacuum loss on a heat riser valve is for it to be stuck open but this happens about half the time.......
I urge you and anyone else to rid you life of that terrible thing and replace it with an open spacer........available from the Vette vendors.....
Even if I was restoring one of these cars to original condition......this would not be on the vehicle......
Jebby
I agree,
This is something I watch for on any cold start.
The car doesn't leave the stall until I know for sure the TVS switch has changed state and the heat riser opens.
I'm still using the Hot Air Choke system so it does help reduce cold start runtime to about 3-4 minutes....
Here's the odd part,it fluctuates with RPM. Raise the idle it moves,it's not open or closed.
Post a photo of how it is plummd, you either have a faulty TVS switch or you have some vacuum hoses mixed-up or a combo of both.
The original TVS switch has what looks like 3 ports but one of them should remain open. It is the exhaust port.
Last edited by bmotojoe; Jul 10, 2023 at 08:38 PM.
Post a photo of how it is plummd, you either have a faulty TVS switch or you have some vacuum hoses mixed-up or a combo of both.
The original TVS switch has what looks like 3 ports but one of them should remain open. It is the exhaust port.
I'm thinking the switch is bad. This car sat for 30+ years in a garage and I'm slowly bringing it back to life. I've followed the vacuum lines based on diagrams I've found online and the line seems to be going to the correct port at the bottom right of the carb. I'm going to replace the switch anyway so I'll know if this was the issue.
I'm thinking the switch is bad. This car sat for 30+ years in a garage and I'm slowly bringing it back to life. I've followed the vacuum lines based on diagrams I've found online and the line seems to be going to the correct port at the bottom right of the carb. I'm going to replace the switch anyway so I'll know if this was the issue.
JMO, the way you describe what's happening you are plumbed to a ported or the EGR port. The EFE TVS switch should be connected to manifold vacuum...
Is it open or is it closed?
