Overheating at Idle after Hard Driving - Radiator Clogged?

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10-01-2010, 11:59 AM
cleave

Overheating at Idle after Hard Driving - Radiator Clogged?
In normal driving, even when it's been hot outside this summer, my water temp gauge never moves past the 12:00 position. After some spirited driving or autocrossing, my water temp needle sometimes gets slightly warmer than normal (12:00 or 12:30 position... usually it's in the 11:30 position). Then, if I am idling the car, the needle swings pretty quickly to the hot end of the spectrum (1-2 minutes or so). When I see it start to move, I usually turn on the heat and it starts to cool back down. The needle has never been pegged and I never boiled any fluid. Things I've done or checked:
- Completely flushed coolant. Did the drain/fill procedure about 5 times until everything came out clean. It was pretty nasty when I bought it. Running 50/50 coolant/water.
- Replaced radiator camp.
- There are no coolant leaks and coolant level has not changed since filling.
- Fans seem to be working fine. I haven't checked the amperage, but when I put my hand behind the cooling fan it is turning very fast and pulling some serious air out.
Things that are left to possibly fix the problem:
- New thermostat
- New radiator (could be clogged?)
- New hoses (but wouldn't bad hoses give me problems more often?)
- New water pump
- Did I leave something out?
Given my symptoms, it seems to me likely that the radiator itself could be the likely culprit. It cools efficiently enough 90% of the time but it isn't efficient enough to cool the engine down when it is being stressed. And then when it is idling the fan just can't pull enough air fast enough once it reaches the point of no return. So, I think I'm going to go after the radiator first.

Has anyone had any luck with any radiator gunk cleaners? Thoughts about the best way to try and get a radiator unclogged? Or other thoughts related to my symptoms and diagnosing?
10-01-2010, 12:09 PM
sammm

Miata's eat thermostats. Make sure it's not stuck closed.
10-01-2010, 12:17 PM
Rob®

2nd. And realize the stock gauge does not show actual temps. If it's running "slightly warm" it's pretty hot.

Oh, and if you find the radiator is a bit blocked, be sure to check the metal coolant pipe bolted to the inner fender - those get all kinds of crapped up. And I've always just pulled the radiator and back flushed it for quite a bit - I've never actually used a product designed to remove deposits.
10-01-2010, 01:04 PM
POS Racing
Quote:
Originally Posted by cleave

Completely flushed coolant. Did the drain/fill procedure about 5 times until everything came out clean. It was pretty nasty when I bought it. Running 50/50 coolant/water.
If it wasn't maintained I'd bet you have some blockage. Is it still the original 16 year old radiator??

If it was me I'd get a new radiator just because! That is what I did when I rebuilt my '94!

YMMV
10-01-2010, 01:17 PM
Sharpie

Some people disagree with me, but miatas really like closer to a 70% water 30% antifreeze ratio. Try it, it's cheap.
10-01-2010, 01:34 PM
Robert

Quote:

Originally Posted by Sharpie

Some people disagree with me, but miatas really like closer to a 70% water 30% antifreeze ratio. Try it, it's cheap.

What does that put the freezing point at?
10-01-2010, 01:48 PM
Rob®

In Florida I ran all water, just about 2 cups worth of antifreeze, and a drop of dish soap. Changed that once I got out here and started seeing freezing temps, but I fully plan to run that again this coming summer.

* Aftermarket radiator, cap, ducting, AC and intercooler
10-01-2010, 01:49 PM
Rob®

Quote:

Originally Posted by Robert

What does that put the freezing point at?

If you try it, you are more than welcome to use my coolant tester to find out. Or if Sharpie lives in Denton, I can meet up with him before/after my classes on Monday or Wednesday and see what it says about his.
10-01-2010, 02:16 PM
cone-cerned

Quote:

Originally Posted by Robert

What does that put the freezing point at?

70-30 puts the freezing point at about +5*F

67-33 puts it at about 0*F

50-50 puts it at about -34*F

I use the 2:1 mix, even way up north here in Woodbine.
10-01-2010, 02:35 PM
cleave

Quote:

Originally Posted by Rob®

Oh, and if you find the radiator is a bit blocked, be sure to check the metal coolant pipe bolted to the inner fender - those get all kinds of crapped up. And I've always just pulled the radiator and back flushed it for quite a bit - I've never actually used a product designed to remove deposits.

Good tips, thanks.
10-01-2010, 02:38 PM
cleave

Quote:

Originally Posted by POS Racing

If it wasn't maintained I'd bet you have some blockage. Is it still the original 16 year old radiator??

That's kind of what I'm thinking. Judging by how ugly the coolant was when I drained it, there's got to be some crap still in there. I don't know if it's the original radiator. It's not brand-new, I do know that.
10-01-2010, 02:40 PM
cleave

Quote:

Originally Posted by Rob®

2nd. And realize the stock gauge does not show actual temps. If it's running "slightly warm" it's pretty hot.

Aye, and it bugs the crap out of me. I'd like to have some gauges that give me the real numbers. Heck, part of the reason I bought a '94 was just because of the oil pressure gauge.
10-01-2010, 02:42 PM
black roadster

Quote:

Originally Posted by Sharpie

Some people disagree with me, but miatas really like closer to a 70% water 30% antifreeze ratio. Try it, it's cheap.

We don't disagree. All customer cars leave TDR wit a 70-30 mix. Perfectly fine in the metromess. Besides, who wants to drive a Miata in sub-zero temperatures. ::Blue::
10-01-2010, 02:42 PM
cleave

Quote:

Originally Posted by Sharpie

Some people disagree with me, but miatas really like closer to a 70% water 30% antifreeze ratio. Try it, it's cheap.

I have considered going that route eventually, but I want to fix whatever is really wrong first before masking the problem.
10-01-2010, 02:45 PM
cleave

Anyways, thanks everyone for chiming in. I think I'm going to see if I can flush the radiator out first before buying new parts. I'll test the thermostat too and see if it's a culprit.
10-01-2010, 02:46 PM
Rob®

Are the end tanks green or black? If green, I'd just replace the thing. I'd probably say to replace it anyway.

Flush the system real well with the old radiator in there, remove radiator and all hoses, remove and clean the metal pipe (2 bolts to remove when the hoses are off), new thermostat, install new rad, hoses, and cap. Unless you've got a bad headgasket or fan(s) issue, you should be worry free after all that.

Oh if you have AC, I would *highly* recommend taking a hose to the backside of the condenser when the radiator is out. Use a normal hose end on the high pressure stream setting, and go to town. You'd be shocked to see how much dirt and bugs get in there over the years/miles.
10-01-2010, 02:49 PM
Rob®

Quote:

Originally Posted by cleave

Aye, and it bugs the crap out of me. I'd like to have some gauges that give me the real numbers.

So mod yours. If you can solder, it's cake. If you can't, I know mr brg offered to do it to someone's gauge here, and I did as well. I haven't done a 94-97 gauge in about 2 years, but I still have all the pics to do it.
10-01-2010, 03:16 PM
farphle

Quote:

Originally Posted by Rob®

...be sure to check the metal coolant pipe bolted to the inner fender - those get all kinds of crapped up.

You can also get a single silicone lower rad hose from FM that eliminates the two hoses and metal pipe altogether. I put one in my '92.
10-01-2010, 03:39 PM
Rob®

From what I've seen here, I think you can just get the hose from an NB. But I have yet to see that for myself, so I chose not to mention it.