I bought a manual 2004 RX8 about two months ago.
CARFAX Vehicle History Report for this 2004 MAZDA RX8
This is the carfax report for those of you that want to dig deep.
I've put 2k of my own miles on it since this was made, and replaced the battery due to some starting problems, which resolved.

The car heats to 102 C (about 216 F) at idle and has gotten as hot as 107 C (225 F) in traffic (it has only gone this high on two occasions). It runs hotter with the heater off, and I have been keeping the heater on lately to keep it at a safe temperature. It is happiest (running 93-95 C, 200-203 F) around 3K rpm, whether at 20mph or 40mph, with the heater on full heat and fan speed at 3.

At first, I thought this could be a rust build-up, so I ran some cleaner through the rad and did a full flush and fill. When I realized the dramatic effect the heater has on the running temperature, and after the problem persisted, I realized it could be something else entirely. Because the problem was pronounced at idle and in traffic, I suspected the fans. After diagnoses:
Fans are installed correctly and spin freely.
Relay comes on at the correct temperature.
Fans spin at full speed.

So, not the fans. I'm not that familiar with the heater core and how it works, but my theories are:
Mother-of-god air bubbles in line, which migrate to heater core when it is active.
Water pump is failing, running best at 3k rpm. Heater helps pump coolant?
Thermostat is sticky or faulty.

I'm running with a 2/3 distilled water 1/3 peak coolant mix.
I bled the system a week ago and got a few tiny bubbles but the problem persists.

Any ideas on where I should start?

 

How did you bleed the system? Did you drain the engine block? Why are you running so much water? I would definitely replace the tstat and water pump just so you know you are starting fresh there, they are cheap enough.

 

I bled the system by parking on the most ridiculous hill I could find, letting the car completely cool, removing the pressure cap, warming the car to operating temp with heater on full blast, revving back and forth from 4k to 2k, and then replacing the cap.

When I flushed I did not drain the engine block. When I perform any repairs or replacements, I will be sure to do that as well.

I'm running so much water because it is what the owner's manual recommended, and I am in no danger of freezing. I will, obviously, change the concentration in the fall.

Thanks for the quick reply! I'll probably order the ReMedy thermostat and a stock water pump, since the water pump is $270 at Mazparts and $80 with Duralast. Is there a steep performance cliff here?

 

You can get an Accessport which will turn your fans on sooner.

 

I don't know where you get that the owners manual states to run water, FL22 (Mazda long life premixed coolant) is what is recommended. If you did not drain the block then you missed about a gallon at least.

You really need to think of the system as a whole, the tstat, hoses, radiator, water pump, sealing around radiator, coolant, cap, reservoir, etc. One kink in that system and it can affect everything. If you are unsure of the history of the car then start fresh and replace everything.

 

Fans don't come on until about 216F stock. Get a fan controller or access port to lower the turn on point. Don't go more than 70% distilled water. You can have freezing problems in the winter. Is the stock temp gauge moving at all?

 

Stock is 207F Fan A, and 214F Fan B IIRC.

 

It says Protection (Above 16*C) Antifreeze 35% Water 65%, this is to what I was referring.
When I perform any necessary repairs I will be sure to drain the block completely and flush several times. I'll even try to get my hands on some of the FL22.
While I might eventually replace everything, I'm trying to avoid a total rebuild of the cooling system if possible so I can spend my money on other things like plugs, coils, winter tires, etc.

Where is the best place to find one of these access ports? What is the name of yours?
I plan to change coolant in the fall, so I don't have any freeze problems. Obviously, if I use the FL22, I won't have to consider this. The stock temp gauge moves to the middle as the car warms but never past the middle, thank god. I log the temperature with my OBD2 and smartphone.

I've seen this fix before a few times, but I'm not really confident finding an Accessport. Where did you get yours, and what is it called?


I've seen the COBB accessports, but they don't support RX8s. If I can't find one, I'm considering modding for fans always on low speed.

Thanks!

