Need Advice: Overheating Miata: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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May 26, 2008 12:46 p.m. Team1CG None

Hey Folks, What are the basic steps to go through in diagnosing an overheating problem? Our patient is a 99 Miata.

Background: The PO had a problem about a year ago where the car overheated. The dealership flushed the cooling system and replaced the thermostat. No problems in daily driving since. I've had the car for a month or two. Last week, I had the dealership (a different one) do a 60K service, including cooling system flush and a timing belt. They also wanted to replace the thermostat while they were in there. I declined since it was only a year old and hadn't been a problem.

Yesterday's Adventure:

Did an autocross school, that included sessions of several runs back to back. Weather was mid 70's. At one point I looked down to see the temp gauge pegged on H. Running the heater on full blast brought the needle back to normal. Took a break then finished the last set of runs with the heater still running on full blast. Temp gauge read normal throughout.

Packed up at the end of the day and headed home thinking we were OK -- started out without the heat on. Temp gauge started climbing soon after getting on the highway. Put the heat on and pulled off at the next exit. Temp guage quickly dropped back to normal.

Hit the highway again -- with the heat on. Started running hot again. Exited the highway again. Temp guage fell to normal. Parked and let it idle for a while, then turned it off and let it sit for 10-15 min. Listened to the coolant gurgle in the overflow tank. Considered calling a wrecker for the Miata and getting my better half to come fetch me (an hour + from home). For reasons involving domestic tranquility, decided to try and limp it home.

Stayed mostly on local roads with the heat on. Made it home. Temp gauge stayed on normal. Was even able to turn the heat off eventually. When I popped the hood at home, however, the coolant was gurgling in the overflow tank again.

Thanks in advance for your input.
May 26, 2008 1:05 p.m. GameboyRMH Dork

Sounds just like when my water pump started to give out on the AE92...
May 26, 2008 4:05 p.m. littleturquoiseb New Reader

Is it possible you have an air bubble trapped in the system? I really don't know much about the Miata Cooling system but have had it happen to other cars.

Jeff
May 26, 2008 6:18 p.m. Woodyhfd HalfDork

I have a 96 that has only seen 15,000 miles over the last 8 years. The last time I changed the coolant and hoses, there was a lot of corrosion in the aluminum tubing. Check there too. Low milage is not always a good thing.
May 26, 2008 6:46 p.m. wlkelley3 Reader

My 99 Miata did the same thing shortly after I bought it. Temp was fine around town, as long as I didn't go over 45 mph. Highway driving and/or using the AC it would overheat. I always caught it before it pegged. I went through the complete cooling system, new waterpump, a couple thermostats, flushed it a couple times. Nothing fixed it. A new radiator did fix it. I've been told that sometimes some of the cooling fins would plug up and not allow adequate coolant flow. Something about the aluminum core and corrosion. On a sad note, that 1 1/2 year old AZ special waterpump gave up on me on the highway out in the boondocks, miles from anyplace, cell phone battery almost dead and poor reception. I gambled and tried to make it to the next exit. Barely did but fried the engine. It's at a shop getting a used engine installed right now. On the bright side of that, the crank/woodriff key slot issue will be fixed with the replacement engine. I got my radiator from a place on eBay stores. They treated me right when there was a shipping issue, I think the name of the place was eRadiator or something like that. It's a Koyo factory replacement.
May 27, 2008 9:13 a.m. confuZion3 Reader

I like my Koyo radiator. It's holding up just fine and I've had it since November. I have one track day and one auto-x on it as well. No overheating issues.

Have you ever replaced your water pump?
May 28, 2008 7:58 a.m. scotaku None

My money is on a broken impeller on the water pump. From the outside, everything looks great. No leaks. Only overheats when driven. Cools off if left to idle... that's because the engine speed drops low enough to let the impeller move the coolant. Bring up those revs and the coolant slows or stops the impeller though nothing seems wrong on the outside.
May 28, 2008 10:29 a.m. petegossett Dork

FWIW, if you - or anyone doing a cheap Miata build - need a good used waterpump/timing belt/tensioners, I just changed them on my 96 last weekend, and(to my surprise)they appear to be pretty new. Of course, I'm a packrat & can't just throw them out, even though I'll never use them!
May 28, 2008 11:45 a.m. nickleone None

I,ll go for the water pump also. My older escort and now my sons protege(same type engine) both had problems when the timing belt was replaced and not the pump. The problems started with a slow leak that escalated to a major out poring of coolant.

Nick