Overheated on track - now what?

[Pete_89T2]

Originally Posted by infernosg

After the first "event" I filled up the radiator and managed to drive 2.5 hours home averaging 70 mi/hr with no overheating, but the buzzer would come on occasionally during tighter turns and lower speed driving. Once I got home and the car cooled off I topped off the radiator and bled the system the best I could. At this point the radiator was completely full and the overflow bottle was at the full level as well. I was able to drive to and from work (only 8 miles) with no issues on Monday and I let the car sit most of the day yesterday. Yesterday afternoon, after driving more recklessly than I'd like on the street, I came home and revved the engine in neutral in the driveway. No buzzer while doing all this so I turned it off and let it sit for about 30 minutes. Before starting it up again I noticed the overflow bottle was now ~1" above the "full" mark. I made it to the end of the driveway when the buzzer came on. Since it was still fully warmed up I just pulled back up and turned it off for good. The buzzer did go off as soon as I started to move back up the driveway.

The 1" extra coolant in the overflow bottle 30 minutes after a hot shutdown sounds about normal. When you shut down the engine the radiator cap allows pressurized/expanding coolant to flow into the overflow tank, raising its level. When the system cools down enough, the resulting vacuum should suck the coolant back into the radiator, leaving the overflow bottle around the full mark.

Originally Posted by infernosg

What has me most worried is the constant bubbling I'm getting in the overflow bottle. To ensure it wasn't be caused by engine vibrations I removed the overflow cap and tube and placed it in a cup of clear water. With the car fully warmed up there was a constant stream of tiny bubbles coming out of the pickup tube. Only once did a moderately large amount of coolant get burped into the cup. After that it was back to the bubbles.

If air is getting into the system, from either a leaky radiator cap or anywhere else in the system, it could explain the bubbles you're seeing. And it would explain the added difficulty in getting the system fully "burped" and keeping the buzzer quiet.

Originally Posted by infernosg

Pressure testing is my next step before throwing new parts at it (rad. cap, thermostat, water pump). The plan is to swing by an auto parts store after work to see if they have anything I can rent.

If you're going to throw parts at it, I'd suggest getting a new Mazda radiator cap & thermostat regardless, especially if you don't know what's in there now. Don't bother buying a Stant or aftermarket thermostat - for some reason the ones the make for rotaries are crap.

Originally Posted by infernosg

The one thing I can't seem to explain is the original overheating event. IIRC, the cooling system holds like 2 gallons of water and I know I didn't add nearly that much after it overheated. The fact the buzzer came on just as the temperature gauge started to rise also has me stumped considering it has come on many times since that and the temperature gauge hasn't moved.

If your system is all stock, the cooling system is barely sufficient for track duty - it has very little excess cooling capacity, and a just a little bit of pump cavitation due to the prolonged high revs can easily boil things over. The buzzer sensor needs to be immersed in coolant at all times to keep it quiet - just one air bubble under it and BUZZZZZZZZZ. The temperature of the coolant has no effect on the buzzer - it's a level sensor.

[infernosg]

Originally Posted by Pete_89T2

The 1" extra coolant in the overflow bottle 30 minutes after a hot shutdown sounds about normal. When you shut down the engine the radiator cap allows pressurized/expanding coolant to flow into the overflow tank, raising its level. When the system cools down enough, the resulting vacuum should suck the coolant back into the radiator, leaving the overflow bottle around the full mark.

When I get home I'll check it out again to see if it went down. I know the radiator pulled a good amount back in at the track as it cooled down, which isn't looking too good for the "there's a leak somewhere" theory.

Originally Posted by Pete_89T2

If air is getting into the system, from either a leaky radiator cap or anywhere else in the system, it could explain the bubbles you're seeing. And it would explain the added difficulty in getting the system fully "burped" and keeping the buzzer quiet.

I have noticed there is a tiny tear in one of the rubber seals in the radiator cap. I though it too small to make a difference but who knows. I always figured with the car running the pressure generated by the water pump would be more than sufficient to keep air from being sucked in, even at idle. If air is getting into the system with the car off, I'd expect to see the bubbles stop at some point as well.

Originally Posted by Pete_89T2

If you're going to throw parts at it, I'd suggest getting a new Mazda radiator cap & thermostat regardless, especially if you don't know what's in there now. Don't bother buying a Stant or aftermarket thermostat - for some reason the ones the make for rotaries are crap.

