Understanding the Atomic 4 late model bypass cooling system

If you’re a fairly recent owner of a late model Atomic 4 and find all the discussion about bypass type cooling systems more than a little confusing, you’re certainly not alone! You won’t find many examples of automotive engines using a by-pass type cooling system today other than (I’m told) in a few vintage British cars. So, even if you come to our fraternity as a bit of a motor-head, your prior experience may not be of much help.

An original late model Atomic 4 bypass cooling system will have two main features that distinguish it from more “ordinary” systems; (1) it will have a double-acting thermostat under a small dome shaped housing in the front corner of the cylinder head, and (2) it will have a bypass hose leading from a “T” fitting on the starter side of the block (under the alternator) up to an inlet fitting on the thermostat housing. More about that stuff later.

Late model cooling systems are sometimes also referred to as “Full Flow” systems in recognition of one of their major design features; e.g. all of the water from the water pump moves through the cooling system and out through the exhaust system at all times, regardless of the action of the thermostat. This full flow feature is important in marine applications where engine cooling water is used to cool the rubber hoses and plastic mufflers in modern water-lift exhaust systems, but please note that the term “full flow” does not mean that there is a full flow of coolant from the pump moving through the block and head unless you would manually completely block the bypass hose.

This flow schematic (elegant in its simplicity) was inspired by one of our long time Community Forum members (Rigspelt). It does a great job of laying out the basic coolant flow within a late model bypass type cooling system.

As shown, the schematic is specifically applicable to Raw Water Cooled (RWC) engines. Fresh Water Cooled (FWC) engines will have their coolant (usually 50/50 water and permanent antifreeze) routed from the back of the manifold through a heat exchanger to be cooled and then returned to the inlet of the coolant pump. FWC systems will also have a second pump to pump raw water through the “sea water” side of the exchanger and out through the exhaust. The operating concepts of RWC and FWC cooling systems relating to coolant flow through the engine are essentially the same.

If we ignore the role of the thermostat for a moment, we can see that all water from the pump is able to take one of two paths. It can enter into the engine block through the stem of a “T” fitting and flow up through the thermostat housing, or it can flow across the branch of the “T” and bypass the block, going directly up to the inlet port on the thermostat housing. In both cases, the bypassing water and the water from the block and head combine in the thermostat housing and flow together over through the manifold and out with the exhaust.

The role of a double acting thermostat:

The first function of the thermostat in a late model bypass system is to regulate the movement of coolant up through the block and the head. This first function is not much different from a typical automotive cooling system where a single-acting thermostat simply regulates the flow of coolant going from the engine to the radiator.

The second function of the thermostat is to regulate the bypassing flow as it enters the thermostat housing. Controlling the bypass flow at the thermostat housing has the effect of increasing or decreasing the flow of cool water entering through the “T” fitting, which is important in maintaining a constant temperature as the thermostat opens and closes during power changes.

The following photo sequence illustrates the functioning of our MMI double-acting thermostat during warm up, at normal operating range, and during an overheat condition in case a problem develops somewhere within the cooling system.

The temperature ranges given are for our 150 degree version which is typically preferred for raw water cooling systems. For our 160 degree thermostats, you can add 10 degrees. The 160 degree units are typically preferred for fresh water cooling systems which can benefit from somewhat cleaner combustion associated with slightly higher operating temperature.

During engine warm-up (below 150 degrees), the main valve of the thermostat is closed which is preventing coolant from passing up from the block and head. At this time, all flow of coolant from the pump is bypassing through the branch of the “T” fitting and flowing through the thermostat housing directly to the manifold and out with the exhaust.

As coolant temperature through the thermostat rises through 150 degrees, the main valve will start to open and allow hot coolant to flow up through the thermostat from the block and head. At the same time, the conical shaped center of the main valve will begin to attenuate the incoming bypassing coolant which forces more cold coolant to backup and flow into the block through the “T” fitting. This simultaneous blending of hot water leaving the block with cool water entering through the “T” fitting provides very accurate temperature control over a wide range of operating conditions.

If anything happens to interfere with the normal functioning of the cooling system (a blockage occurring somewhere in the system, pump problem, etc.) the coolant temperature will rise until the thermostat is fully open at approximately 185 degrees. Notice that the bypass loop is completely blocked in this condition to provide maximum cooling within the (reduced) capability of the cooling system.

How and where is engine temperature measured?

The factory location for measuring engine coolant temperature is a 1/2” NPT boss area in the front of the head, just below and slightly to the right of the thermostat.

During our dyno tests, we install a manual temperature gauge in that boss area which has a sensing probe that extends approximately an inch in past the inside of the front of the head. In this location, the sensing probe of the gauge is very close to the lower part (the sensing part) of the thermostat, from where we’re able to directly monitor the ability of the thermostat to regulate the coolant temperature at various power settings.

