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So…I have a 72 Cutlass with a 350 engine, recently bought. I’m just starting to get into it. Looking at the intake manifold by the oil fill tube I see that there is a device with three vacuum ports, and a two prong electrical connector on top of it. Learned it is, I think, the Distributor Thermal Advance Switch. All vac lines are missing, and the connector is broke and not attached, yet the engine runs reasonably well. Researching this a little online while I wait for my eBay FSM to get here. I see that vacuum lines run various places. One being the vac advance on the distributor. That vac line is now direct connected to the carb vac port.
So I am led to think that this is an emissions device that is abandoned? Is this correct? What is the purpose of the electrical connector? Where does the info it supplies go? Is it worthwhile to get it re plumbed and electrically connected again? If not, will a pipe plug fit the hole?
Yes, it is an early emission control device. The overall function is to reduce NOx by preventing distributor vacuum advance until the engine is up to temperature and the transmission is in high gear, effectively by retarding the ignition timing.
The bypass for this is what you described - connecting the distributor vacuum advance directly to a vacuum source (either ported or manifold - there are lots of arguments for each). This makes the engine run much better (power wise and efficiency wise).
Yes, you can remove the DVCS & install a standard pipe plug. Cover the plug (electrical tape will work) which would normally attach to the top of the DVCS then tie-off that wire loom w/ the plug somewhere by the firewall out of sight.
Fun71 and Vintage Chief, thanks! Nice pic on the vac line routing. It will help me out. And that brass plug looks way better than that ornament! I just replaced coolant today, so my timing is off just a little on that brass plug install! I’ll drain the coolant down. Easy enough.
So I would imagine that the 2 inch flex pipe from exhaust manifold to air cleaner is also part of the emissions elimination? And also the vac line going to the air valve on the air cleaner snorkel?
No, not really. That pipe is an air heater - hot air vaporizes the gasoline better, which increases efficiency (and also helps with cold weather operation). The vacuum diaphragm on the air cleaner snorkel will open to allow cooler outside air.
No, not really. That pipe is an air heater - hot air vaporizes the gasoline better, which increases efficiency (and also helps with cold weather operation). The vacuum diaphragm on the air cleaner snorkel will open to allow cooler outside air.
Where does the vac line for the snorkel valve connect? Seems like it would be controlled by a valve that recognizes heat?
Before you start ripping out anything that you think is evil emissions control hardware, you should first understand what the part actually does. The TVS has TWO purposes. The solenoid controlled part of the valve is part of the Transmission Controlled Spark system, which is intended to reduce NOx emissions by inhibiting vacuum advance in lower gears. A switch in the trans activates the solenoid valve in the TVS to block the vacuum connection to the distributor when the trans is not in high gear. Simply disconnecting the electrical connector on the TVS deactivates the system and provides vacuum to the distributor in any gear. Note that the initial timing and advance curves in the OEM distributor are designed to use PORTED vacuum.
The thermally-controlled part of the TVS is actually a protection against engine overheating and is NOT an emissions-control item. Since the distributor uses ported vacuum normally, if the engine starts to overheat at low speeds, the TVS switches from ported to manifold vacuum to increase idle speed and timing advance to cool the engine. Disabling this function without compensating by reworking the vacuum advance and initial timing can cause low speed overheating.
Thanks Joe for that explanation. The electrical part of it was what I was trying to figure out. Looks like some delete this system, and some do not. Kinda like a lot of late 60’s early 70’s cars. I remember my 68 corvette was missing a lot of the emissions stuff when I got it, and it never was replaced.
Yes, you can remove the DVCS & install a standard pipe plug. Cover the plug (electrical tape will work) which would normally attach to the top of the DVCS then tie-off that wire loom w/ the plug somewhere by the firewall out of sight.
Norm
Where did you get that pipe plug with the nice hex top? What I see at the local big box plumbing dept has a square drive protruding top.
We have Ace, I’ll see if they have one. My car project took a severe departure from the runway. I have the radiator at a shop in Dubuque getting a new core. Meanwhile, the whole front of the engine is now easier to cleanup and paint. With fender and bumper off car I can stand and work on engine. So….pulling ac brackets, alt brackets and other stuff and have some brushable gold paint coming from Summit. So the top of the engine is gonna get a facelift. I remembered the brass plug, and not surprised you were the one that showed it on your engine. I really like it. If I could find a new original part I would but I think it an impossible search. Must admit I have not tried.
But I just finished the metalwork on the left fender and will put it in epoxy tomorrow. That leaves me with left door that needs a lot of work, although I already have the hinges rebushed and the door rehung to check panel gaps. They are gonna take some work. I am having so much fun working this car.
