Weird coolant temp spikes.
 

Curly finished up my rotrex (75mm pulley) and intercooler install on the VVT motor about a week ago. He also did water pump and pulleys, drained, filled and bled coolant system, also new 180 degree thermostat.

Attachment 230967

Here's the symptoms.
If I start the car and immediately start driving, just normal driving, no hard pulls. About 8-10 minutes in the drive, as the temp warms up (which is does now way faster than before) it will continue to climb to 220 degrees and stay there about 10 seconds and then drop really quickly to normal temps.
If I warm up the car at idle in the driveway, car warms up to normal temperature.
After driving a while there's nothing odd, even under heavier street driving and pulls accept sometimes 8-9 degrees fluctuation around 185-195. It's a CX racing radiator and OEM cap.

I read in an older post that someone else had this problem due to tstat housing being gunked up.

Imo, it sounds like a faulty stat opening too slow :dunno:

Once fully open it drops down to normal temps.

I should note the thermostat was replaced because of these symptoms. So 1 old tstat and now 1 good one. Could be bad out of the box, but it's worth mentioning.

I had similar symptoms with a collapsed lower radiator hose. Not sure why that was though. All I know is I replaced the hose and this problem went away.

The little bypass line in the housing needs to be clear. Easy enough to check. It's just like the little hole in the M-Tuned reroute T-stat. It allows the hot water in the head to get to the wax element. Otherwise, opening of the T-stat will be delayed when you first get going.

My OEM VVT motor in Acamas would do the same thing. I assumed it was a sticky thermostat but never bothered to diagnose further. Did a reroute which fixed it.

Interesting, anyone care to care why it doesn't seem to spike when I let it warm up at idle?

If it's the collapsed hose (oops- typed 'hoes' first :giggle:) , lower flow at lower rpm won't collapse it and there's no problem.

It's worth a check, the lower hose is all sorts of squished around the hot side piping and power steering stuff. I mean, I'm not saying I blatantly ignored a flattened radiator hose, but I might of missed something.

We have considered the drilled tstat, and reroute.

Sure. What is happening is that there is a volume of stagnant water that resides in the thermostat neck. This water will warm up slowly. The little bypass nipple and hose that takes a little bit of water back to the waterpump inlet is designed to break up this stagnant water and allow the water in the area of the thermostat wax bulb to warm much more quickly. If the bypass is clogged, you will see a temp spike and eventual opening of the thermostat when you do an aggressive warmup. When you do a slow warmup (idle only), heat transfer into the stagnant water is able to keep up.

This is a well-known issue with Miatas, and one that I have personal experience with. Both my Miatas have since been converted to back-of-the-head reroutes where the heater hose performs the bypass function. So I don't have to worry about that stupid little nipple and hose anymore.

I shoulda had a re-route. Smh.

What should be the normal temp range with 180 degree tstat?

210-220F or less if I remember... that being the maximum. Should run at 200F or less in normal conditions.

Normal temp range really should be under 185 I would say, if you're using a 180* tstat. If you're going over that, the system is struggling to keep up.

Hornetball nailed it. I had this same problem on my MSM. Drilled a little bleeder hole in the tstat, problem solved. This was on a reroute car.

I expect the bypass hole thingy is clogged. But could also be the lower rad hose as mentioned.

I need a ride.

New tstat didn't change anything. Maybe drilling small hole in thermostat is an easy solution to try. I've got a couple of them now.

Thanks guys for the info!

My car runs at 185 on the freeway and track with a 180 degree tstat. Idle i let it up to 205, then fans pull it to 195 pretty quickly.

Good news, I drilled out the little bleeder valve and then opened the hole with a 1/4" bit.

Temps are around 185, highest I've seen so far is 195.
Thanks to Curly for the recommendation.

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