After arriving at home after a drive tonight, I noticed a boiling sound from my FD. This is not somehting I am used to hearing. It was coming from the back f the engine bay passenger side, near the intake pipes.

I just recently did the Miata switch fan mod. So I began by checking the ECU and fanswitch sensor. I used a blue LED type light, and found the green glow on the ECU coolant temp sensor. it was a VERY minimal amount. I also just replaced the radiator cap after doing the fan mod.

Coul this small leak be the cause of the boiling? any advice on where to begin checking ? I really hope I'm not destroying my engine... I'll tighten up the ECU sensor immediatley.

 

near to your intake pipes ? well, afaik the water always starts to boil after a while in he turbo area.
got an aftermarked gauge? if so, what temp?

 

check the coolant hose end to end..see if there's any worn..i'll recommand get a pressure test..it'll show u where it leak

 

Check the coolant level in the overflow bottle - it may be very full. Follow my dissertation:

Think of the engine as a coolant pump to and from the overflow bottle.
When the engine warms up the coolant expands and forces its' way through the overflow tube into the overflow bottle. When the engine cools down it creates a vacuum, which sucks coolant back from the overflow bottle.

This works fine until one of your coolant caps develops a vacuum leak at the rubber seal. Then all that happens is some percentage of the coolant in the engine is pumped into the overflow bottle and remains there, leaving the engine a little low. This allows the pressure to drop after shutdown (because air is compressible), which in turn allows the coolant to boil after shutdown. The metal in the engine holds a lot of heat when the engine is running, and this is ok as long as there is coolant flow to carry it away. Once the flow stops, the coolant will heat up more than normal, and if the pressure is low
enough, it will boil.

The classic event run is you notice the engine coolant level is way down (checking when cold, of course) when you look in the water pump fill tube. You pour in more water. It gets pumped into the overflow bottle (to stay as you have a vacuum leak and can't get it back to the
engine). You are now nervous, and check the level in the engine every day (morning when the car is cold), and fill it. Soon (two to three days) the overflow bottle is full to the top and leaks out where the filler tube (removeable) joins the bottle. You see the puddle and
panic.

It happened to me. Solution - replace both caps (they were old), replace all small coolant hoses to/from overflow bottle (they are not designed for pressure use as there is no pressure in this part of the system, so they are thin and can be collapsed under suction - stopping the coolant from returning to the engine. I replaced them with suitable pressure coolant hose routed so it would not kink (it's thicker so care must be taken when routing it - it will not bend as well).

 

All the above is true but if changing the caps and hoses dont change anything, you might have a coolant seal problem. Thats what just happened to me. I thought the pressure in the system was the problem and I changed the caps and checked all my hoses. Instead it was air getting into the cooling system via the motor. The air was pushing coolant outa my overflow tank and making me run low on coolant everyday. Eventually the seal totally gave way and drained into my motor. Also, you can trouble shoot alittle more and start the car with the water pump cap off and check to see if any bubbles come up. It doesnt matter what the bubbles look like as long as there a bunch of them. If you see continuous bubbles, might be rebuild time.

The motor was ripped apart and the culprit was a coroded rotor housing.

SO....get you cooling system pressure tested FIRST. It will help you determine if it's you hoses or a bigger problem.

 

JDMrx7:

The very same thing happened to me. My engine was rebuilt at 114,000 and, after about 15,000 miles, I experienced the very same symptoms as you.

I had the system pressure tested and it checked out....but I still lost it.

My engine failed on the Interstate exit ramp coming home from work just about one mile from my house. (The night before I lost my job!)

It is now 4 months later, and I am now driving with a new Mazda Reman. Cost $3,700.

I did make one additional addition.....added a Delphi Water Temp gauge. So I could monitor coolant by exact temp, as the stock gauge is blank between C & H...By the time it registers H, It's too late.

I got a center speaker twin gauge pod and moved my boost gauge from the A pillar to the gauge pod. It's pretty cool, functioanl and easy to see.

Now, My RX-7 is purring like a kitten.

 

First off, the coolant is not boiling. Secondly, what you hear is normal. Call M2 Performance, they will tell you the same thing.....

 

if the boiling sound is down near the turbos then what's happening is what they call percolating. do a search on percolating and you'll get a good thread about it. percolating is not a problem and is not bad for your car

 

Uhh....I have had boiling coolant before. So somebody is wrong and its not me.

I can't wait till my FD is puring like a kitten again. I have a huge fiberglass sub enclosure to start on and I am waiting to get my car back.

