L-79 running hot

[Tom Merkel]

This is a follow-up to my post from last week.

The temp (L-79) ordinarily holds at 180, but recently decided it wants to reach 200 - 210. I swapped-out thermostat (new 180) and it still does same thing.

Simple experiment: remove thermostat, and see what happens. (Yet to do)

Someone suggest it might be fan clutch. To my way of reasoning, if a motor is 'hot', then the fan should be 'locked' pretty good onto the shaft, and thus want to turn in synchronous with motor rpm. But mine still 'free wheels' (that is, able to spin easily with hand) even after running for a while 'at temp'.

Is this a valid conclusion for 'bad fan clutch'?

Thanks.

Tom

[Duke W.]

Re: L-79 running hot

Your fan clutch definitely sounds suspect. With the engine cold turn the fan to get a feel for how much drag torque it has. Then get the engine good and hot on a warm day - over 180 if possible by letting it idle for an extended period. Shut down the engine and check the fan drag torque again. It should be signficantly more. You can also subjectively test fan air flow cold and hot at idle and 2000 RPM. When the engine is hot you should perceive signficantly more air flow.

You should also check that your vacuum can is funtioning to spec. It should provide 16* @8", so it should be fully deployed at idle. If you check your timing at idle with the vacuum can connected the total idle timing should be 16 plus whatever initial you run.

Don't waste your time removing the thermostat and running the car without one. That's not a valid test. Your car didn't run hot when it was new, but something is probablly not performing to OEM spec. Find the faulty component and repair it, thus returning your cooling system to design performance.

Duke

[Bill Clupper]

Re: L-79 running hot

Yes, it is.

[Tom Merkel]

Re: L-79 running hot

Thanks, Duke. Good input, but my basic understanding of timing translates very simply to (1) removing vacumn advance, and (2) hitting the harmonic balancer with the strobe, turning the distrib. until I see 10 deg. before TDC for #1.

So, given that, could you explain a little more your timing line-of-thought?

Appreciate it.

Tom

[Duke W.]

Re: L-79 running hot

Yes, you set you initial timing with the vacuum can disconnected. So how do you check if the vacuum can is functioning. Most people never do!!!

One way to check it is to see if it holds a vacuum. There's and organic diaphram inside the can, and though they are long lived, they don't last forever.

Since your L-79 has a can that provides 16* @ 8" you should be able to easily pull the plunger all the way to the stop with oral vacuum. If it's an original or Delco replacement can it will have "236" stamped on the bracket that connects the can to the dist. housing.

Assuming your initial timing is set at 10 degrees you should measure 26 degrees TOTAL IDLE TIMING with the can connected.

The vacuum can is totally ignored by 99.99 percent of owners and few really understand its purpose. A malfunctioning or incorrect vacuum can is often a source of idle overheating, but it's rarely checked.

As part of any normal tuneup, the can should be checked for leaks and performance versus specification. The former can be done by testing to make sure it will hold vacuum. and the second can usually be checked by verifying that is provides full advance at idle on pre-emission engines.

Duke

[Gary Schisler]

Re: L-79 running hot

AS others have stated, your conclusion is correct about the fan clutch. However, you didn't give enough information about the problem. Is this an idle or low speed problem or a highway overheating problem. I fought a similar situation in my 66 L79. However, my overheating was only at highway speeds.

If you have a low speed or idle overheating, your fan clutch AND your radiator my be suspect. If you have a highway overheating problem, then it may well be the radiator. I fought this problem and did all that you did: new fan clutch, thermostat, checked timing, flushed radiator, new upper/lower rad hoses, etc. I finally broke down and bought a repro Harrison radiator and, once installed, I was able to cruise at highway speeds again.

Isolate the problem and fix or replace the faulty component. As a note, I bought an aftermarket, totally incorrect, fan clutch to replace my original one. It looked incorrect but it did meet the specs for the car and I never had a low speed problem. Others on this board will mention various brands of thermostats and other components. Some of the "remove and replace" compenents are cheap and easily replaced to see if that solves the problem. There are very few accurate ways to check radiator condition, until you get to the point that nothing else solves the problem.
Gary