Thermostat and radiator cap maintenance

[Sylvert]

Hi all,

My 2018 Mazda 3 now had 255 000 km (158 000 miles ish) and I never replaced or touched the radiator cap and engine thermostat (which is a standard regular one).

Are those two items need to be changed? Some say radiator cap should be changed to maintain de pressure in the cooling system, I personally never changed it, same for thermostat, no problems so far. All I did for the cooling system is coolant change.

Any inputs? Thanks!

 

[Kira]

Working eight year old OE cap and stat in a vehicle of mine would be left alone. Known lifespans excepted.

I renewed my truck's cap because its 17 year old coating of oxidation caught my eye.

 

[TmanP]

I've never changed either proactively, but I do keep a close eye on the temp gauge and coolant level (I tend to check oil and coolant for something to do when filling up) and have a very good understanding of the warmup behavior of the car. If I noticed that it was taking any longer to warm up or anything like that, I'd swap the thermostat for sure.

 

[mclasser]

I only replaced a rad cap once and that was because it was leaking.

Never replaced a t-stat. Main risk is if they fail closed, the car can overheat. Failed open is just engine won't warm up, poor heat, etc. This will usually trip a P0128 code.

 

[slacktide_bitog]

The car will rust before you have any problems with the thermostat

 

[walterjay]

I would leave it alone if it were my car.

 

[2010Civic]

Unless having an issue not a chance I would replace either.

 

[The_Jeff]

Another vote for not changing anything.

Do recommend throwing a scan tool on it to see if it hovers within a few degrees of thermostat temp when you're driving on flat land (eg normal load.) That way you can spot trends before they hit the heavily buffered dash gauge.

In my experience radiator caps fail in a way that they don't suck coolant back from the overflow/ surge tank when the engine cools back down. When new, the flap of rubber is flexible and lets this happen. The spring is reliable so you're very unlikely to overheat because the cap "jams." You can easily evaluate the flap of rubber yourself holding the cap up against a new one.

As an aside, I like the little bit of Japanese on OE rad caps that isn't on parts store replacements. For that vanity alone I try to keep them intact.

 

[ARB1977]

25 year old f150 is on it’s original cap and thermostat. I have a t stat just too lazy to change it.

 

[Chris142]

25 year old f150 is on it’s original cap and thermostat. I have a t stat just too lazy to change it.

I guarantee that cap is not working correctly.

STANT used to recommend replacing them every 2 years as the rubber seals would harden or swell and not seal.

 

[zzyzzx]

Look at the seal on the radiator cap.

 

[dogememe]

I'd leave it alone if it's working fine. And if you ever do need to replace the thermostat, get a genuine one from the dealer.

 

[ARB1977]

I guarantee that cap is not working correctly.

STANT used to recommend replacing them every 2 years as the rubber seals would harden or swell and not seal.

How so? Doesn’t over heat or loose coolant. Stant told you to jump off a bridge, you do it. I’ve had 14 year old, a 9 year old trucks…all with original caps.

 

[Chris142]

How so? Doesn’t over heat or loose coolant. Stant told you to jump off a bridge, you do it. I’ve had 14 year old, a 9 year old trucks…all with original caps.

The rubber seals get hard,cracked etc and no longer hold the correct pressure.

 

[bdc101]

Another vote for not changing anything.

Do recommend throwing a scan tool on it to see if it hovers within a few degrees of thermostat temp when you're driving on flat land (eg normal load.) That way you can spot trends before they hit the heavily buffered dash gauge.

In my experience radiator caps fail in a way that they don't suck coolant back from the overflow/ surge tank when the engine cools back down. When new, the flap of rubber is flexible and lets this happen. The spring is reliable so you're very unlikely to overheat because the cap "jams." You can easily evaluate the flap of rubber yourself holding the cap up against a new one.

As an aside, I like the little bit of Japanese on OE rad caps that isn't on parts store replacements. For that vanity alone I try to keep them intact.

This is my assumption as well.

I also have a 2.5l Mazda6 coming up on 100k miles. I've never changed the coolant and have noticed that I've had to add about a cup of coolant twice in the past 5 years. The car's never been overheated or had any other problem or even a CEL, just a teensy bit of coolant has disappeared. I think it's far more likely that the 12 y/o radiator cap is just old and not working exactly right than it is truly eating coolant. Could be wrong but I would assume this is what's happening.

 

[The_Jeff]

^ Sounds like normal coolant loss to me. You can lose "a little", I've heard that adding it more than every three months is a problem. It's not a perfectly closed system; air has to get into and out of the overflow/ surge tank and that humidity is going to mix with the air that comes and goes.

That said, check the cap like Chris142 wrote. You can feel the difference; they use pretty cheap rubber that loses its rubbery qualities.

 

[Chief2025]

Id get a new OE rad cap from the dealer - - - - ALL aftermarket caps are trash that will last a year or 2 at best.

That is a lot of heat cycles on that Rad cap you have there.

Good consistent pressure control is just as important as antifreeze maintenance..........

But, I'm probably in the minority here......

 

[bdc101]

^ Sounds like normal coolant loss to me. You can lose "a little", I've heard that adding it more than every three months is a problem. It's not a perfectly closed system; air has to get into and out of the overflow/ surge tank and that humidity is going to mix with the air that comes and goes.

That said, check the cap like Chris142 wrote. You can feel the difference; they use pretty cheap rubber that loses its rubbery qualities.

I know some or maybe all water pumps use a mechanical seal as well, which will seep a tiny bit of coolant out over a long period of time. Could just be that and nothing else, I don't know. Talking about maybe 20 ounces of fluid over a more than ten year period.