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I'm trying to troubleshoot why my water temps quickly get to 220F when I'm only shifting at 4k rpm and taking it easy (reading is from a gauge pod). Redlining a few shifts will put it at about 226F.
My engine is technically failing compression at about 5.7 and 6.7 bar @ 250rpm, but I want make the engine last a bit longer before I pull it out.
What troubleshooting have you done? A/C condenser looks fine, is there anything stuck in between it and the radiator? Does it have coolant? Any coolant leaks? Water pump good or is it leaking from the weep hole? Have you checked thermostat? Where are you getting temperature reading? Does it overheat at idle? Do you have foam around the radiator?
What troubleshooting have you done? A/C condenser looks fine, is there anything stuck in between it and the radiator? Does it have coolant? Any coolant leaks? Water pump good or is it leaking from the weep hole? Have you checked thermostat? Where are you getting temperature reading? Does it overheat at idle? Do you have foam around the radiator?
I'm just starting the troubleshooting now. Coolant was just below the F line, so I poured some distilled water in to bring it to the line while the front was jacked up in the air and running. After driving, the line rises above the F line.
I did the test where you start the car, and turn the AC on. The radiator fans click on for about ten seconds, then click off for about then seconds, and repeat off and on like that. (Note: when turning the car off, the fans both stay on loudly full blast for five minutes due to versatuner)
The evo gauge pod gets the temperature reading from a sensor that I think is tapped into a hose underneath the battery. A previous owner installed it.
I can't see any coolant coming from the pump as far as I can tell.
I can test where it goes to while idling after driving. Likely it might get down to about 217F at idle after being at 220 while driving.
Last edited by GuyWithRx8; Dec 31, 2025 at 12:02 AM.
You know what, it could be the thermostat. I'll have to troubleshoot that more tomorrow. I'm researching it now. There was one time recently that the gauge kept reading around 110F to 130F after 20 minutes of driving. It just never warmed up. So maybe the thermostat is mostly sticking closed, but occasionally sticking open too
What troubleshooting have you done? A/C condenser looks fine, is there anything stuck in between it and the radiator? Does it have coolant? Any coolant leaks? Water pump good or is it leaking from the weep hole? Have you checked thermostat? Where are you getting temperature reading? Does it overheat at idle? Do you have foam around the radiator?
Looks like there is foam on the sides, should there be foam underneath as well?
You know what, it could be the thermostat. I'll have to troubleshoot that more tomorrow. I'm researching it now. There was one time recently that the gauge kept reading around 110F to 130F after 20 minutes of driving. It just never warmed up. So maybe the thermostat is mostly sticking closed, but occasionally sticking open too
If the gauge gets the temperature under the battery, that's after the thermostat and if the thermostat is closed it's not reading actual coolant temp. Because the coolant isn't circulating.
Use an OBD reader to get actual stock temp reading to confirm your gauge isn't lying to you.
If the gauge gets the temperature under the battery, that's after the thermostat and if the thermostat is closed it's not reading actual coolant temp. Because the coolant isn't circulating.
Use an OBD reader to get actual stock temp reading to confirm your gauge isn't lying to you.
Okay, so it's actually worse. The gauge pod sat at 80F until the obd read about 160-170, then it finally starting climbing. The OBD consistently rose from the time I started the car until about 220, even while sitting at a red light, it continued rising. Then it slowly went to 230 while the gauge pod read 220. Total trip time was about ten minutes.
I didn't go past about 5k rpm. Light driving.
Okay, so it's actually worse. The gauge pod sat at 80F until the obd read about 160-170, then it finally starting climbing. The OBD consistently rose from the time I started the car until about 220, even while sitting at a red light, it continued rising. Then it slowly went to 230 while the gauge pod read 220. Total trip time was about ten minutes.
I didn't go past about 5k rpm. Light driving.
Are both fans turning? It's not unusual to find one jammed or burnt out.
What color is your coolant?
Any bubbles in the overflow bottle?
If you still have contact with the previous owner, ask if they ever put stop-leak, water wetter or other junk in there (and if so, for what reason). Hard to draw conclusions yet, but one problem could be the rad is clogged and flowing poorly or exchanging heat poorly. The gauge not moving until OBD is near thermostat temp sounds expected for a temperature probe installed after the thermostat.
