Overheating at idle

[Stanley S.]

I'm having some engine overheating at idle on my 65 396. Engine temp stays normal while driving at 30 MPH or faster. I just discovered my fan is the 327 version. Does anyone think replacing it with the correct higher pitch fan used with the 396 would solve this problem? I'm also questioning the fan clutch operation but have not tried replacing it yet. Thanks.
Stan

[Michael J.]

Re: Overheating at idle

Oh boy, this is a familiar subject. There are numerous threads on this, I had the same issues with my L71 right after I got it. Here is one thread with some tips:
https://www.forums.ncrs.org/showthre...block&uid=8579
There are others, but Duke is the expert on this, he has helped me and many others here get cool running BBs. First, get the proper BB fan, then check the clutch, check your timing, if it is below 10-12 degrees BTDC it will cause overheating, then convert the ported vacuum advance to full time, then check the vacuum advance to see how much advance you are getting. I had to have my radiator rodded as well as changing coolant to a non-silicate Zerex brand. Lots of things to do, just take it one step at a time. Good luck.

[John H.]

Re: Overheating at idle

I'm having some engine overheating at idle on my 65 396. Engine temp stays normal while driving at 30 MPH or faster. I just discovered my fan is the 327 version. Does anyone think replacing it with the correct higher pitch fan used with the 396 would solve this problem? I'm also questioning the fan clutch operation but have not tried replacing it yet. Thanks.
Stan

You might want to review this article on Corvette cooling systems:

http://www.lbfun.com/warehouse/tech_...em/cooling.pdf

[Jim T.]

Re: Overheating at idle

Nice article John. Lots of work goes into writing articles, it is appreciated.
I learned in 1970 about the importance of full time vacuum with my then brand new 1970. Sitting at idle one hot day in SEP with me out of the car and leaving the air on I noticed how how the engine picked up RPM's on its own. It functioned just like the engineers designed it to. Having a sensor to detect higher operating temp it permitted full time advance instead of restricted vacuum to the vacuum advance unit permitted the increase in engine RPM's. I went full time vacuum advance right away using a length of vacuum hose to bypass the TCS and its been that way ever since, coming up on 40 years ago now.

[Stanley S.]

Re: Overheating at idle

Thanks for pointing out the excellent prior posts and the article on the cooling system. They are a great help. I will order a fan and clutch-my clutch turns as free when engine is hot as when cold. Could someone explain how to convert to full time vaccum advance or point me to some literature to read to learn how. This process is still confusing to me. Thanks.

Stan

[Joe L.]

Re: Overheating at idle

Thanks for pointing out the excellent prior posts and the article on the cooling system. They are a great help. I will order a fan and clutch-my clutch turns as free when engine is hot as when cold. Could someone explain how to convert to full time vaccum advance or point me to some literature to read to learn how. This process is still confusing to me. Thanks.

Stan

Stan------


The biggest cause of overheating at idle is a worn-out fan clutch. In your case, I expect it's a combination of incorrect fan and worn-out fan clutch. Finding a correct 1965 L-78 fan will be difficult and very expensive if you succeed. You can obtain a reproduction of the 66-67 big block fan, though, and it should work as well or better than the 65 L-78 fan. I think these things cost about 200 bucks. Combine that with a new GM #3916141 fan clutch and you should have you fan and fan clutch problems solved. Save your existing clutch (assuming it's actually the original clutch) for possible later rebuild for show purposes.

As far as conversion to full time vacuum advance, there is a lot in the archives on this subject.

[Michael J.]

Re: Overheating at idle

Thanks for pointing out the excellent prior posts and the article on the cooling system. They are a great help. I will order a fan and clutch-my clutch turns as free when engine is hot as when cold. Could someone explain how to convert to full time vaccum advance or point me to some literature to read to learn how. This process is still confusing to me. Thanks.

Stan

Here is a good thread, it goes through it and shows pictures.
https://www.forums.ncrs.org/showthre...acuum&uid=6331

[Jim T.]

Re: Overheating at idle

Thanks for pointing out the excellent prior posts and the article on the cooling system. They are a great help. I will order a fan and clutch-my clutch turns as free when engine is hot as when cold. Could someone explain how to convert to full time vaccum advance or point me to some literature to read to learn how. This process is still confusing to me. Thanks.

Stan

Stanley I am not familiar with how a 65 396 left the factory, I do know my 64 Corvette left the factory with full time vacuum advance. Quick check to verify that you have full time vacuum advance is start your 65 and remove the vacuum hose from the distrubutors vacuum advance unit. If the engine rpm's decrease this indicates you have full time vacuum and the vacuum advance unit is working.

