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It is my opinion that more Renesis engines are lost due to cooling system failures than anything else. Perhaps it was not always the case, but as a larger and larger percentage of RX-8s are well past the normal cooling system lifetime, there are more failures occurring from that than anything else.
The rotary generates a LOT of heat. All this heat has to be shed somehow, and it's the cooling system that needs to do it. The major parts of the cooling system are:
If even one of these components fails, at all, even in a slight way, it is possible for you to lose your engine.
Our coolant seals are really only trustworthy to a coolant system temperature of about 220F (104C). Past that, you have an increasing risk of having one fail from the heat. If you pass about 240F (115C), you have an additional risk of warping the rotor housings, which is even worse. Making this all much worse, is the fact that our factory coolant temperature gauge (the one in the instrument cluster) won't start moving off of 'center' until 235F-240F (113C-115C)
So far too many owners expect to rely on that gauge to tell them if there is a problem, and expect that there is only REALLY a problem if it gets all the way to the right. All the way to the right is 300F+. If you think about his combination a bit, you can see why so many engines are lost due to cooling system failure.
OBD2 is the answer. OBD2 is an emissions compliance standard that has existed within the US since 1996, and ALL cars that are sold in the US since then are required to be compliant with it. I believe this means that most foreign versions of cars sold in the US are OBD2 compliant as well, though I am not 100% sure about that. Emissions? Yes, Emissions. An engine that is operating near 200F produces less emissions than one that is operating lower than 170F, or higher than 240F. However, the end result of the standard means that the actual coolant temperature is reported through the CAN BUS network (a communication standard), and is available on the OBD2 port. You can get an OBD2 device that reads live data, and get the actual engine coolant temperature. The ECU reports it in C in increments of 1 degree, so conversions to F will be in increments of 1.8 degrees. But it's close enough. The best way of getting at this data is through a bluetooth OBD2 adapter. Plug it in, no wires to deal with, and then pair your smartphone or tablet to the OBD2 adapter. Then download any one of the numerous free OBD2 apps ('Torque' is the best for Android), and you can get a full readout on nearly everything that the ECU can report. You can read/reset CEL codes, read live data, and record data logs. And you can do this with any car you own, or any friends and family that want help. Highly recommended. Bluetooth OBD2 adapters start around $20, and go up to $150. They all generally work, though the higher cost ones are typically longer lasting and/or have faster communications. Exceptions exist however.
Proper maintenance of your cooling system is CRITICAL. Annual cooling system flushes are important if you aren't using Mazda's FL-22. (Why FL-22? Read this thread: RX8Club.com: Issue Many Are Ignoring: Most Coolants Contain 2-EHA (which 'eats' silicone)) Every 2 years is acceptable if you use that. Mazda says their coolant is good for 5 years, but $40 in coolant isn't worth the risk of a $4,000 engine is it? DEFINITELY replace the coolant by 60,000 miles if it hasn't. Regular coolant flushes will help prevent other cooling system components from failing, however, they will still fail in time.
The thermostat is often the first to go, and is a $65 part (at most). I personally recommend upgrading to Mazmart's 172F thermostat to give your system a bit more head-room, but don't go lower than 170F for the thermostat, or you will be causing CELs, poor gas mileage, and power loss (the ECU is expecting at least 170F when up to temp). Remember that this is the target temperature of your cooling system. I won't be able to go lower than this temp, and the radiator will only see coolant that is hotter than this target temperature.
The coolant bottle is also a common failure point, though usually in the coolant level sensor before a pressure problem with the cap. The sensor is integrated into the bottle, so if your coolant light keeps coming on and your coolant is full, then the sensor has just failed. Replace the bottle with a new bottle and cap for $130ish from Mazmart. This is the main point that keeps air out of your coolant system, and maintains pressure in the system.