Good call on the FL22, you may want to get some extra distilled water to run a flush between the 2 coolants.
Your rad and coolers don't look bad at all. Usual wear and tear but I've seen much much worse on cars that didn't overheat.
If you turn on the heater to full blast, does it help bring the temperature down? Does it produce heat at all?
If you still have contact with the previous owner, ask if they ever put stop-leak, water wetter or other junk in there (and if so, for what reason). Hard to draw conclusions yet, but one problem could be the rad is clogged and flowing poorly or exchanging heat poorly. The gauge not moving until OBD is near thermostat temp sounds expected for a temperature probe installed after the thermostat.
Good call on the FL22, you may want to get some extra distilled water to run a flush between the 2 coolants.
Your rad and coolers don't look bad at all. Usual wear and tear but I've seen much much worse on cars that didn't overheat.
If you turn on the heater to full blast, does it help bring the temperature down? Does it produce heat at all?
I turned the A/C on fan speed 2, with temperature on max heat. And it had zero effect. I doubt turning the fan speed on level 4 would do much, if anything. Yes, the A/C works fine. Heat and cold.
Do you know if the Megan Racing Radiator on ebay is as good as the OEM one? I'd be willing to spend the $160-200 to see if it solves the problem. I don't really want to drop $400 on a new radiator when this engine may only last a few more months anyway.
Last edited by GuyWithRx8; Jan 1, 2026 at 10:57 PM.
Diagnose, don't throw parts at it. That's an expensive way to find out what wasn't the problem. Also not Megan, most of their products are junk. CSF 3143 (i think?) is a great rad upgrade but it's more, and would be a good idea given you're in Arizona. But I would root cause the current problem first. The stock cooling system should be to handle Arizona winter.
Turn off A/C while troubleshooting cooling, it creates extra heat load and changes fan behavior. Your cabin heater is basically another radiator so if putting it on full fan full heat 1/ provides good heat and 2/ has any effect on cooling, it tells us the coolant is at least circulating.
Basically here are possible causes, work to confirm or eliminate them:
- blown coolant seal, perform block gas test or look for bubbles in the overflow while the car is running
- water pump failure, coolant not circulating in sufficient volume. There are modes of failure where the impeller vanes wear down. Not much to do but remove the water pump and have a look. It's not difficult to do, can do it during your coolant change. You'll need a new gasket.
- radiator clogged with something. Your coolant change may hold clues. If the heater on full had significant effect it would point to this too.
- Rad cap not holding pressure or the reservoir is cracked. Not uncommon and a cheap fix for the cap. Is there dry coolant spray around and under the overflow?
- fans not doing their job for whatever reason. Can you get it out on the highway and see if it can cool itself better at speed?
Diagnose, don't throw parts at it. That's an expensive way to find out what wasn't the problem. Also not Megan, most of their products are junk. CSF 3143 (i think?) is a great rad upgrade but it's more, and would be a good idea given you're in Arizona. But I would root cause the current problem first. The stock cooling system should be to handle Arizona winter.
Turn off A/C while troubleshooting cooling, it creates extra heat load and changes fan behavior. Your cabin heater is basically another radiator so if putting it on full fan full heat 1/ provides good heat and 2/ has any effect on cooling, it tells us the coolant is at least circulating.
Basically here are possible causes, work to confirm or eliminate them:
- blown coolant seal, perform block gas test or look for bubbles in the overflow while the car is running
- water pump failure, coolant not circulating in sufficient volume. There are modes of failure where the impeller vanes wear down. Not much to do but remove the water pump and have a look. It's not difficult to do, can do it during your coolant change. You'll need a new gasket.
- radiator clogged with something. Your coolant change may hold clues. If the heater on full had significant effect it would point to this too.
- Rad cap not holding pressure or the reservoir is cracked. Not uncommon and a cheap fix for the cap. Is there dry coolant spray around and under the overflow?
- fans not doing their job for whatever reason. Can you get it out on the highway and see if it can cool itself better at speed?
Update: I simultaneously performed an oil change, coolant flush/change to FL22, and OEM thermostat change. Now the car runs at 180F instead of 225F like before. I can drive like an absolute madman now and not go over 190F. Incredible. If I sit in traffic, it can creep up over 200, and I saw 210 once while sitting. But whenever I'm moving, it's below 190. (ambient air temp 60F)
The only issue now is I have intermittently received a coolant warning light on the dash. It will stay on for a minute and then go off for ten minutes.
