CorvetteForum - Chevrolet Corvette Forum Discussion - Corvettes, Summer, and High Coolant Temperatures

Edited/upgraded thanks to Photobucket

For years I've seen hundreds of threads during the summer months. The responses are always the same, I've searched, and created this thread based on the typical answers from many CF members. I hope this helps out the regular nOOb who comes here asking the same old questions.

Summer is here, and this means it is hot outside 90°F~120°F.
“My Corvette is running hot” is the number one thread of all time in the forum since the day it was created. Forget about you been old school, these cars do not have a carburetor, so be ready to change your opinion about these cars. On the C4s, the fans start around 235°F. This can be changed by reprogramming the car’s computer or chip.

Before you attempt to fix the issue with your keyboard asking the obvious ensure you take these “quick-fix” recommended steps before starting an irrational thread.

1. Average operating temperatures are between 190°F to 220°F, and it stretches to 235°F.

2. Is the cooling system filled up with antifreeze, and when was the last time you replace the cooling system cap?

GM used green antifreeze 1984-1995, and orange Dexcool in 96 Vettes.

3. Are the fans working?

4. Is the water pump operational?

If you replace the thermostat, a 1/8" hole in the thermostat flange will help purge air during the burp/cooling cycle. Performance thermostats like the famous 160°F 'stats already come with a small orifice in them.

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5. Is the radiator shroud area clean? This would include the radiator and condenser.

The radiator shroud collects all sorts of road debris like; newspapers, leaves, plastic bags, anything that it may fit, and make its way in there. Those things and dead bugs will prevent proper airflow through your radiator.

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6. Are the front spoilers in place?
The three-part front spoilers are part of the C4's cooling system. They must be in place, not on top of that speed bump or driveway.

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7. Has the cooling system been burped?

On the LT1 engines, it is done by opening the screw on the thermostat housing.

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On the L98, few members here recommend to max-lift the front of the car. The majorities do this; fill up the cooling system,
start the engine and when the thermostat opens up and the coolant level in the radiator drops, keep filling the radiator with coolant, raise the engine RPMs to around 2,500 RPMs, keep adding coolant. Once the system is filled up, put the pressure cap back on and release the throttle!

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8. Can the radiator be upgraded?

If you would like to run cooler than average, visit the Dewitt web page and buy their two-row aluminum radiator. Your Corvette will run around 180°F on a hot weather day. There are other vendors, I have Dewitts in The Ghost, had it in Betty Boop and in The Stallion; and I'm delighted with their performance.

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Reading Material to magnify your LT knowledge.

LT1 Reverse Flow Cooling System
By Scott Mueller.

One of the greatest features of the '92 and up Chevrolet LT1 engine is the reverse flow cooling system. In fact, it is reverse flow cooling that is truly the key to the incredible performance of modern LT1. Reverse flow cooling is vastly superior to the conventional cooling systems used on virtually all other engines. This is because it cools the cylinder heads first, preventing detonation and allowing for a much higher compression ratio and more spark advance on a given grade of gasoline. A fringe benefit is that cylinder bore temperatures are higher and more uniform, which reduces piston ring friction. Because of this new cooling system, the LT1 can quickly meet ever-increasing emissions standards with significant gains in power, durability, and reliability.

Conventional Coolant Flow:
In conventional engine design, the coolant enters the front of the block and circulates through the block's water jacket. The coolant is first heated by the cylinder barrels. Then hot coolant is subsequently routed through the cylinder heads and intake manifold before returning through the thermostat to the radiator.

Because the coolant from the radiator is first directed to the cylinder bores, they run at below optimum temperatures, which increase piston ring friction. The heads subsequently get coolant that has already been heated by the cylinder block, which causes the heads to run well above optimum temperatures. The hotter cylinder heads promote detonation (spark knock) and head gasket failures. To combat the increased tendency to detonate, compression ratios have to be lowered and spark advance reduced, which significantly reduces engine power output and efficiency.

Besides promoting detonation, causing gasket failures, forcing reduced compression, spark advance, and significantly reduced power output, a conventional cooling system causes several other problems. Since the thermostat is on the exit side of the system, it does not have direct control over the cold coolant entering from the radiator. This is especially true when the thermostat first opens after reaching operating temperature. As the thermostat first opens, allowing hot coolant to exit the engine, a rush of very cold coolant enters the block all at once, shocking the engine and causing sudden dimensional changes in the metal components. The extreme thermal shock experienced by the engine causes head gaskets and other soft parts to fail much more quickly.

