Disclaimer - We culled these comments and observations related to the vortec spikes from various internet forums to help people chasing their tails.  We are not able to diagnose your vortec engine overheating on the telephone.

Why am I seeing erratic temperatures in my Vortec engine?

Before we get to that; a brief history lesson. To prevent hot spots forming a standard small block Chevy has a hole in the pass side of the water pump that routes water through the head to allow water to circulate when the thermostat is closed.   This avoids a pressure build up when the thermostat is closed, and allows the thermostat to open properly.

The bypass hole is a passage that extends from the water pump diagonally through the block and through a hole in the head and head gasket. This passage provides some coolant circulation while the thermostat is closed to prevent localized hot spots. The bypass hose on a traditional big block runs from the top of the water pump to the intake manifold does the same thing for the big block. 

How is the Vortec block and/or Vortec heads different?

They don't have this bypass hole. A classic symptom of a Vortec without a bypass is erratic temperature spikes or swings e.g. 20 degrees or so. Some people even report Vortec head engine’s with 50-70º temperature swings on an analog gauge until the coolant warms up which suggests steam in the system.  

What is the cause of the temperature spike?

As the coolant temperature approaches the thermostat set point, boiling occurs in the combustion chambers in the head. As soon as the thermostat opens, it all settles down.  This will drive you crazy.

What is the solution?

GM issued instructions state "The RAMJET 350 engine includes a standard rotation water pump. This is a cast iron, long leg water pump; the same pump that is installed on the GM Performance Parts ZZ4 crate engine. Any small block engine, regardless of year, that uses Vortec heads, will require an external coolant bypass line from the intake manifold to the 5/8" hose nipple on the water pump (passenger’s side). Suggested routing is from the 3/8" boss on intake manifold to the water pump."

In summary, an engine with a thermostat will not allow any water to move in the front of the intake if you don't run a bypass. The coolant will get rather hot in the heads during warm up and since the water is not circulating, the heads just keep progressively getting hotter and hotter until enough heat build up to the front of the intake and causes the thermostat to open.

Thermostats open and close during engine operation based on temperature. In the Vortec engine, temperatures in the heads may swing wildly and out of control absent a bypass. The internal bypass in the older blocks is only on one side because that is often all you need to let the water move.

In short, radical temperature swings are managed by adding a bypass to the front of the intake.

Other things people tell us they have done in their effort to resolve the problem

1.  Some engine builders have found that improved cooling efficiency results when this bypass hole is plugged (with either silicone or a standard plug) and four holes are drilled in the thermostat. Typically, four 1/8" diameter holes are adequate.  People also report mixed or limited success and we believe in solid thermostats for managing temperature.
2. "Some" racing engine builders perceive that the bypass circulation over cools that front cylinder with extra coolant and many block off the bypass.
3. Using the heater core as a bypass. The heater core can act as a bypass but merely having heater hoses hooked up may make little difference. You need the circulation in the block and heads, not just circulation. Additionally, if a heater valve is used, then it will cut off the flow and the added pressure may shorten the life of the heater core.

The history of the Vortec 

Vortec is a trademarked name for a line of small block engines for General Motors trucks. The name first appeared in a 1984 advertisement for the 1985 model year 4.3 L V6 that used "vortex technology" to create a vortex inside the combustion chamber, creating a better air/fuel atomization

GM introduced its Vortec line of engines in the mid-1990s, in I4, I5, I6, V6 and three V8 configurations in various displacements from 2.2L (I4) all the way up to 8.1L (V8). When enthusiasts speak of "Vortec" engines, they're typically referring to the three V8 lines. The first Vortec V8s were 5.0L and 5.7L versions based on the venerable Small-Block Chevrolet V8 that was introduced in 1955. GM also created the 7.4L and later 8.1L Vortec V8s that were based on the updated Gen-V and Gen-VI updates of the classic Mark IV Big-Block Chevy V8.

In 1997, GM introduced its all-new LS-series small-block, which shares little except a bore-spacing dimension in common with the classic (Gen-I) Small-Block Chevy; Vortec versions of the LS were installed in trucks, though their design differs little from non-Vortec LS engines (like those in the Corvette or Camaro), since the LS-series design benefited from the Vortec lessons and all LS engines employ some of the principles pioneered in the original Vortec engines.

When auto enthusiasts speak of "Vortec" engines, they tend to be referring to the 5.0L/5.7L engines based on the original 1955 Small-Block Chevy V8, so that's what the following info is primarily about, unless noted otherwise. Again, there are fewer differences between LS-based Vortec truck engines and Corvette/Camaro LS engines, since the Vortec lessons are incorporated into all LS engine designs.

Differences between Vortec and other SBCs

The main difference between a Vortec engine and its predecessors were the cylinder heads, which were redesigned to improve combustion efficiency. Reshaped intake ports improved both fuel atomization and cylinder filling by promoting higher air flow velocities through the ports to the combustion chambers. The chambers themselves were redesigned to create a swirling, vortex-like flow of the air/fuel mixture within the cylinder, which kept the fuel more evenly mixed with and distributed throughout the combustion chamber so that when it was ignited the burn was a faster, more thorough burn that produced more heat and thus more cylinder pressure, which then exerted more pressure on the piston, forcing it down the cylinder with more force, which equates to more power.

You can have Vortec heads, but you don't have a Vortec block. The original all-Vortec assembly block would have no 3rd hole, as it is the thermostat bypass water passage not present in Vortec heads and blocks.

Reverse Rotation

The serpentine belt that runs all accessories off one belt uses a "reverse rotation" water pump, because of the way the belt is wound around everything they use the back side of the serpentine belt wrapped in the opposite direction around the pump. Ford V8 serpentine belt is the same way, pump has to be a reverse rotation compared to earlier v-belt system pumps.

Engine driven cooling fans also have to match the pump, reverse rotation pumps have to have reverse pitch fans because they are spinning in the opposite direction, make sure the fan is also correct for your application or it will impact cooling also. Using the wrong fan will try to push air forward through the radiator, fighting road airflow and also leading to highway overheating.