Edit: apparently the thermostat does need to be oriented a certain way when installing. https://foxed.ca/rx7manual/2003mazdarx8/
I spun it around in there and I didn't notice any notch for it to lock in to. I oriented it roughly the way the old one was, but I could have done it 180° or so in the wrong direction. Is it critical to take it back apart and orient it into the supposed notch?
Last edited by GuyWithRx8; Feb 19, 2026 at 01:03 AM.
On the thermostat there should be a bleeding hole with a jiggle valve on it. It should face up so air can bleed.
If I have it oriented the wrong way, would no air bleed at all? I filled it up until no more coolant would enter the system (using a large no-spill coolant funnel), and then more bubbles came up while idling the car and letting it warm up and it took a bit more coolant into the reservoir from the funnel.
I'll drive the car a few more times and see if the light continues to come on. If it does, I'll take it back apart and check the thermostat rotation position.
Side note: Does anyone know exactly what size the 12mm nuts are that hold on the alternator bracket? One of them disappeared into the engine bay, and I don't think I'll ever see it again.
The light sensor is located in the overflow container, I know it at least triggers when low on coolant. I'm not sure if it triggers if something is overheating.
The light sensor is located in the overflow container, I know it at least triggers when low on coolant. I'm not sure if it triggers if something is overheating.
chatgpt says air can move into and out of the overflow tank and through the system if it isn't escaping through jiggle valve. And that can cause the light to come on temporarily.
chatgpt says air can move into and out of the overflow tank and through the system if it isn't escaping through jiggle valve. And that can cause the light to come on temporarily.
Chatgpt says a lot of things. There's a common issue with the sensor in the overflow bottle, they eventually start triggering without reason. You can visually check the that the bottle is full while the light is on. It's a simple floater, it only measures the fluid level.
I can tell you by experience that my engine psi was also that low and I kept running for many months until *Boom*. Just a heads up, when it does go out - it will most likely be catastrophic to where the engine might not be rebuildable. Mine sounded like the bearings went out as the engine was vibrating and grinding before it finally shut down. The dipstick contained metal sharks and glittery oil. I plan to drop in a JDM low mileage long block engine and so far have been gathering all the items needed and that were recommended for the swap. The main ones are new oil lines and either having the oil coolers professionally cleaned or replaced. I have also purchased a new radiator with new coolant hoses. A new coolant tank since the sensor is bad on the current one. Due to the motor going out the way it did; i have to ensure that there are no metal shards/debree and contamination to the parts I will be using on the new engine.
We had a hot day yesterday, and some coolant spewed out. I believe it was from the thermostat housing. So I took it back apart and rotated the thermostat correctly. I definitely had it turned slightly the wrong way, because the little tab on the gasket was sandwiched and squished. That's probably how the coolant came out.
Originally Posted by TopGunM2k
I can tell you by experience that my engine psi was also that low and I kept running for many months until *Boom*. Just a heads up, when it does go out - it will most likely be catastrophic to where the engine might not be rebuildable. Mine sounded like the bearings went out as the engine was vibrating and grinding before it finally shut down. The dipstick contained metal sharks and glittery oil. I plan to drop in a JDM low mileage long block engine and so far have been gathering all the items needed and that were recommended for the swap. The main ones are new oil lines and either having the oil coolers professionally cleaned or replaced. I have also purchased a new radiator with new coolant hoses. A new coolant tank since the sensor is bad on the current one. Due to the motor going out the way it did; i have to ensure that there are no metal shards/debris and contamination to the parts I will be using on the new engine.
Oh dang, sorry. Are you saying it's not a good idea to drive too far from home?
I'm not expecting the engine to be re-buildable anyway. I talked to Lucky 7 Racing, and they said it's pretty rare for the renesis housing to be re-usable. If I decide to stay rotary, I may buy an engine from someone who lives near me. The person he got it from told him it's a reman engine, although I thought those haven't been assembled for at least the past decade. If I buy that, I'll test the compression first. He's had the engine sitting out in a hot garage for a couple years, so I'm not terribly confident in it.
I'm leaning more towards swapping in a piston engine. Probably the Keisler LFX kit.