 

I'm not saying you didn't, but out of curiousity, how did you come to the conclusion that the coolant was boiling???

 

b/c the sound was coming from my coolant hoses by the radiator and water pump. I can't see it being anything else.

I understand your info about if it comes from the turbo area but it wasn't. I had just drove the car home from buying it and the previous owner was a ****. From the looks of the coolant, he just poured it in straight and no water. That would make the boiling point lower than what it shoud be. I am tired and drunk, so I might be wrong about all of this.

 

I cannot tell you for sure about whether or not your coolant will boil if you use the wrong dilution, but given a ratio of the neighborhood of 50/50, the coolant would not boil. That sound of people hear boiling does indeed sound like boiling but it is not, that I can tell you for sure. Whatever it may be- (the contraction of liquid sucking air from the overflow tank back into the cooling system, then the air rising thru the galleys? ) I dont know, but when I asked m2 about this they said it was not boiling, and that the sound was normal for the fd's...
So basically I cannot explain what makes the sound, but I figure that m2 would know.....

 

Boiling coolant is indicitave of air in the cooling system. It got there through a leak that occured when the coolant system was under vacuum (cool). It's not a good thing. Boiling creates localized hot spots.

I have only ever had this occur twice. Once, when my turbo coolant line had a crack, and two, when i forgot to completely screw on the AST cap. Both caused coolant to leave the system when the pressure got high, which allowed air in when it cooled, and hence, the boiling, when it heated back up again.

The percolating, or boiling can't occur without the air. The system is under pressure from the atmosphere (14.7 PSI) and from the coolant caps (13 PSI).

From my thermodynamics tables:
Water begins to turn into steam at 211 degrees at atmpshperic pressure. At 27 PSI water requires about 245 degrees to boil.

Now coolant has an even higher boiling point. Usually around 260 degrees at atmoshperic pressure. I'm not going to do the math, but if you have boiling with a 50/50 mixture, the temperature at that spot is probably above 260 degrees. That's far into the danger zone.

 

Just to clear up my reason for posting. As already stated, if there is air (actually -too much- air) in the system the coolant can percolate/boil. The scenario I posted is what usually allows too much air into the system. My car has not "boiled" since I fixed the problem I posted about. It is not "normal", it indicates a problem that should be addressed. I doubt any harm would occur if nothing was done, providing you don't run out of coolant. Other than to your sanity, hearing your car boil every time you shut it off ;-).

 

to give you a simple answer. yes, a leak can cause the boiling. my car has made that noise on three separate occasions. every time, a leak has appeared shortly thereafter. once each leak was fixed, no more noise. try fixing the leak first. alot of people on this board are quick to diagnose it as your engine going south. i got the same diagnosis a while back and it turned out to be a tiny leak that eventually was visible, not a bad coolant seal, etc.
andy

 

I'll replace the AST cap tonite, and take car of the leak on the ECU temp sensor.

The Coolant system was tested by my friend who owns a garage, and he replaced the main radiator cap with a new 13psi cap. he is no RX-7 expert and may have over looked the AST cap.

I'll see if this will change it. I am also working on linearizing the stock gauge right now.

 

Another question: Can I use the same kind of cap on the radiator and AST? The stock AST has a large od rubber gasket on it, but all the caps I find at the parts store have a slightly smaller one.

What did you use when you changed the ast cap?

-I did some searching and found I should use a standard 13psi .9 bar radiator cap on both filler neck and AST-

 

i dont want to hijack this post but this is another quick coolant problem story.

It is time for a new engine when you check your coolant levels and it looks exactly like checking the oil level. haha The guys from mazda installing my engine (extended warranty) had got on my car 5 miles into breaking it in and were suprised at how easily the car boosted to 10 psi. 1000 miles later, the car rained oil and i had a blown oil seal on one of the housings. lol The coolant looked like a slimey oil. So that is a bad sign.

 

I bought my caps at a Mazda dealer (imagine that). Only downside to doing that is they are "Mazda parts" (very expensive). The engine fill cap should have no spring on it. The AST cap should. Unless you have eliminated the AST, in which case the spring cap goes on the engine fill tube (from a 2nd gen, as I recall). Not sure what a spring cap will do on the 3rd gen fill tube if you have the AST, and would rather not experiment for no reason. The Mazda cap is expensive, for a cap, but much less than the damage that I can imagine might occur with the wrong part. It probably will not harm anything, as I'm sure I've read of people accidentally switching them. But anyway, do it right!