Thermostat?

David Spratte · **Posted:** Tue Sep 06, 2005 9:09 pm

So, on a cold start, about 5-10 minutes in the Miata has been getting hotter than usual. Like, up into the 3/4 range. The first time it happened, I started to pull over, but then the needle started diving to rest where it usually does, a bit below straight up.

It does this independent of my driving behavior at the time.

It's doing it on a fairly regular basis now, radiator is new and the fan behaves correclty, so I'm guessing it's my thermostat.

Any other suggestions before I wander over to THoff tomorrow.

jimpastorius · **Posted:** Tue Sep 06, 2005 9:31 pm

David Spratte wrote:

So, on a cold start, about 5-10 minutes in the Miata has been getting hotter than usual. Like, up into the 3/4 range. The first time it happened, I started to pull over, but then the needle started diving to rest where it usually does, a bit below straight up.

It does this independent of my driving behavior at the time.

It's doing it on a fairly regular basis now, radiator is new and the fan behaves correclty, so I'm guessing it's my thermostat.

Any other suggestions before I wander over to THoff tomorrow.

You can simply pull the thermostat out and see if that cures the problem. But my money is on the water pump starting to go.

George Bright · **Posted:** Tue Sep 06, 2005 9:43 pm

My old 510 used to do this. It turns out the thermostat would stick. I had several do this. replacing the thermostat was the answer.

DickRasmussen · **Posted:** Tue Sep 06, 2005 11:32 pm

Speaking of "water pumps starting to go", how does this happen? I've had to replace a few water pumps over the years due to a leaking shaft seal (resulting in a slow coolant leak which I've always spotted before overheating resulted) but I've never had one "pump less". FYI I have heard of the BMW, Contour, etc. plastic impeller failure which I've assumed was a sudden failure.

Alway trying to learn.

Thanks!

David Spratte · **Posted:** Wed Sep 07, 2005 12:09 am

I might take your money Jim. The water pump was replaced in the last year or so when the timing belt was replaced.

I'll give the thermo replacement a shot and see what happens.

jimpastorius wrote:

David Spratte wrote:

So, on a cold start, about 5-10 minutes in the Miata has been getting hotter than usual. Like, up into the 3/4 range. The first time it happened, I started to pull over, but then the needle started diving to rest where it usually does, a bit below straight up.

It does this independent of my driving behavior at the time.

It's doing it on a fairly regular basis now, radiator is new and the fan behaves correclty, so I'm guessing it's my thermostat.

Any other suggestions before I wander over to THoff tomorrow.

You can simply pull the thermostat out and see if that cures the problem. But my money is on the water pump starting to go.

Kevin Butler · **Posted:** Wed Sep 07, 2005 12:38 am

I'm with Dick on the water pump symptoms. They usually fail by the seals or bearings failing. My experiences have always seen water pump failures manifested by overheating due to coolant loss. Maybe I've just been lucky they were obvious and easy to spot.

If your coolant level was low due to a leaking water pump, I don't think it would heat up and then drop to normal running temp. It would stay hot/overheated.

The thermostat could be sticking closed a bit long before finally opening. That's why you see what you do on the gauge.

A third and quite often overlooked culprit could be your radiator cap. With a pressurized cooling system, if it's leaking a bit before sealing to build up pressure, I believe you'll see symptoms similar to what you are describing.

Both a radiator cap and thermostat are cheap. Change the radiator cap first and see if it resolves the situation. If it does, you don't have to bother getting coolant all over your hands

(hate the stuff) Better yet, you can return the thermostat and buy an extra gallon of gas!

If the cap swap doesn't work, then you'll have the peace of mind after changing your thermostat that you know the radiator cap is good.

I hope the cheap parts solve your problem. :thumbsup:

AndersGreen · **Posted:** Wed Sep 07, 2005 8:20 am

Kevin Butler wrote:

Both a radiator cap and thermostat are cheap. Change the radiator cap first and see if it resolves the situation. If it does, you don't have to bother getting coolant all over your hands

(hate the stuff)

Coolant is my most hated of all car liquids! Slimey, stays around forever, slippery and dangerous on the floor, and it's not thick like oil, so it splashes on everything! Rrrrrrr. Even gasoline is better behaved! (Because it will evaporate and go away.)

I'd only reverse Kevin's suggestion but otherwise I agree with him: thermostat _then_ radiator cap.

Cheers,
Anders

jimpastorius · **Posted:** Wed Sep 07, 2005 8:28 am

David Spratte wrote:

I might take your money Jim. The water pump was replaced in the last year or so when the timing belt was replaced.

Sorry David, you withheld information

It just might be your sensor going bad too.

Frank Catena · **Joined:** Wed Aug 27, 2003 1:08 pm **Posts:** 418

David,

Wouldn't hurt to read this :

http://miata.net/garage/cooling_system.html

Especially the part at the bottom.

Frank

Ryan Holton · **Posted:** Wed Sep 07, 2005 9:56 am

I know where my money is:

Air bubble.

From what you typed, it seems you replaced the radiator VERY recently. I am also assuming that the overheating started with the new radiator.

If so, an air bubble is your problem, IMHO.

My CRX was a PITA to bleed. I had to jack the front of the car up as high as I could and let let it idle for 15 mins.

David Spratte · **Posted:** Wed Sep 07, 2005 11:54 am

Frank Catena wrote:

David,

Wouldn't hurt to read this :

http://miata.net/garage/cooling_system.html

Especially the part at the bottom.

Frank

Nice pointer Frank. I looked through the garage under thermostat R&R and searched the forums. Didn't think to hunt through the general cooling FAQ.

Looks like it's time to play in the lime Kool-Aid. I'll check that out, but for the money while I have it off, I think I'll toss a new cap and thermostat on it.

Radiator has been awhile and either I've just noticed it or it's started in the last 2 months. Last radiator replacement was about 6 months ago.

Kevin Butler · **Posted:** Thu Sep 08, 2005 3:54 pm

So David,

Inquiring minds want to know...what was your problem solution?

David Spratte · **Posted:** Fri Sep 09, 2005 9:46 am

That's this weekend. I'll let you guys know.

But I have to do something about the fact that I won't remember how to autox by the time I get back out there.

Looks like I'm missing Sanford, but it'll be hell or high water to have me miss the school on October. . . which reminds me I need to register for that.

Chuck Frank · **Posted:** Tue Sep 20, 2005 5:03 pm

Did you check the coolant level? Is there coolant in the overflow tank both while hot and cold? a small amount of air in the system can produce these symptoms.

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