After38Years

Must be August...

L79 stock everything.

Recently installed a heater cutoff valve in the hose coming from the intake manifold. (the 5/8 inch hose). Problem encountered is the same whether heater control is open or shut.

Took it out on a 95 degree day and temperature was fine until I came to a stop light and it quickly began to climb, approaching the yellow area of the gauge. Moving again, temp drops. Checked fluid levels - all is good in the overflow tank.

Bought an IR gun, put some flat black tape on the thermostat housing and went for a ride to bring up to temp (ambient temp 82 degrees). Get back home idling at 750, check thermostat housing, showing about 212 degrees, gauge showing 210.

Same thing happens - idling for a few minutes, gauge shows rising temp, getting into the yellow. Shooting the black tape on the thermo housing shows 233 degrees.

Fan (finned as original type) spins for about 2 - 3 seconds after shut off of engine. Fan easily moves, when hot, with one finger. Spinning fan with one finger gives about 1/2 revolution to stop.

Radiator is new repro, water pump is new/rebuilt, radiator hoses are not collapsed and upper hose is very firm when warmed up, indicating pressurized. No puking of fluid after shut down or while running.

I've driven the car this summer in 90 degree weather with no problems. The only change is the cut off valve but I don't see how that would enter into this issue.

Any thoughts?

Bluestripe67

Is the reproduction rad a DeWitts? If not, that is a player in your problem. Did you birp the cooling system after you put the valve in? Do you know if your vacuum advance is working? What are your timing setting? What plugs are you running? Dennis

After38Years

Don't know if the radiator is a DeWitts. I don't have that level of detail from the rebuilder - was not educated enough to ask...

Vacuum advance is working - timing set to spec per maintenance manual. Plugs are AC45S.

What is the burping protocol?

GTOguy

Since you are ok until the car stops moving, you are not pulling enough air through the radiator core at a stop to dissipate the heat. It sounds like the fan clutch is the issue. A lot of the newer fan clutches are set to engage at higher temps, and the clutch is a long way from the core on a Corvette. There are articles on this forum that cover adjusting the thermostatic spring, etc. You do have a shroud, don't you, and it's sealed up tight? In any case, that fan should be locking up tighter than it is, and at a lower temperature. 230+ at the T-stat housing is getting pretty warm.

After38Years

Do have the stock shroud installed, will check seal and will pull the fan clutch to look at it. Was not aware it was adjustable. I thought the aluminum finned hub just seized on the steel shaft as it got hot...

Vipergtrs

I had exactly this on my 66 427.

It could only be the efficiency of the fan and it's components causing the problem since it was fine when moving

I fitted a pushing electric fan in front of rad and all is well

Incidentally, My fan didn't have a clutch fitted so how would this affect the efficiency of the fan?

Mike67nv

The two things that matter most in engine cooling are heat transfer and air flow. Original radiators installed in midyear small blocks were a stacked-plate aluminum design with a heat rejection rate of 4600 BTU/min. Typical copper/brass replacement radiators have about a 3300 BTU rating. So, if you don’t have a stacked-plate aluminum replacement (like a DeWitts), you have roughly a 25-30% reduction in cooling capacity.

Thermal fan clutches don’t really “lock up” when engaged. Standard duty clutches run at about 20-30% of shaft speed when disengaged and 60-70% of shaft speed when engaged. The engagement temperature of the clutch is important since it is activated by radiator air temperature which is about 30* lower than coolant temperature – a clutch engaging at 180* means coolant is about 210*.

leif.anderson93

Might I recommend a new fan clutch from K&B. Their's are fully engaged at 165 degrees and are a Schwitzer reproduction...exceptional quality. They are sold by LIC and Paragon. It was the final piece of my overheating puzzle

Bluestripe67

I think the problem is solved with...a new DeWitts aluminum rad, a correct fan shroud, seals for same, correct GM 5 blade fan or 7 blade, new correct fan clutch, new Flow Cooler water pump, and a 180* thermostat. Dennis

After38Years

Think I'll start with a check of the thermostat operation. Did one other test by pushing a heavy roll of newspaper firmly into the rotating fan - does not stop the fan, but does make a lot of nice confetti. .

