cooling off that c4 corvette


Staff member
as always, posting clear detailed pictures of any custom mods, you do to any of your cars, from several perspective angles , along with a detailed description of what you were trying to accomplish and the theory behind any mods being done would be VERY WELCOME!
85 and 87a -> Ground

86 -> Power

30 -> Output

-30 = constant [positive (+)] power (usually wired directly to car battery)
-85 = coil ground (wired to the negative (-) battery terminal or any grounded metal panel in the car)
-86 = coil power (wired to the control source. could be a switch, or it could be the car's IGN or ACC circuit.)
-87 = switched [positive (+)] power output. (when the relay coil is powered, lead/pin 87 is connected to lead/pin 30)
-87a = [on 5 lead/pin relays only] this lead/pin is connected to lead/pin 30 when the coil is NOT powered.

The CTS (coolant temp sensor) is located in the front of the intake manifold on the 84-91 C4's. It has two wires (yellow and black). The yellow wire might have a black stripe, it's been awhile since I looked.

84-85: The aux fan (if equpped) switch is located in the passenger side head between #6 and #8 spark plugs. It has a single green wire. If the aux fan is not equipped, the hole where the switch is will be plugged.

84-85: The coolant temp sender for the dash gauge or (IP) is located in the driver side head.between #1 and #3 spark plugs. It sends resistance signals to the dash gauge where the digital coolant display is.

86-89: The aux fan (if equipped) switch is located in the driver side head between #1 and !3 spark plugs. Single green wire.

86-91: The coolant temp sender for the dash gauge or (IP) is located in the passender side head between #6 and #8 spark plugs. It sends resistance signals to the dash gauge where the digital coolant display is.

90-91: Do not have a aux fan. They have two fans side by side..same as the 92-96. Both are controlled by the (90-92 ECM) or (94-96 PCM).

92-96: The CTS for the ECM or PCM is located in the water pump housing. This sensor sends signal to the ECM and the digital readout on the dash gauge.

Someone can correct me if wrong, but I believe the coolant temp sending unit for the analog guage is located in the driverside head between #5 and #7 spark plugs.

BTW most early TPI corvettes had an oil cooler mounted between the engine block and oil filter that would reduce the oil temps in the engine, by circulating the engine coolant and oil thru separate,parallel passages , this had the advantage of more rapidly warming cold oil but keeping its max temp lower than without its use.

yes they work, they easily drop the oil temps 15 plus degrees F or more, by running the oil flow alongside but separated from the engine coolant. now they are NOT as effective as a aux cooler because they can,t lower temps to quite as low as engine coolant temp levels,and if you pay attention to your gauges youll generally see oil temps tend to run 15-30 degrees over coolant temps, on most engines. while if you install the AUX oil or trans fluid coolers , with the electric fans ,those can, at least in theory lower temps to outside air temps, or at least to significantly lower levels than the coolant temps. and yes that can easily be 100F LOWER. I,d also point out that a 7-8 quart baffled oil pan, the longer length oil filters and an adding an fan cooled oil cooler can reduce oil temps very rapidly , so get a few gauges installed to help verify actually operating oil temps.













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solid fixture here in the forum
as soon as we go thru with the custom air box i will definitely post up some pics, it kind of took a back sat to the blown out turbo seal.


My old 1990 TBI chev truck had coolant crossover at the rear of the intake with the heater core pulling coolant off of it- returned to the cold side of rad. Changed over to aftermarket FIRST TPI and there is no coolant crossover at the rear- the heater core is supplied off front of intake and returned to low pressure side of water pump. Any affect on towing capacity here- was thinking about putting SS T-line from rear intake to front.