This I know. The rad. cap is the original I believe, but I'm almost certain the thermostat was replaced at one point (I have the records from the previous owners) and it's likely it's not an OEM Mazda unit. I'll do the pressure test and if that passes, change the radiator cap and go from there. If it fails, well I've got some exploring to do...

Originally Posted by Pete_89T2

If your system is all stock, the cooling system is barely sufficient for track duty - it has very little excess cooling capacity, and a just a little bit of pump cavitation due to the prolonged high revs can easily boil things over. The buzzer sensor needs to be immersed in coolant at all times to keep it quiet - just one air bubble under it and BUZZZZZZZZZ. The temperature of the coolant has no effect on the buzzer - it's a level sensor.

Yep, cooling system is all stock. I had no issues during the other runs over the weekend but I did stay in 3rd gear a lot more for this particular run so perhaps that was the proverbial straw. Aside from an upgraded radiator what other modifications would bring the cooling system up to track-ready levels? It's not a track-only car and I maybe drive it 25% of the time. I've seen underdrive main pullies but those make me nervous with the stock alternator's output.

Is there any way to "officially" bleed the S5 cooling systems? I open up the heater, turn the car off, then fill the radiator and overflow bottles. I squeeze the upper radiator hose to get as many air bubbles out as possible.

[Pete_89T2]

Originally Posted by infernosg

I have noticed there is a tiny tear in one of the rubber seals in the radiator cap. I though it too small to make a difference but who knows. I always figured with the car running the pressure generated by the water pump would be more than sufficient to keep air from being sucked in, even at idle. If air is getting into the system with the car off, I'd expect to see the bubbles stop at some point as well.

Definitely replace the cap with a new Mazda cap then. If it's not sealing properly, it won't hold its rated pressure, and this in turn will make it more likely to boil over.

Originally Posted by infernosg

This I know. The rad. cap is the original I believe, but I'm almost certain the thermostat was replaced at one point (I have the records from the previous owners) and it's likely it's not an OEM Mazda unit. I'll do the pressure test and if that passes, change the radiator cap and go from there. If it fails, well I've got some exploring to do...

If it's not a Mazda T-stat, I'd just replace it. The Mazda T-stat & gasket for it is pretty cheap, like under $15

Originally Posted by infernosg

Yep, cooling system is all stock. I had no issues during the other runs over the weekend but I did stay in 3rd gear a lot more for this particular run so perhaps that was the proverbial straw. Aside from an upgraded radiator what other modifications would bring the cooling system up to track-ready levels? It's not a track-only car and I maybe drive it 25% of the time. I've seen underdrive main pullies but those make me nervous with the stock alternator's output.

Besides an upgraded radiator, just make sure the fan shroud and all the ducting is in place & functional, including the plastic undertray. Key point here is to make sure that air only flows thru the radiator & oil cooler, and not around them. As for the fan, it only matters when you're at idle or driving at low speeds, but just check it per the FSM procedure to verify the clutch on it works - when it's hot, fan should turn but resist freewheeling when you try to spin it by hand. The underdrive pulleys will help reduce waterpump cavitation at high RPMs on your track days, and if paired with a similarly sized underdrive alternator pulley (i.e., to keep the drive ratios the same/close to stock pulleys) would solve any alternator output issues. Anything else is just maintenance - flush out your system regularly, use fresh coolant & distilled water at proper mix for your climate, check/replace all hoses as required.

Originally Posted by infernosg

Is there any way to "officially" bleed the S5 cooling systems? I open up the heater, turn the car off, then fill the radiator and overflow bottles. I squeeze the upper radiator hose to get as many air bubbles out as possible.

Not that I can find in the FSM. What I do whenever I flush & fill the system is make sure the heater is set to full heat before starting, and after I do the initial filling of the radiator with coolant, I'll let it idle with the rad cap off. When the engine reaches operating temp and the upper hose is hot to the touch, I'll run the engine in neutral at a constant 2~3K RPMs for a couple of minutes, then I'll quickly rev the engine up past 5K & back down to idle a few times, still with the cap off. When the level in the radiator drops (it will as trapped air pockets escape), add more coolant till its full. While you're doing this drill, the coolant level buzzer will sound off from time to time - it's normal. I keep repeating this process until I can't add any more coolant to the radiator. Then I top off the overflow tank with coolant to the full line, put the radiator cap back on, and take it for a drive. Let it cool down fully, check the level in the overflow tank, add coolant if needed to get it back to the full mark.

