I’m a new FD owner and last night my car got a little too hot after 10 minutes of hard highway driving in 85° weather. I got off the highway, smelled coolant, and saw that my temp gauge was about 2/3 up. I pulled over and shut the car off as fast as I could, but it was climbing so quickly it touched the top white mark on the gauge before I got it shut off. Coolant was spilling out the overflow tank.

I let it cool for 10 minutes, then drove it another 1/2 mile out of the way without it getting quite that hot again. After it sat for 2 hours and I refilled the car with water, the temp gauge still started climbing above half about 5 minutes into the drive, so I parked it and had it towed home.

My car is almost stock with 66k miles on the original engine with an aluminum AST, 2 oil coolers (r1 style), and vented hood.

I have no trouble starting the car, and I only get some smoke on startup but not worse than my FB. So far, I haven’t seen evidence of coolant loss, but when I bought the car the coolant was filthy and I flushed it 3 times. It does make gurgling and boiling sounds after I shut the car off, which I thought was normal?

My best guess right now is that the thermostat is stuck, or that the radiator is blocked. My plan is to start by replacing the thermostat, radiator (something aluminum), and cap but I am open to suggestions.

Couple questions:
1. Did my engine get too hot? Is there a way I can check for damage from overheating?
2. Am I on the right track to fix the overheating or are there other things I should check?

 

You should get the compression test done.
Its the best method to find if the motor is good or not.

 

1. Cooling system pressure test to determine if you have any leakage.
2. If not and the engine runs smoothly, then radiator, thermostat, etc. are suspect.

 

With the age and mileage you have on your engine, I suspect that your coolant seals are bad. Most likely its rebuild time for you. A coolant system pressure test should tell you if the seals are bad. A compression test wont tell you anything.

 

Are both fans operating?

 

What often happens if the coolant has not been changed often enough is that the very thin coolant o-ring retainer walls rust through and coolant can leak into the combustion chamber or combustion gasses can be forced into the cooling system. The latter will result in the symptoms you just had.

A coolant pressure test will aid the diagnosis.

 

Thanks, I’ll start with the pressure test tomorrow to and see. I guess if I have good pressure, then the coolant seals are probably ok? I was hoping my engine would last a little longer since it had good compression when I bought it, but I knew a rebuild wouldn’t be far off either way.

 

pressure test will tell you if you have a leak somewhere too.

the radiators are plastic and could be cracked. basically the entire coolant system is suspect after 20 years.

 

I doubt coolant seals are bad.
1. Something is blocked.
2. Fans are not turning on.

 

1 and/or 2 probably killed the coolant seals

 

I would start with the simple things first before going nuts. Make sure the fans kick on, make sure there isn't a trash bag stuck in the nose of the car blocking airflow and check for coolant leaks.

If that's all good do the champagne test and look for bubbles.

Hopefully it's something easy, rule out the simple explanations first.

Dale

 

Make sure you did not switch the caps on the water pump and the AST. Pressure cap goes on the AST. Pressure cap on the WP can also cause this condition which you have described.

 

I haven’t touched those caps so that’s not it. My fans kick on like normal and keep running when the engine is hot. I did have the filler cap off when I started the car in order to top it off and saw no bubbles, but I’ll have to look into the correct way to do the champagne test - that’s a new one for me.

 

I don't think you should be hearing gurgling. I thought it was normal as well but after replacing my coolant for the 2nd time I spent the time squeezing hoses to get all the air pockets out and Inow have no gurgling.

 

Gurgling isn't good. Pressure test and coolant flush!

 

I got a pressure tester tonight, and at first pressurized it to 16psi, which dropped. Then I realized .9 bar == 13psi and I tried again. It is still slowly dropping from 13psi with no visible leaks as far as I can tell.

I also started the car 3 times and never saw any smoke, though the coolant is probably 70% water at this point so I’m not sure if I should expect smoke.

The last thing I did is pull the EGI fuse and crank the engine for a second, then pull the leading plugs. Both were dry with no trace of coolant. This was after I pressurized the system and let it drop to about 12psi, about 15 minutes later.

I also did the champagne test by running the car for a minute with the filler cap off, and didn’t see bubbles either at idle or when I held the rpm higher. I only did this for a minute or two though as the coolant started backing up out the filler neck as it warmed up.

So, with the pressure loss but no evidence of coolant in the engine or on the floor, what’s next? I’m not sure what else to check at this point

 

Sounds like you don't have a catastrophic failure. That's good. How about the thermostat? Maybe it's failed closed.

 

The thermostat is what I was wondering about. It sure acts like the car just isn’t circulating coolant. Would that cause it to back up into the overflow tank as it heats? When I overheated, that’s what happened.

It’s been about 2 hours and I’m down to 8psi though, so I’m guessing something is still wrong, I just don’t know what

 

This is probably a dumb question, but I realized the neck on my overflow tank is cracked. The plastic where it connects to the tank is shattered. Is the overflow tank relevant to the system holding pressure?

 

The overflow tank will allow coolant to back flow into the system as the car cools down. If it doesnt hold coolant (cracked), coolant wont refill the system. This could lead to not enough coolant in the system, causing overheating. Fix what you know is broken and see if the problem goes away.

My guess is that you have bad coolant seals

 

Based on the fact that it isn’t holding pressure?

I feel like I’d have other symptoms though. If my system isn’t holding pressure due to a leak somewhere else, it could cause the coolant not to circulate and then I’d overheat. I want to fix the overheating issue first and drive it for a while if at all possible. Then I’ll be able to see if it loses coolant or smokes more than before.

 

After sitting for 12 hours, I had 4psi left. I started the car this morning and saw a decent cloud of smoke, but not much more than usual. I’m wondering if I’ve had a coolant seal issue all along. The coolant was basically black when I bought the car.

After about 15 minutes running the temp gauge crept up past the halfway mark again. Correct me if I’m wrong, but bad coolant seals wouldn’t cause the car to quickly overheat like that right?

 

I would suspect a stuck thermostat if it's overheating quickly. But, you may have bad coolant seals as well as a stuck thermostat.

Dale

 

Bad coolant seals can leak one way and not the other. Based on your symptoms it sounds like combustion pressure is getting into the cooling system but little coolant is getting into the combustion chambers. Combustion gasses in the coolant will cause coolant to be forced into the overflow tank and an air pocket to form in the engine, leading to overheating that starts slowly as the air pocket builds up and then rapid overheating once the coolant can no longer circulate due to the air pocket getting too large.

It also may explain the black oil & carbon contaminated coolant.

 

On the overflow tank, the bottom half of it is really all that needs to be water tight. There is a "straw" going down to the bottom. When the system heats up, the radiator cap lets some of the extra coolant out into the tank, then it sucks it back in as the system cools. The tank isn't pressurized.

It's worth replacing if the top is cracked and leaking, that may be from the tank barfing coolant. They're $100ish new from Mazda and look WAY better in your engine bay.

Dale