The Grumpy Grease Monkey mechanical engineer.
Staff member
Grumpy, I'm going to change my coolant hoses and fan belts because their still the original components on a 1996 corvette thats now 20 years old..
I've got 106 K miles on the car.
So I'm curious ,how many miles did you have when the trouble with your water pump?
I don,t want to get stuck or over heat!
and I hear about other damage like warped heads,leaking radiators,
and trashed opti-crap ignitions when the LT1 corvette water pumps fail?
I'm Thinking it would be easier to do it now than later. Thanks !
its CRITICAL to keep the trans fluid clean and ideally changed about every 70K miles and use of a auxiliary cooler that keeps the fluid temp under about 170F is going to extend service life a good deal longer
1995-96 corvette cooling hoses



its generally a combo of time and mileage that kills water pumps ,
as both wear and corrosion take a toll,
but at 102K your certainly well past the 3/4 -7/8 or later point in most chevy water pumps expected life spans ,
Ive seldom had a water pump exceed 100 k,
and yes failing to change the 50%/50% water antifreeze mix of coolant every few years,
or failing to use anodes and good electrical grounds,
and use of anti freeze free water accelerates wear.
yes you'll probably get more use and it will last longer , but how long and whats the peace of mind or security worth to you knowing its in good condition and not likely to fail at the worst time, miles from help, as it was recently replaced, if it was my car and I had well over 100 k miles and had the time and money I'd replace the belts hoses and water pump at over 100K miles while you have a good opportunity and the parts time and tools, like brake pads,and rotors ,if your smart they get changed BEFORE they wear out

my 1985 corvette came with a factory oil cooler, that runs engine coolant through separate but contacting internal passages, this warms the oil faster getting it flowing but tends to reduce the heat engine oil can reach as it absorbs oil heat effectively transferring it too the engine coolant on the car, where its transferred too air flow through the radiator







those aluminum finned tube coolers work, are reasonably cheap, very durable ,but a bit restrictive

but they don,t cool trans fluid or oil no where near as efficiently/fast as the larger fan equipped coolers with the AN#8 line size
and in either case finding a place to mount any cooler where you can keep it out of sight and still easily access fresh outside air flow,
is usually a problem for most people

one of the most common mistakes less than experienced performance enthusiasts, face and very commonly over-look, is the fact that the internal cross sectional area on many hydraulic and fuel line fittings are considerably more restrictive to flow that the fuel limes or hydraulic lines inside diameter they were designed to be used with, and it varies a great deal between different manufacturers, now ideally the fittings internal passage cross sectional area is both consistent and the same or greater that the tube or hydraulic line size, it listed to match, , so a 1/2" inside diameter fuel line, or hydraulic lines?hoses, for example should have components for the connections and fittings that have significantly smaller internal cross sectional areas, it does you very little good to use lets say, AN#8 or half inch fuel lines if the internal cross sectional area of the connections and fitting used with those lines is only 3/8" or smaller in cross sectional area,this is an area where dealing with a local hydraulic supply shop that has the correct tools and fittings to custom fabricate your fuel lines, coolant or lubrication lines is a very good idea!
talk to a local professional at your local hydraulic supply, measure accurately, take the time to explain what your trying to accomplish and take several pictures to show them what your doing, and get them too fabricate any high pressure fuel or coolant lines and related fittings




Up to 45 GPH= 3/4 GPM = 5/16" or -04 AN
Up to 90 GPH = 1.5 GPM= 3/8" or -06 AN
Up to 250 GPH =4.2 GPM= 1/2" or -08 AN
nearly ideal for transmission and oil coolers :D
Up to 450 GPH =7.5 GPM= 5/8" or -10 AN
Up to 900 GPH = 15GPM 3/4"or -12 AN






sources for parts

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The Grumpy Grease Monkey mechanical engineer.
Staff member
I love this thread, but I have a 1995 corvette that over heats,
if its stuck in traffic, yes the fans do come on at about 230f ,
but that seems to be awfully late and too high a coolant temp, before the fans come on.
I re-read the links and threads and see where I may have a partially blocked radiator , so how do you get to clean those,
its obvious that you can,t easily get access to the radiator too flush out the fins with a garden hose like most cars,

anything in coolant temps much over 220f is higher than ideal,
and if you think about it the clue here is that it maintains temps ,
while moving but not if stationary,
verify BOTH fans are coming on, not just one fan,
If the engine cools while moving but over heats up and over heats,while stationary,
this strongly indicates the lack of significant air flow rates when the car/truck,
is not forcing additional air flow through the radiator through its movement,