[infernosg]

Originally Posted by Pete_89T2

Definitely replace the cap with a new Mazda cap then. If it's not sealing properly, it won't hold its rated pressure, and this in turn will make it more likely to boil over.

If it's not a Mazda T-stat, I'd just replace it. The Mazda T-stat & gasket for it is pretty cheap, like under $15

I'll take a picture of the tear and post, but you're right, might as well replace it as they're only like $10. It looks like regardless of the condition of the engine I'll be needing a new thermostat so might as well pick up one of those as well.

Originally Posted by Pete_89T2

Not that I can find in the FSM. What I do whenever I flush & fill the system is make sure the heater is set to full heat before starting, and after I do the initial filling of the radiator with coolant, I'll let it idle with the rad cap off. When the engine reaches operating temp and the upper hose is hot to the touch, I'll run the engine in neutral at a constant 2~3K RPMs for a couple of minutes, then I'll quickly rev the engine up past 5K & back down to idle a few times, still with the cap off. When the level in the radiator drops (it will as trapped air pockets escape), add more coolant till its full. While you're doing this drill, the coolant level buzzer will sound off from time to time - it's normal. I keep repeating this process until I can't add any more coolant to the radiator. Then I top off the overflow tank with coolant to the full line, put the radiator cap back on, and take it for a drive. Let it cool down fully, check the level in the overflow tank, add coolant if needed to get it back to the full mark.

That all makes sense, but the problem I'm having is with the car on and the radiator cap off I get a lot of coolant coming up and spilling out. Maybe there was just a large air bubble pushing it all out but the pessimist in me says this is the pressure from the combustion chamber. Maybe another picture will suffice...

[Pete_89T2]

Originally Posted by infernosg

That all makes sense, but the problem I'm having is with the car on and the radiator cap off I get a lot of coolant coming up and spilling out. Maybe there was just a large air bubble pushing it all out but the pessimist in me says this is the pressure from the combustion chamber. Maybe another picture will suffice...

I forgot to mention that I remove the radiator cap after the coolant is at operating temp and circulating thru the engine - this helps avoid most of the spilling, since once it's all circulating, the level in the rad should drop a bit. But yeah, if you remove the cap while its still cold and run the engine with the cap off after just filling the radiator, lots of coolant will barf out. Just use a thick rag to keep from burning yourself, crack open the cap to the first stop to release the pressure, let any excess coolant bleed out then remove it all the way.

[infernosg]

Originally Posted by Pete_89T2

I forgot to mention that I remove the radiator cap after the coolant is at operating temp and circulating thru the engine - this helps avoid most of the spilling, since once it's all circulating, the level in the rad should drop a bit. But yeah, if you remove the cap while its still cold and run the engine with the cap off after just filling the radiator, lots of coolant will barf out. Just use a thick rag to keep from burning yourself, crack open the cap to the first stop to release the pressure, let any excess coolant bleed out then remove it all the way.

I did this at the track before heading home and a good amount of coolant came spilling out. Kind of like a slow geyser. I didn't leave the cap off long enough to see if it was going to stop though so maybe that's normal?

So more updates. First a picture of the radiator cap. The tear is hard to see but it's "inside" the "ear" of the cap toward the bottom:





I managed to rent a cooling system pressure tester from the local Autozone as well. First the good news. I checked the radiator cap - it opened at the appropriate 0.9 bar (about 13 psi) but it would not hold any pressure above ~9 psi and even then it still fell slowly. Outright, this makes me believe the radiator cap is dead so there's at least one thing.

Next, the not-so-good news. I attached the pressure tester to the radiator and pressurized the system up to 15 psi. Within 10-20 minutes the pressure had dropped to 14 psi or so. I brought the pressure up to 20 psi next to see if it fell off any faster but it dropped ~1 psi in about the same amount of time. I did not see any external leaks, so something's got to be off internally. Then again, perhaps the seal on the tester wasn't perfect and the engine's fine as I'm not detecting any gas or exhaust fumes in the overflow bottle or radiator

[RETed]

I see the tear - it's trash; replace it.
The system has to be airtight - a tiny leak like that will compromise it's effectiveness.

15psi is about as high as I would go.
20psi is way too much pressure - in fact, most cooling system will start to leak all over the place.
Remember, the stock radiator cap is only rated to 0.9 bar(?), which is like 13psi.

15psi drops to 14psi is okay in my book.
This pretty much eliminate a major water jacket o-ring failure.

Replace the cap...
Burp all the air out of the system...
I think your engine should be okay...
Cross fingers!


-Ted