Troubleshooting RX-7 Cooling Problems

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It seems that every spring when the weather turns hot, the RX-7 forums and mailing lists are flooded with questions related to overheating and running hot. The aim of this document is to help you troubleshoot any cooling problems you may have, thus avoiding the complications (ie. engine rebuilds) that come with overheating.


There is nothing particularly special about the RX-7 cooling system, except the fact that there are actually two in the car.

The first system is the one we are concerned with. This is the water system. Like any other car, it contains 4 key components; the thermostat, the radiator, the water pump and the fan. All are pretty standard, and overall fairly reliable.

The second cooling system found in the car is the oil cooling system. It consists of the two oil hoses at the front of the engine, the oil cooler, and the bypass thermostat. The oil cooling system is very reliable, and rarely causes problems outside of leaks. Thus, we are not concerned with this system and will be focusing our attention on the water system.

For the purposes of this document, normal operating temperature is defined as 1/4 on the stock temp gauge for '86-'88 car, and 1/2 on the stock gauge for '89+ cars. Note that the stock '89+ gauge is truly terrible and has basically three positions: cold, normal, and "new engine". If you see the gauge start to creep up, immediately shut down the car and deal with the problem. In '89+ cars, by the time the gauge reaches the "H" position, the car has been running way too hot for way too long.


The actual troubleshooting process is pretty straightforward. As the cooling system is fairly simple, the problems are also fairly simple, with easy (if not necessarily cheap) solutions.

Before You Begin
Replace all your hoses and flush the system. Replace both the upper and lower rad hose, as well as the heater hoses located at the passenger and drivers side of the engine. The drivers side heater hose is a very common failure item due to the oil saturation that results from it being directly under the oil filter. Any cracks in these hoses will result in loss of coolant and system pressure, which will most likely result in the car running hot or overheating. Flushing the system is important as it helps clean out the system, and replace old and tired coolant. Use a 30/70 mixture of ethylene glycol and water. Never use "red" coolant (Dex-Cool) as it is very corrosive to the seals inside the rotary engine.

If flushing the system and replacing hoses has fixed your problem, congratulate yourself and sit down with a drink of your choice. Otherwise, continue reading.

Overheating In Traffic (Stop and Go, Low Speed Driving)
Overheating in traffic and low speed driving has one common cause: the fan. During stop and go, low speed "creeping" and low speed driving, there is not sufficient flow of air through the radiator. Therefore, the engine has a fan to draw air through the front of the car. This fan operates off of a viscous fan clutch. Basically, the fan clutch is mounted between the fan and the engine. As cold air flows over it, the clutch loosens up, allowing the fan to freewheel on the shaft of the engine. The fan then hardly turns. As the air flowing over the clutch heats up (as the engine transfers more heat to the radiator) the clutch tightens up, spinning the fan faster and drawing more air through the radiator. The cycle continues as long as the car is running. Over time, this fan clutch can wear out, causing the fan to freewheel whether it is hot or cold. This causes reduced airflow through the radiator, and therefore high operating temperatures during low speed driving. This theory is easy to test, because once you get on the highway the car will cool right down.

Testing the fan clutch is pretty easy. Simply start the car and allow it to warm up to normal operating temperature. Shut it off, then give the fan a good flick with your hand. If it rotates more than about 1.5 turns, then odds are the fan clutch is worn. However, I have found this test to be a very bad indicator of the true condition of the clutch. A much better test is to simply pick another one up from a local wreckers (they are cheap and available) and swap it on. If this solves your problem, then the fan clutch was obviously the culprit.

As mentioned, replacements are available at many wrecking yards. You can also purchase a new unit from Mazda, but I don't even want to think of the cost. If your fan clutch is bad, you might want to put some serious thought into just replacing the stock fan with an electric unit. It will cost less than a new fan clutch from Mazda, and offer slightly less drag on the engine resulting in better throttle response. Good fans to use are made by Flex-A-Lite and are available from Summit Racing.

Overheating During High Speed Driving
High speed driving in this document is defined as highway or freeway driving. Overheating during high speed driving has two common causes: a blocked radiator and/or a missing underbelly pan.

As radiators get older, mineral deposits and corrosion build up and clog the core. If the coolant is not regularly changed, this problem is common as old coolant looses it's corrosion resistance after a few years. Checking for a clogged radiator is fairly easy. Remove the fan and fan shroud from the engine, then start the car and pay careful attention to the temperature gauge. Without the fan, the car basically has no cooling, so you don't want to overheat it. Allow the gauge to reach normal operating temperature, then immediately shut off the engine. Using your hand, feel the radiator for cold and warm spots. A good rad will be uniformly hot, whereas a clogged rad will have cool/warm and hot spots. Be careful, because the hot spots can be very hot, and the fins on the radiator can cut your hand. This test is not completely certain, as a clogged rad may be so bad that it uniformly shows "warm", even though very little heat is being transferred to the coolant. A rad shop can help you diagnose a clogged rad, and can mostly likely repair your bad one by either rodding it out or re-coring it.

