The more I dwell on this, some long forgotten memories come back.  Remember I worked on our C3s in a Chevy dealer when they were much newer.  Mid 70s to early 80s.   It took a while to fish through the grey matter, but it finally came to me.  

In the mid/late 70s there was a problem with some of the heat risers rattling without any real problem.  The TSB from Chevy told us to put a vacuum delay valve in the line between the thermo vacuum valve and the heat riser vacuum motor.  It worked for the condition at hand.   Finding a vacuum delay valve may be a bit of a challange now, let alone the one metioned in the TSB.   Ford had many vacuum delay valves, and one of them may work.  A few seconds delay or so is enough.

I remember that, Ken!! The problem was the valve wanting to stay partially closed, but the exhaust wanting to flop it open. The vaccuum actuator didn't know what to do, so it said, "I'll just let this sucker buzz until it gets hot enough to get full vaccuum from the thermo valve". We always just propped them wide open with a big ol spring!!

If you do not have an EGR intake but do have the EFE, where does the heated exhaust go during warmup?

Disneyland?


It will back up into the head. The pressure will push the efe valve open, anyway.

 Do a search on ebay Seller corvettemack   or item 8071233981 It's a buy it now.

So if the exhaust will just backup, sounds like the EFE should get full time vacuum not time delayed to keep open, or spacer in its place?

They do have "spacers" that take the place of heat riser valves, that are completely open.

If you have any version of a heat riser valve, then your car should have an EGR. If the intake has been changed, or the EGR passages in the intake/head have been blocked, then the heat riser should be removed, or blocked in the open position.

So what do you think, will this fit my 78?

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem& ;ru=http%3A%2F%2Fsearch.ebay.com%3A80%2Fsearch%2Fsearch.dll% 3Ffrom%3DR40%26satitle%3D8071233981%26fvi%3D1&item=80712 33981

It'll fit, if that's what yours looks like.
It looks like one of the "ears" for the studs is broken off. There should be three holes for the studs. Might shoot the guy an e-mail, and ax aboot that.

 Nothing wrong with that valve. They only used the two holes to locate the exhaust pipe and flange. The third stud was on the outside of the riser. Kiltime38869.9326736111