Putting your finger over the removed vacuum hose and feeling the vacuum pull will confirm full time vacuum at idle. If you did not have a rpm loss and no vacuum on the removed vacuum hose your vacuum hose is most likely connected to a ported vacuum source that supplies vacuum above idle speed.

I am assuming you have a Holley carburetor. The external vacuum connections at the base of the carburetor are a source of full time vacuum that can be verified with the engine at idle.
If your vacuum advance unit is not operating with full time vacuum applied, a good source of replacement is from NAPA.

[Duke W.]

Re: Overheating at idle

I think the L-78 has full time vacuum advance, BUT I think it also has a ...201 15.5" VAC as OE, and the engine doesn't generate enough vacuum at idle to keep it locked at full advance. Is the carburetor the correct original type or an exact OE replacement?

Not too many L-78s out there, so it would be nice if you would determine if it's full time or ported vacuum advance and also tell us the number on the VAC, and whether the carburetor is correct.

I recommend a 12" VAC (stamped B20 or B26) for ALL Corvette big block engines. Search those ID codes for the brand part numbers.

Does it have the original type aluminum radiator and shroud? The radiator looks like the type used on SBs, but it's wider. Anything other than the OE design radiator, shroud, and fan/clutch could certainly be a cause of overheating.

It's impossible to diagnose a problem unless we know the configuration and have some data. For example, "overheating" is very vague. What does that mean? Is it boiling over? What's the highest reading you've seen on the temp gage? Have you checked engine outlet temperature with an IR gun?

Duke

[Stanley S.]

Re: Overheating at idle

Duke,
My radiator part number is 3007436 date 65B. It has a harrison aluminum decal on right side on top. Carb is Holley 4150 list 3124 which is correct for the car. The cap on the expansion tank is a 15 lb RC26 which I believe is for the 327. I wonder if the cap is bad. The highest I have ever let the temp gauge go is half way between 180 and 240 so I guess about 210 but I don't have a IR heat gun yet to document this. While driving, the temp gauage stayed on 180 but after idleing for only about 2-3 minutes and watching the guage slowly build to 210 I shut down the engine and watched 2-3 qt of antifreeze boil out on to the floor. I recently installed a 180 thermostat and removed the 160 that was in there. I also put in G05 antifreeze. I can check the number on the VAC can but I'm not sure where to look. I plan to start the engine today and pull the vac hose off to test for engine RPM decrease and vac pull.
Stan

[Timothy B.]

Re: Overheating at idle

Stanley,

Sounds like the radiator cap is not holding pressure, try a new radiator cap. Let the engine cool and check/refill the cold fluid level and run the car with a new cap.

Also, verify the #'s on the vacuum advance control and post them for more comments.

[Edward S.]

Re: Overheating at idle

Paragon sells a repop 396 fan blade - I can tell you the guys doing the fan clutch rebuilding are almost working far behind, I waited alittle over a year to get mine back from Oliva. Good luck.

[Stanley S.]

Re: Overheating at idle

I just started my engine and after letting it warm a little and come off of fast idle, I pulled off the hose on the VAC and the engine RPM did drop a little . There also was significant suction on the hose attached to the carb. So I guess this means it has full time vacuum advance?

I'm still looking the the number on the VAC can. Do you have to pull off the distributor to find it?

[Jack H.]

Re: Overheating at idle

Yes, and the Paragon 396 reproduction fan was a direct copy for the original on a long time NCRS member's car who specialized in 396 (Stewart).

The vac advance on my 396 is a '201 and it doesn't overheat at idle. Fan clutch is a Fred Oliva restoration. BTW, the last I talked to Fred, he'd made progress in his backlog and was quoting 6-month turnarounds.

On 396 rad cap, while the erratta sheet that came with the car says 15#, NCRS and the JG book say 13#--identical to the SB cars.

From later posts in this thread it doesn't sound like there's a bona fide overheating problem here. I see the concern is the temp gauge drifting above 180F during idle (no time period specified) and there is NO report of the rad cap 'popping' with coolant spewing out the overflow hose. So, I'm forced to conclude we don't have a case of 'overheating' to diagnose...

[Paul L.]

Re: Overheating at idle

I just started my engine and after letting it warm a little and come off of fast idle, I pulled off the hose on the VAC and the engine RPM did drop a little . There also was significant suction on the hose attached to the carb. So I guess this means it has full time vacuum advance?

I'm still looking the the number on the VAC can. Do you have to pull off the distributor to find it?

Stanley,

The answer is no. Just pull off the cap. You can see B26 in this pic.