WARNING: Regardless of if you or someone else does the replacement, there is a little hose that runs from the front of the bottle to the top of the radiator. This hose is stronger than the radiator nipple it connects to. CUT the hose in half and remove the two halves independently, because breaking that radiator nipple means you HAVE to replace the radiator. It can not be repaired. Mazmart has a bottle listed that includes this small hose for this very reason.
The radiator is next, and typically when it starts to fail it will tend to just slowly clog up, with less and less surface area actually shedding heat. A replacement OEM spec radiator is available from Mazmart for $150 (MT, the AT is slightly more). Avoid Mishimoto, it is a well known brand, but it is inferior to OEM, AND more expensive. If you want to upgrade, talk to BHR. They developed an upgrade in the Arizona heat that will outperform OEM and last longer. Remember, the radiator will only shed heat that the thermostat lets it see. If you can't get down to thermostat temperatures, you need a better radiator. It won't ever let you go below the thermostat temperature, no matter how good it is.
If you are replacing the radiator, might as well replace the coolant lines too. They can sludge up in several points and cause their own problems. Typically something else will fail first, but if you just replace other parts and not the lines, the lines won't be all that far behind, and might destroy the new parts you installed. Silicone lines will run you around $300, rubber lines are far cheaper. Keep in mind that there are 9 major coolant lines, so most coolant line kits you see that are 3 piece obviously aren't sufficient. Even the 6-piece kits won't have critical lines replaced. The best bet is to order all new OEM lines individually, rather than a kit.
If you are getting an engine rebuilt, then there are improvements that can be made to the coolant passages in the engine which help reduce localized hot spotting and improve the transfer of heat from the block to the coolant. Talk to your builder for details. I believe Racing Beat pioneered those modifications, but I may be wrong.
Just because the needle moves and you are in danger territory doesn't mean that you automatically have engine damage, but it's likely. Cases have been known where people have avoided damage. It isn't likely though. Once you have fixed the cooling system, you need to test for coolant seal failure. Pressure test the cooling system is a good way (most corner shops), as long as you know that every single other part of the system is fine. Otherwise, get the oil tested (Blackstone Labs) for coolant, get the coolant tested for combustion gasses (most corner shops), and inspect the plugs for the presence of coolant after it sits overnight. Failing any of these means you need a new engine or an engine rebuild.
If you are only overheating when stopped or at a low crawl, the problem is most likely a fan airflow problem. Either the fans are running when they should be, or your radiator is getting blocked. Safely stop the car and shut the engine off and see if you can find an obstruction to the radiator or fans.
If you suffer a catastrophic cooling system failure that starts spraying coolant everywhere, shut off the car immediately. It is highly unlikely that you will avoid engine damage, but it's possible. As little as 10 seconds of running without coolant in it can be terminal for the engine.
This is the troll under the bridge. There is a significant amount of evidence that has been slowly and quietly building beneath the surface of community knowledge that points to one of the primary causes of early reman engine failure, and that is the excessive amount of sealant used in the Mazda remans. This sealant starts to break off as the engine heat cycles, and the bits and pieces and chucks start circulating around the cooling system, eventually clogging the radiator, coolant lines, thermostat, and/or coolant bottle. The engine overheats as a result, and takes the engine with it. A delayed suicide if you will. If you have your engine replaced with a dealer installed reman, you need to flush the engine as much as possible. Even flushing is only going to be moderately successful however, as that will most likely just move the crap into other areas of the cooling system. I advocate that you should really plan to replace at least the radiator and thermostat about 3,000 - 5,000 miles after a dealer installed reman goes in. Sooner and you might not get all the floating sealant with the replacement, later and you may have waited too long and lose your engine. An additional $300-$500 + labor cost after replacing the engine doesn't sound that pleasant to most people, but then, it's a lot cheaper than another engine failure.
This is not a problem with any hand built engine built by a reputable builder, such as Rotary Resurrection, Racing Beat, BNR, Pineapple Racing, or BHR. Mazmart's reman engines are also flushed prior to shipment, so there is less risk with those engines, though not non-existant.