Electric gauges will typically read approximately 10 degrees hotter than our manual gauge because their probes are shorter and they tend to conduct a bit of additional heat from the front of the head. If you mount your temperature sending unit on a fitting to accommodate a limit switch from a warning system, your cockpit indication will be approximately 10 degrees cooler than the temperature shown on our manual gauge.

If you have access to a hand-held IR temperature gun, the temperature on the outer face of the cantilevered part of the head (the part that extends out past the side of the block) measures very close to the temperature we read on our manual gauge. This means that you could use the temperature of that spot on your engine to calibrate your electric cockpit gauge to the temperature that your thermostat should be maintaining.

So what should the coolant temperature be?

Relying on information from Universal Owner’s manuals from the 1970s, the short answer seems to be 140 to 160 degrees. Earlier recommendations were slightly lower at 140 to 150 degrees.

These temperature ranges are consistent with the temperature rating of the “three-spring” Holley thermostats which Universal used throughout the 1970s and 1980s. The Holley thermostats (as well as current O.E.M. thermostats) start to open between 140 and 150 degrees, but they don’t do much to attenuate the bypass flow until they are almost fully open at around 180 degrees. For this reason, they tend to maintain temperature between 150 to 170 degrees at normal cruising power settings in reasonably well maintained cooling systems.

In terms of routine monitoring and trend analysis on your cooling system, we recommend that you pay attention mostly to the temperature you see on your cockpit gauge (plus or minus whatever conversion factor you may be using) while stabilized at your normal cruising power setting. Stabilized temperature values at normal cruising power can be used to make useful comparisons in analyzing trends between seasons as well as working through troubleshooting exercises.

Troubleshooting suggestions:

There is an abundance of specific troubleshooting suggestions on our Community Forum, also in the cooling system chapter of our MMI Service and Overhaul Manual, and in the instructional videos in our online catalog.

By far, most cooling problems involve a restriction somewhere within the system, or a problem with the water pump. Here are a few suggestions to help you get into the right neighborhood to locate and correct such a problem:

Check first to see if you have a normal flow of cooling water coming out through the exhaust system at idle. At idle, a normal flow of engine cooling water will usually be sufficient to build up a head within the water lift muffler to overpower the exhaust pressure and cause the water to sequentially “batch” out of the exhaust every second or so. At normal cruising power, the cooling water should come out with the exhaust steadily.
If you observe a normal flow of water out of the exhaust, you will have to look for a restriction that is preventing water from entering the block and causing it to preferentially move up through the by-pass loop. Examples of such a restriction are a blockage within the “T” fitting, the diverter cap on the inside of the “T” fitting, or a thermostat stuck in the closed position.
NOTE: You can restore normal flow through the “T” fitting by removing it and pushing the diverter cap away from the back of the water jacket side plate. The cap will fall harmlessly to the bottom of the water jacket. Quite a few Atomic 4s are running around with no diverter cap with no profound ill effects, so you can operate the engine for a season or so until you have time to remove the water jacket side plate to retrieve and clean (or replace) the diverter cap.
If you do not have a normal flow of water through the exhaust system, your water pump could be too weak to provide sufficient water flow; or you could have a restriction in a location that can limit the total flow of water. Examples of “full flow” restrictions include a blocked raw water through-hull, a piece of broken impeller in the elbow on the outlet of the water pump, or a blockage in the water jacket of the manifold (frequently within inches from the rear water outlet).
If normal coolant flow can be determined from the pump and no restrictions can be found, check for a thermostat being stuck in the open or closed position. A defective thermostat will usually result in overheating, but if the thermostat is stuck in an open position an overcooling problem will develop.
Inspect the condition of your thermostat housing for two important features: (1) a clean sharp groove around the bottom of the housing. This groove insures the proper centering of the thermostat as well as preventing coolant from passing up around the base of the thermostat, and (2) a clean inlet port coming down from the center inside of the housing. A double-acting thermostat cannot seal adequately against the inlet port to control the bypass water if the inlet port is irregular from corrosion.
If you have an FWC system, the basic engine part of your cooling system is usually less problematic, but you will have to add your raw water pump and the raw water side of the heat exchanger to your troubleshooting checklist.
You can sometimes improve coolant flow through the engine by clamping off the bypass loop with a pair of vise grip pliers or a small “C” clamp. This procedure will insure that you’re getting the maximum amount of cooling from your system in its reduced capability until you can get to a safe harbor and make a more permanent repair of the cooling system.

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If you do not have a normal flow of water through the exhaust system, your water pump could be too weak to provide sufficient water flow; or you could have a restriction in a location that can limit the total flow of water. Examples of “full flow” restrictions include a blocked raw water through-hull, a piece of broken impeller in the elbow on the outlet of the water pump, or a blockage in the water jacket of the manifold.

… we can see that all water from the pump is able to take one of two paths. It can enter into the engine block through the stem of a “T” fitting and flow up through the thermostat housing, or it can flow across the branch of the “T” and bypass the block, going directly up to the inlet port on the thermostat housing.