Hey all,

I have the assembly manual but I can't tell for sure. Is the shroud supposed to tuck into the gap between the side tank and fins or does it sit on top of the ridge on the side tank?

I'm chasing an issue with the car getting hot at idle with the AC on (230+ after 5 minutes of idling on an 85 degree day, verified with a laser thermometer). It hasn't boiled over but 85 is not where we top out here in Texas. Once I'm moving, the temp hovers between 200 and 205 whether on surface streets or the highway. I'm pretty convinced that if I got stuck in traffic on a 95+ degree day, I'd be in trouble. Radiator support, radiator (standard brass) and shroud have all been replaced by PO within the last 5 years or so. Radiator to support seals are there as is the hood seal. The bottom seal on the shroud is there but the top seal is missing and the side seals aren't making proper contact with the radiator. One side of the shroud is tucked into the gap between the fins and and tank but the other side is not and there's half inch gap between it and the radiator. I'm trying to get the radiator bits sealed up correctly to reassess the situation before traveling down the path of electric fans with the requisite electrical upgrades that go along with that. I have a stage 1 Stewart high flow water pump and a 180 degree t-stat that I put in this year. The fan is a 6 blade fan installed by PO with an unknown clutch but the clutch "seems" to be working correctly as it only spins a few rotations when I kill the engine after it's hot. The fan IS a little deep into the shroud I think, a bit more than half way due to me having to shim it out to keep the clutch from bottoming out on the water pump shaft. All of the air has been purged from the system and the timing has been meticulously set following Lar's papers after verifying TDC with a piston stop tool. The carb was rebuilt by the man himself late last year so I'm 100% confident it is setup correctly.

I'm also missing the rubber flaps that cover the control arms. Are those at all relevant to cooling the engine compartment?

Appreciate any insight on what else I should check after the shroud is sealed up if I've missed anything obvious. I've just rewired the whole car with Lectric Limited parts and I hate the thought of having to dig into my brand new harnesses to run upgraded wiring to support a larger alternator and electric fans if I don't really need to.

Thanks!

 

Before I would consider an electric setup Id completely go through,replace the cooling components. You're going to spend money regardless so why not go with a new radiator,seals and anything else you will need. Getting the radiator/shroud out is a total pain in the ***,ask 10 people what's the best was to get it out and you will get 10 different answers. I've done it,send me a message if you need some tips.

 

Flaps on a arms I don’t think would matter.
how about some pics??
no reason correct OEM setup should not work.
look close at fins inside radiator for openness ..an aluminum radiator i think would help everything..
they will gain temp at idle and ac…that condenser preheats the air pretty good so your ac must be working good!

 

Will definitely hit you up for hints if I end up going that route. Thanks for the offer!

 

Forgot to mention. The AC is a Vintage Air install done by PO so yes, it does work very well. Nothing on the cooling system is more than 5 or 6 years old and is in pretty good condition. Right now I'm just trying to figure out how the shroud is "supposed" to fit because it's definitely sitting in there catywampus at the moment and not sealing like it's supposed to.

Appreciate the comments. Thank you! I'll take some pics and post up.

 

What's your thermistor temp? . The radiator feeds from the top of the tank on the driver side thru the veins to the tank on the passenger side . I had plugged veins.

 

Sorry to be thick… thermistor temp?

 

Post some pics im sure someone can share their 75 on fitment

 

Anytime I hear "my Vette is running hot" my first question is what is not OEM original to the car or what has been changed. A 1975 350 Corvette did not run hot when it left the dealer lot and was used as a daily driver for a lot of owners back in the day. I had a 75 back in the late 80's that was a daily driver and it never ran hot, a 74 that was a part time driver that also never ran hot and currently own a 73 that also never runs hot. What those three cars all have in common is they are OEM original in the sense that any replaced parts were GM or reproduction to GM specs. Example, my 73 has a Dewitts OEM radiator.

Data point --> Water boiling point is 212F, at 15 psi (radiator cap) that boiling point is increased to 249.8F. A 50/50 mix of water and antifreeze will add 11F buffer to those number so in reality the car pukes out the overflow when the water temp is 260F or higher.

You stated the car doesn't puke out the overflow so we're looking at reducing the operating temperature vice fixing something that is broke causing an over temp condition. I would start with verifying the engine timing is correct first and then move on to what is not OEM correct. Couple thoughts right out of the box based on your statements is the fan being too far into the shroud and a stage 1 Stewart high flow water pump. If water is being pushed through the radiator too fast it doesn't have time to cool or in simple terms it's not letting the radiator do its job. There should be about a 20 degree difference in the inlet and outlet temperature if the radiator is doing its job.

Because you don't have a boiling out the overflow problem I would also verify or the best you can verify that the temperature of the coolant exiting the engine block matches the temperature gauge or is close. An IR gun will prove to be your best tool for this. Not unheard of for a mismatch between the sender and gauge causing a 10 to 20 degree difference.

Keep us updated on this as we all can learn from these over temp issues.

