The thermostat in my 67 427/435 seems to be sticking. When first started and allowed to idle for a while to warm up the car will start to overheat. Then once on the road, not at speed but just moving up the street a block or two the temperature will drop dramatically to 160 degrees. I think this is the thermostat opening up. Before I go out and buy one is there a specific make that might be more accurate or efficient than another. Any recommendations would be very much appreciated.
67 Big Block thermostats.
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Re: 67 Big Block thermostats.
I question whether the thermostat is at fault. You may recall that there have been many discussions about hot running '66 L-72s. It turns out that both CA and 49-state versions have a "ported" vacuum advance, which does not allow vacuum advance at idle, and converting to a full time vacuum advance by rerouting the vacuum can signal line to the choke vacuum break or an adapter to the port (OEM configuration has a pipe plug installed) on the inlet manifold usually eliminates the tendency to run hot at idle.
Does the L-71 have ported vacuum advance? You can easily check by disconnecting and plugging the line. If there is no change in the idle speed, it probably does. You can also check for a vacuum signal at the line at idle and observe the idle timing with the vacuum signal line connnected and not connected, which will also answer the question.
Chances are the 160 thermostat was installed as a crutch in a attempt to solve the overheating. It would be better to use a 180 thermostat. If it's a CA model it would have been equipped with an AIR pump and a 195 stat.
Duke- Top
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Re: 67 Big Block thermostats.
Yes, I know. I'm trying to get to the bottom of the '66 L-72 configuration. Some say it was exempt from CA emissions, but I question that. My current opinion is that the '66 L-72 ported vacuum advacne was sufficient to meet '66 CA emission regs, and this version was sold nationwide - probaby because GM did not want to develop two L-72 configurations since it was a relatively low volume option. The only engineering reason to implement a ported vacuum advance is emission control as it increases EGT, which would help oxidize HC and CO in the exhaust system.
I understand that '67 CA L-71s were equipped with AIR systems, and I want to establish whether or not they had ported vacuum advance. To the best of my knowledge the early AIR equipped engines did not have ported vacuum advance, but they did have lower initial timing specs than 49 state versions.
Ported vacuum advance (with or without AIR) became very common in the early seventies as emission standards tightened along with TCS, which blocked vacuum advance in all but top gear.
Duke- Top
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Re: 67 Big Block thermostats.
Rick,
Did you check the antifreeze/water mixture level in the circulation loop? If you are low, similar simptoms may surface. I also don't think the problem is the thermostat. At least this would be an simple fix. Good luck.
TonyTony- Top
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Re: 67 Big Block thermostats. 427/435.
Ported vacuum....manifold vacuum....which is which....am I the only one having a hard time understanding these technical descriptions? The line from my vacuum can runs to a fitting on the manifold just below the carburator....so I'm guessing it's not "ported" vacuum. The car does not run hot. As far as timing goes, it detonates if I set it any higher than 6 degrees and I'm using the Napa vacuum can recommended by Duke. I changed the thermostat yesterday. The one I took out was a 180 and I installed a new 180. This seems to have cured the problem. The coolant was down a little as well.
Thanks for the help, it is always very much appreciated.- Top
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Re: 67 Big Block thermostats. 427/435.
Rick - your application as you describe it is full manifold vacuum since the connection is below the carburetor throttle plates, and see's the vacuum in the intake manifold. The ported vacuum is tapped into a venturi in the carburetor above the throttle plates, so, at idle, the plates being mostly closed, this port sees no vacuum. When the throttle is opened, the port will see vacuum...Craig- Top
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