The conventional cooling system design also allows isolated engine hot spots to occur, which leads to the generation of steam pockets and coolant foaming. The coolant, which is full of air and foam, reduces cooling system performance and can even lead to engine overheating.

LT1 Coolant Flow:
The LT1 is completely different since it uses reverse flow cooling. The incoming coolant first encounters the thermostat, which now acts both on the inlet and outlet sides of the system. Depending on the engine coolant temperature, cold coolant from the radiator is carefully metered into the engine. This allows a more controlled amount of cold coolant to enter, which immediately mixes with the bypass coolant already flowing. This virtually eliminates the thermal shock present in the old system.

After entering through one side of the 2-way thermostat (at the appropriate temperature), the cold coolant is routed directly to the cylinder heads first, where the combustion chambers, spark plugs, and exhaust ports are cooled. Then the heated coolant returns to the engine block and circulates around the cylinder barrels. The hot coolant from the block re-enters the water pump and hits the other side of the 2-way thermostat, where it is either recirculated back through the engine or directed to the radiator, depending on temperature.

The main concept behind reverse flow cooling is to cool the heads first, which greatly reduces the tendency for detonation and is the primary reason that the LT1 can run 10.5 to 1 compression and fairly significant ignition advance on modern lead-free gasoline. Reverse flow cooling is THE KEY to the Generation II LT1s increased power, durability, and reliability over the first-generation small-block engine.

Thermostats:
All LT1 engines utilize a special 2-way acting full bypass thermostat. This means that the thermostat regulates coolant flow both in to as well as out of the engine, while the bypass portion of the thermostat circuit supplies the water pump with a full flow of liquid coolant at all times. This is unlike a conventional engine thermostat, which only regulates coolant flow at the engine outlet, and which does not allow full flow through the water pump when the engine is cold, and the thermostat is in bypass mode.

Both sides of the 2-way thermostat used in the LT1 are linked together, and a single wax pellet actuator operates the spring-loaded mechanism at a pre-set temperature. When the designated temperature is reached, the wax pellet expands, opening the dual-acting valve. All current LT1s come from the factory with a relatively low 180-degree temperature thermostat. Most conventional engines today use 195-degree thermostats to meet emissions specifications at the expense of power, durability, and reliability.

It is important to note that the 2-way thermostat is unique to the Generation II LT1 and is not interchangeable with older Chevrolet small-block engines. This is particularly important if you decide to change to a colder 160-degree thermostat, make sure it is the proper dual-acting type required by the modern LT1.

Additional LT1 Cooling System Improvements:
Also, to reverse coolant flow, there are several other improvements in the LT1 cooling system over conventional engines.

Dry Intake Manifold:
The LT1 has absolutely NO water running through the intake manifold! Conventional cooling systems have passages in the intake manifold which allow coolant to crossover from one side of the engine to the other. In the LT1, coolant crossover occurs in the water pump, which is also where the thermostat is located. Since there are no coolant passages in the intake manifold, a major source of leaks has been eliminated. Overall engine reliability is improved since an intake manifold leak allows coolant to enter the top of the engine, which can quickly wipe out the camshaft, lifters, and other major engine components. Designing a dry intake manifold without either coolant passages or a thermostat housing also allows a much lower profile. The LT1 engine is 87mm (nearly 3.5 inches) lower than the previous L98 Corvette engine.

Gear Driven Water Pump:
One big problem with conventional cooling systems is the water pump, which simply cannot last a targeted minimum 100,000-mile reliability figure without experiencing leaking gaskets or seal failures. This has traditionally been caused by the excessive side loads placed on the bearings and seals of a conventional water pump through the belt drive mechanism. In the LT1, this problem is solved by driving the water pump directly via a spur gear driven by the camshaft sprocket. This results in a dramatically more reliable water pump that should easily last 100,000 miles or more.

Since the water pump is no longer belt driven, the vehicle will still be drivable even if the serpentine belt fails. This is a major safety factor as it allows one to drive the partially disabled vehicle to the nearest service center.

Steam Vents:
The LT1 has strategically placed steam vents at the back of both cylinder heads. Since the heads are the hottest part of the engine, pockets of steam can be more easily generated there. The steam vents are connected together by a crossover vent tube at the back of the heads, which directs any steam and a small flow of coolant to the front of the engine where it flows through the throttle body, warming it for improved cold-weather performance. After passing through the throttle body, most of the steam is condensed back into the liquid coolant and returned to the system.