856666

See attached.

I agree on the air flow (lack of) at idle.

Quick checks:

Plug the gaps between the radiator and shroud

Make sure the fan is at lest halfway inside the shroud

Good Luck

Phil

Dave Cunningham

This is what I did on my L71 with stock radiator:

-changed the vacuum advance source to full manifold from ported.
- did the adjustment to the thermostatic spring on the fan clutch.
- added an electric fan to pull through the rad , set to around 200*
- added water wetter to the 50/50 antifreeze mix, and filled the cooling system with the thermostat removed ( tested the t-stat while removed)
- adjusted idle mixture using a vacuume gauge. Set idle to 900

Had the car out the other day in 100* weather, it now cools at 190 ish, while moving, and never gets a degree over 210* at idle in heavy traffic.

karkrafter

Sometimes I think guys pay too much attention to these freaking gauges...'Idle for a few minutes'....why would you do that in the hottest part of the summer?


230 is not the end of the world, you mention 'almost in the yellow' well almost means NOT in the yellow right?


A heater by pass is a BAD thing not a good one. Open the heater core and turn the heat on and watch the temp go down...for when you are idling for a few minutes...[which I would just avoid in the dog days of summer]


I just don't think you really have a problem here. And you never mention that the car ever burps any coolant out. If it's NOT burping, you really don't have a problem.


If you are really panicked about it use synthetic oil [oh now a whole new thread there] That withstands higher temps which is always a good idea anyway.

tuxnharley

210* ?

After38Years

Traffic

And climbing


I've run cars through the desert with the heater wide open to help with cooling. Dealing with overheated wife is a whole other issue!!

Synthetic can deal with higher temps, but it won't do much to avoid higher temps.

I do appreciate the input karkrafter.
I'm an engineer - I was trained to worry...

Dave Cunningham

Thanks for the correction ,tuxnharley, I did mean to say 210*

GTOguy

Messing around with the thermostat is chasing your tail. So is changing out the radiator. All of that is fine, as evidenced by no overheating driving the car. Since it is only heating up at stop lights, you have a FAN problem....not enough airflow thru the radiator core. Period. But, change the T-stat, radiator, etc. if you want.

karkrafter

''I've run cars through the desert with the heater wide open to help with cooling. Dealing with overheated wife is a whole other issue!!''


You didn't mention that!!!


You cannot spend too much money or make too much effort to keep Mama happy...happy wife , happy life.

SWCDuke

[QUOTE=ow.

Fan (finned as original type) spins for about 2 - 3 seconds after shut off of engine. Fan easily moves, when hot, with one finger. Spinning fan with one finger gives about 1/2 revolution to stop.

[/QUOTE]

It's patently clear that the fan clutch is inoperative.

Also check the VAC. If it's the original 236-16 it should start to pull at 4" and be fully deployed at 8", and total idle advance should be 16 plus 8 initial = about 24.

Duke

After38Years

So the winning suggestion(s) is the fan clutch!!

Cold engine before starting, checked the effort required to rotate the fan assembly. Drove around, then let it idle for about 15 minutes - moving close to the yellow on the gauge. Stopped the engine and checked the fan rotation effort. It was easier to rotate when hot rather than more difficult.

Decided to pull the fan clutch and see if I could do the mod of shortening the brass plunger. Well, that might be possible except that the brass plunger is seized - can't move it. It's obvious this is the original clutch. Damn - you'd think these things would last for more than 50 years.

Going to find a good reproduction. Also, the coolant is now really nasty looking - guess the overheating stirred up some gunk, so will change and flush. Found some Zerex green equivalent anti-freeze.

Thanks for all the help - will confirm the operation when the new clutch gets installed.