The 2nd gen rads have plastic end tanks, so don't be surprised if it cannot be fixed. Luckily, aftermarket copper core rads are available from places like Absolute Radiator. If you have the money to burn, I would suggest installing either a Fluidyne or Koyo replacement. These are all aluminum, and offer much more cooling capacity than stock.

The second part of the problem can be the plastic underbelly pan. This is a plastic shroud that goes from the front of the bumper to the front subframe. It not only protects the engine bay components from damage, but serves to duct air through the radiator and oil cooler. If this pan is missing, air will hit the radiator, and be deflected down under the car instead of being forced through the car. The result is a 20%-30% reduction in cooling capacity, and increased operating temperatures. If this pan is missing on your car (as they often can be) then it should be replaced immediately. Replacements are available from Mazda, as well as most wrecking yards.

Constantly Overheating
What could be considered constant overheating would be overheating at idle and during any sort of driving. Normally, the car is started cold and the temperature just keeps increasing until the car has overheated. There are 4 main causes of this; thermostat, blocked radiator and water pump/belts.

The most common cause of constant overheating is the thermostat. While Mazda thermostats are very reliable, like everything it will eventually wear out. The thermostat then becomes stuck or sticky. This results in less (or no) coolant flow through the radiator. With no coolant being pumped through the radiator, no engine cooling will take place. Testing a thermostat is pretty simple, though rather pointless. For the minimal cost of a new unit ($20) it makes very little sense to test an old one as it is probably way past it's due replacement anyway. Replacing the thermostat is a standard maintenance item, and should be done once a year (at the same time you flush and fill your cooling system). If replacement of the thermostat cures the overheating problem, then it was obviously the cause. Note: Do not remove the thermostat entirely. This will result in an overheating condition. Always replace the thermostat with a genuine Mazda unit. Generic replacements will be unreliable in this application.

Clogged or blocked radiators must be really bad before they will cause constant overheating, so this is not a common cause. More than likely the condition was found before it got to this point. However, cars that have been sitting for a long time with the improper coolant mixture can have their radiators completely blocked with corrosion. Use the techniques explained earlier in this document to troubleshoot a clogged radiator.

In general, the RX-7 water pump is a pretty reliable unit. It's normal failure mode is leakage, which is indicated by coolant running out of the "weep hole" behind the pulley. However, badly corroded water pumps can result from long periods of sitting, and improper coolant mixtures. The result is that the impeller is slowly eaten away, dramatically reducing the ability of the pump to move coolant through the engine. This obviously results in very little cooling. The cure is to replace the pump. This should be taken as a "last resort", as normally this pump damage occurs over a long period of time and is noticeable by ever increasing coolant temperatures (over a period of months or years).

Slipping or missing belts can have a dramatic effect on the operation of the water pump. As is common practice, the airpump is often removed. The removal of the airpump drive belt from the water pump will cause the remaining alternator belt to slip under high load. This means that the water pump is not turning as fast as it should, reducing it's effectiveness. Of course, if the alternator belt then snaps, you have a water pump that is not turning at all. Presumably, this condition will be noticed by the driver before it becomes an issue (ie. lack of power, all idiot lights on, dropping voltage). Even if you still have both belts in place, the failure of either belt can cause the other to be kicked from the pulley. Again, a failure of this type is hard to miss. Older and rotted belts will also slip. The solution is, of course, to replace your belts as should be done every year.

Overheating At High RPMs
Constant driving at high RPMs will often result in increased coolant temperatures (as expected) and if continued for a long period of time (more than a few minutes), overheating. There are two main reasons. One, the engine is being asked to produce it's maximum power. This of course results in more heat, and is completely normal. The second reason is that at constant high speeds, the water pump cavitates. This means that instead of pumping, the impeller simply "churns" the coolant within it's pump housing, not really moving very much at all. After a number of minutes, this lack of coolant flow will cause increased temperatures or overheating. The solution is to install a set of under drive pulleys if you intend to run the engine at high RPM often (ie. track use). The under drive pulleys slow down the rotation of the water pump and other accessories, making them more friendly to constant high speeds.

Constant Violent Overheating (Coolant Gushing Out Of Overflow Bottle, High Temps, Car May Spew Clouds Of Steam)
This almost certainly indicates a failure of the coolant O-rings. Combustion gasses are being forced into the cooling system, over pressurizing it and causing coolant to be ejected as the gas bubbles force their way out of the system. Normally, the car will also emit clouds of sweet-smelling steam from the exhaust. The only cure is to rebuild the engine to replace the damaged seals.

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