In LT1 B/D-cars, coolant exiting the throttle body is passed directly into a pressurized coolant reservoir where any air remaining in the coolant is completely scavenged. In LT1 F-cars, coolant from the throttle body connects to the heater outlet via a vented T-connector. That's where trapped air in the system can be bled off manually, eliminating steam pockets, and foam in the coolant allows for more uniform cooling system performance, preventing hot spots and potential overheating.

Reverse Flow Radiator:
Unlike a conventional cooling system, the thermostat coolant outlet is connected to the bottom of the radiator. This forces the coolant entering the radiator to push up through the radiator core and eventually emerge through the top radiator coolant outlet. This helps to eliminate air pockets in the radiator and provides a more even distribution of cooling through the core and improving radiator efficiency.

Precision Machined Thermostat Housing:
The thermostat housing is a precision machined component that fits directly onto the top of the water pump without a gasket. Instead, an O-ring is used to seal the thermostat inside the housing. This precision design reduces the tendency for leaks, plus it makes thermostat replacement a very simple job since there is no old gasket material to scrape off. Servicing is further simplified because the thermostat housing is situated directly on top of the water pump, and access is unobstructed. I dare say that the LT1 thermostat is the easiest to change I have ever experienced. Finally, an air bleeder valve is located on the top of the thermostat housing, which allows one to quickly and easily bleed out any trapped air after cooling system maintenance has been performed.

Low Operating Pressure:
The entire cooling system on the LT1 is designed to operate at lower pressures than conventional cooling systems. The maximum operating pressure in the LT1 cooling system is 15 psi for B/D-cars and 18 psi for F-cars, limited by a pressure cap. These limits are similar to other cars, but in the LT1, these maximum pressures are rarely reached. Running at a lower pressure drastically decreases the number of leaks and significantly improves overall reliability and durability.

Coolant Reservoir:
Corvette and B/D-car LT1 applications use a pressurized coolant recovery reservoir instead of a non-pressurized overflow tank used with conventional cooling systems. All of the coolant flows continuously through the pressurized reservoir, which is an integral part of the cooling system. The pressurized reservoir in the LT1 B/D-cars is connected to the cooling system in three places. One inlet hose connects to the top of the RH radiator tank, a second inlet hose is attached through a "tee" connection on the heater inlet hose, and a third outlet hose is connected to a "tee" connection in the throttle body heater outlet.

The pressurized reservoir is mounted at the highest point in the system and provides a place where all air can be continuously scavenged from the coolant. Any steam and bubbles are allowed to rise to the surface, eliminating foam and providing pure liquid coolant back to the engine. Pure liquid coolant is returned to the system via the heater outlet hose connection. The pressure relief/vent cap in these systems are rated at 15 psi and is located in the reservoir rather than the radiator.

LT1 F-cars use a conventional coolant recovery system, which consists of a non-pressurized coolant overflow tank connected to the radiator by a single hose. These cars use an 18 psi rated pressure relief/vent cap on the radiator like most conventional systems. Since these cars cannot scavenge air from the coolant as well as the B/D-car or Corvette systems, they have two air bleeder valves for manually bleeding trapped air from the system. One is in the thermostat housing, which is the same as all other LT1 engine vehicles, and the second one is located in a "tee" where the coolant from the throttle body connects to the heater return hose.

B/D-car LT1 (Caprice/Impala/Roadmaster/Fleetwood) Cooling Systems:
Standard equipment for all LT1 equipped B/D-cars is a dual electric fan setup with a 150-watt primary (RH) fan and a 100-watt secondary (LH) fan. The electronic engine coolant fans are independently operated by the PCM (Powertrain Control Module) based on the inputs from the Engine Coolant Temperature (ECT) sensor, A/C Pressure Sensor, Vehicle Speed Sensor (VSS), and various other information.
The B/D-car coolant fans operate under PCM control at the following engine temperatures and A/C system pressures:

Fan Mode ------------------Temperature----A/C Pressure
Primary (RH)** fan ON----107°C 225°F-----189 psi
Primary (RH) ** fan OFF----103°C 217°F-----150 psi
Secondary (LH) Fan ON----111°C 232°F-----240 psi
Secondary (LH) Fan OFF----107°C 225°F-----210 psi

Additionally, the PCM will turn off the fans at higher vehicle speeds (above 48 MPH, I believe) since running fans can actually impede airflow through the radiator at high speed. Each fan also has a minimum running time.

Once activated, the primary fan will run for a minimum of 50 seconds, and the secondary fan for a minimum of 26 seconds. Finally, specific Diagnostic Trouble Codes (DTCs) may cause the PCM to turn on one or both fans.