it sounds like your either low on coolant, the water pumps in need of replacement,
or the radiator is stuffed full of crap like leaves and trash like grass,plastic bags etc.
the corvette sucked up trash, off the street,constantly,
corvettes suck trash off the road like a vacuum cleaner,

you obviously need a functional combo that sucks more air through the existing radiator when stationary,
if its full of street trash it needs to be cleaned obviously, to regain air flow rates.
adding an aux engine oil cooler with its own electrical fan will also reduce the heat load on the existing radiator.
and adding an aux oil cooler and or trans fluid cooler, always results in a lower heat load on the radiator

read the linked info
pull trouble codes and check your fuses
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Well-Known Member
Did the toggle switch to ground to the fan relay on my old 85. That worked good. Gona go all aluminum Rad on the 93. Future project. R&R'd all the org hoses last year,Cap, Thermostat,TB by pass using hoses for a 96 LT 1. That eliminated 2 T's from the system.Used Prestone Extended life Anti. Cleaned the Condensor & ORG Rad with a bottle of spray Gunk,hosed that off with the garden hose. Blocked the Opti with Card board so it wouldn't get wet.

If you wish to eliminate the throttle body coolant hoses, which are used to warm the unit during cold temperatures, you can use 1995 - 1996 Engine Cooling System Rubber Hose Set [LT1, LT4]
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New Member
Wow!! There's tons of really great info here! I've read through the whole thread, but I still have questions. They might have been touched on in one of the links, I don't know. I clicked on any that had descriptors, and a bunch of others... There's so much info on fans and their systems, a bit less on specific radiator recommendations.

I'm putting a nasty big block in my 1987 C4 Vette. (Please don't lecture or judge me.) I'm in the process of doing restorative maintenance on the car, and doing upgrades that the car will need to support the hot big block soon to come.

So now, it's radiator upgrade time. I need a really great radiator, both to cool that racey big block, and also because I live in the hottest desert in the world; the Mojave. (Look it up; it really is the hottest!) It routinely gets to 130 in the shade here. Keeping any Vette and engine cool, well any car actually, is a challenge. I've been at this for a while, and understand the issues on airflow and such, and trust me; I'll address them. But what about the RADIATOR itself??

Which is truly proven better; more rows, or wider tubes?? How many 1 inch rows can I squeeze into this 1987 Vette?? Any specific recommendations for a really good radiator for this project?? I'm hoping to keep radiator cost under $400. Feasible??




The Grumpy Grease Monkey mechanical engineer.
Staff member
trust me my personal goal has been for several years,
to collect the parts required and knowledge to produce and install a larger displacement, EFI BBC engine,
connected to a 4l80E transmission i a personally owned c4 corvette,
modified with full manual control 4l80e trans in a C4 corvette with a dana 60 rear differential, I personally own,
I've helped 8 other people do similar engine swaps over the last 21 years, installing, chevy big block engines in their personal C4 corvettes ,
so I know what's involved, but I personally just never yet, have had the spare cash, to build my corvette the way I want to build it,
and since I refuse to compromise and slap something inferior, or sub-standard together,
its still a project I'm working on completing.
with the rapidly approaching future move its once more being delayed
if your going to do something I've learned you either do it correctly or wait until you can do so, or you'll always regret the results

related thread



New Member

Thanks a bunch for those links! I already had read the main one on the swap, and had it bookmarked into my "Vette" file.

I had also read through this whole thread:

However, I am still not seeing any specific radiator recommendations, nor definitive answers on which design radiator is more efficient. More rows of tubes? Less rows of wider tubes? Multi-pass? What exact configuration of tube width, row count, etc., is best, and what will fit a C4 without major surgery??

Sorry, but I'm just confused with all the options out there, and all the advertising hype. I can only afford to do this once, and when it's 130 degrees out, the LAST thing you need to have to worry about is those engine temps inexorably climbing. I also have had problems with big blocks with any compression at all being detonation prone, and want to run cool to help with that as well. As you say; it's best to build it RIGHT the first time!