Below is a link from 2008 when I was chasing a temp problem on my 65 Vette.
https://www.corvetteforum.com/forums...ed-for-me.html

 

I replaced the leaky original radiator on my low mile 1976 Corvette (should turn 50,000 this year....maybe) with a DeWitts Direct Fit replacement aluminum radiator and new hoses. It stays at 200 degrees or below with the original fan set up. I'm very happy with the radiator. I used their brand antifreeze made for aluminum radiators. The radiator is not painted black. I replaced the radiator without removing the hood but I did cut the fan shroud in the middle on the top and the bottom. It seemed impossible to get it out without doing that. The old radiator was harder to get out than it was to insert the new one back in! It seems like I had to rotate it side to side to get it out with my wife's help but, since I knew the trick, the DeWitts just went back in real easy. I used some bright flat aluminum stock with some self threading screws to splice the shroud back together on the top and bottom. Doesn't look 100% stock but it is a clean splice and looks fine to me! Looks like a very professional Bubba! Thank you! That shroud is in a really tight area originally and my solution worked for me. YMMV!

 

See Post #21 in the below link on how to R&R a C3 radiator without butchering your fan shroud.
https://www.corvetteforum.com/forums...replace-2.html

 

There is a small tab down by the lower hose that hangs up when pulling the shroud. The easiest,quickest way the remove the radiator is to remove the entire assembly as one piece. 6 bolts for the radiator support,remove the hood and lift the assembly out. I made handles that bolt to the top of the support. Many my view this as a bit extreme but I spent hours getting everything out,5 minutes dropping it back in place. As stated I'd remove everything and go from there. I got a very nice DeWitts radiator,had the support painted and replaced the seals. Car runs cool without the lower valence,spoiler. These are in the process of being replaced.

 

One reason not to replace the entire cooling system with another stock system is even after spending and easy 4-500 bucks to do this the obsolete mechanical system will never cool as effectively and efficiently while the car is sitting at idle which is when the OP's system is not keeping up. A mechanical system is limited at idle by things like engine rpm which directly governs the fan speed where as the electric setup will have full cooling power to the fans here. This is why the factory c3s later came with an auxiliary electric fan which came on with the AC to help with this very shortcoming and issue.
An electric setup can cost as little as $300 with the high amp alternator the OP will need.
Op I would try sealing up the factory shroud better first abd make sure your clutch isnt bad.
And ignore the tip to remove your lower spoiler to help. That will do the very opposite of helping things.

Ill tell you what... Going electric with my fans was one of the best day and night differences I seen with my 74, which I did AFTER replacing my radiator, hoses clutch, seals and multiple thermostats...
And now it's way easier to work on things in my engine compartment such as replacing my A arms/ bushings and adding the a arm brace.
And it's a great way to gain 7-14 dyno proven hp vs a functioning clutch fan

 

Great info and comments, thank you.

I did some additional data collection this morning after reading your post. Keeping in mind that ambient was only around 70, I drove around for 20 minutes and then let it idle for 20 minutes with the AC on and the hood closed. I did put the top seal on the fan shroud because it was easy but one side is still not sealed up properly. Inlet temp as measured on the radiator inlet itself with an infrared thermometer was at 237, outlet temp was at 204.

As far as OEM type parts. This car is definitely a hodgepodge. Engine is not original, the pulleys they used seem to be a mismatch of bits. The part number on the crank pulley for example comes back to a big block and I don't know for certain what the water pump pulley is from. I had a heck of a time finding a water pump that would let me get the pulleys even close to lining up and when I did, the shaft is sticking so far out of the fan flange, it bottoms out in the clutch before the fan can fully seat, thus the necessity to shim it out a bit. The radiator is OEM style and not aluminum and the shroud is re-pop from Corvette America or Zip. It's filled with premixed Prestone antifreeze which is around 6 months old with maybe 300 miles on it. I recently installed a new temp sender from Lectric Limited specifically calibrated for a 75 Corvette. With the previous sender, the gauge was never getting above 150 degrees.

It's hard to get pics that show any relevant information but here's a few just to give us all something to look at.


Fan depth into shroud


Radiator and Shroud installation


Gauge after idling 20 minutes with the AC on.

 

P.S. I don’t know if anyone remembers “lsintampa” who used to post here but this is his old car which I bought from him via BaT.

 

From page 5 of the attached article Corvette Cooling System

Factory fans are very carefully designed for maximum efficiency (and minimum noise, which is why the blade positions are staggered), and are designed to provide maximum efficiency when the tips of the blades are half-in/half-out of the rear edge of the shroud, with approximately 1/2” clearance from the blade tips to the shroud. The radiator/shroud/fan combination on each Corvette is the result of a lot of tedious hot-weather development work by the engineers who designed it, and the original system is tough to improve on, assuming that all the components of the cooling system are functioning properly and haven’t been butchered, altered, removed, substituted, or backyard- engineered to “improve” them. These cars didn’t overheat when they were new, and they shouldn’t now, if the “system” is still composed of the correctly configured components.

 

Have you verified the timing is correct, there are a number of threads on this subject.

 

Thanks for pics..any chance to inspect ndide radiator for fins?

 

My 73 L82 fan is about 3/4 of the way in the shroud and has about 1" or more of clearance to the shroud.

 

Nothing in your pictures jumps out and says oh there is your problem, I would start with verifying engine timing first.