 

Cobb accessport watch on here in the fs stuff they show up a few times a month for about 300 dollars

 

Take a garden hose a nylon brush and some dish soap to your radiator. It's probably just caked with dirt/bugs. If you feel really adventurous then you can remove the air box/battery box and backwash it from the inside. Recently had to do this to mine because of an oil leak. (hint: oil is a dirt magnet.)

Oh and the heater keeps the car cooler because it's a mini radiator. It's an old shade tree trick to run the heater if the car begins to overheat.

 

And often overlooked step and something I tell people to do all the time. In offroading clogged radiators due to dirt is common. Guys ride the trails for years without ever washing out their radiators and then one day they over heat.

 

This is a pretty good suggestion. It fits my budget, too! xD

I was aware of the heater trick. I used to have to run it 24/7 in my truck because of a clog.

Do you remove the A/C condenser or the undertray in order to access the front of the radiator for cleaning?

I've had a nightmare trying to get the undertray out to inspect the radiator in the first place, and ended up not even taking it all the way off before I put it back. Thing is a monster.

 

It's just a few screws, clips and six bolts, helps to remove the bumper first.

 

Update?

I'm suffering from the same problem. It slowly climbs to 105C (221F) when idling. Started happening about a month ago randomly. Now it seems to happen every time I idle. Temps are fine over 40mph.

Just like you, I turn on the heater when it gets up to 105C, and then it would drop down. My fans work. I have flushed the coolant and changed the thermostat.

 

221F is well within what is listed as the safe specs (up to 243F) in the factory service manual. What is the ambient temp when this happens? In the summer here, 221F after sitting at idle or in slow traffic is normal.

 

Anything above fan-on temperature is not comfortable to me. When this was occurring, the ambient was in the 70s F.
Update: I pulled the thermostat and it was gunked up by some black sticky tar-like substance. I'm guessing that it required higher than normal water pressure from the pump to open, which is why I was seeing that behavior. I installed a remedy pump and thermostat, and I see nothing but blue skies and correct temps even up to 80 F. Untested as yet in true summer here. I also had low compression, which could have contributed, but the thermostat replace fixed the overheat before I had the motor replaced.

 

But Ive heard coolant seals can fail at around 226F

Ambient temp is around 90F. It would slowly creep up to 221F, as if it's not cooling properly. I don't know if it'd keep climbing, coz I turn on the heater. Ive noticed that it only happens when under the sun. It doesn't happen during nighttime.

My guess is the fans are spinning slower than normal. A while back the rubber grommet under the air box fell in and jammed the fan. Although it didn't burn the fan, could it be that it damaged the motor and now it's spinning slowly?

I have a thread regarding the issue. Check it out if you have the time

https://www.rx8club.com/series-i-tro...-256820/page2/

Glad to hear you fixed your issue

What were the temps after replacing all that? I just replaced my thermostat and I too found gunk inside the thermostat housing. I scraped them out before putting the new one in.

 

There is no definitive temperature at which coolant seals will just fail, it depends. You need to properly troubleshoot your entire cooling system.

Engine ------> Symptom Troubleshooting.

http://foxed.ca/rx7manual/2003mazdarx8/

 

Where exactly in the factory service manual is the "safe specs" reference? I can't seem to find it.

I do find the thermostat full open at 95 deg C / 203 deg F.

I found the ECT PID (Engine Coolant Temperature Parameter Identification) under engine problem diagnostics for normal temperature to be 82 to 112 deg C / 180 to 234 deg F for "No. 9 Fast Idle". I also found to verify ECT PID was less than 116 deg C / 241 deg F for "No. 13 Knocking".

No luck finding an explicit safe range or any value up to 243 deg F. Not trying to be a jerk, just looking for more margin (if it is to be had).

 

Temps in real summer (80-95F) here have fluctuated with AC on between 200-212f, never rising above fan full-on even in traffic with AC on or sustained near-redline cornering.
Very happy with thermostat and pump.