Engine overheating on 1999 Mazda Miata MX-5

My engine overheats at highway speeds -- originally it was only with the a/c but now it overheats after about 15 minutes of highway driving. I checked the fuses and relay and they are ok (thank you for the prior tip); the fans are also working. The mechanic suggested the catalytic converter could have a backflow problem? I replaced the cat this past fall. I love this car and would love to have it available to me fulltime (it does fine with in-town driving). Thank you for any tips!

by in Pismo Beach, CA on May 10, 2009
3 answers 1 comment
ANSWER by on May 11, 2009
Hi I remember replying to your question. I had a Miata, sorry I sold it. I have never seen the water pump fail (some European cars used to have the water pump impeller fall off or slip).I would check the thermostat, perhaps it does not open fully it is cheap replace it. Make sure the coolant concentration is correct (too much antifreeze will have to much viscosity and could cause over heating).Check the fan comes in at the right temperature a fan switch turns on the fan, this is checked with a non contact infer red thermometer. You want to ensure the radiator is not restricted or blocked either internally or externally. No leaves or road debris lodged in radiator air intake duct front bumper area. Do you have additional lights in the grill opening that would restrict air flow through the radiator? Check coolant circulation in the radiator. Finally check for a blow head gasket carrying out what is called a chemical block check. You get the engine warm carefully remove the radiator cap and draw gas through a device that checks for the prescence of carbon monoxide in the cooling system. Carbon monoxide is a by product of combustion and will only be present in the cooling system if the engine has a blown head gasket.
COMMENT by on July 08, 2010
replace thermostat
ANSWER by on July 08, 2010
replace thermostat
ANSWER by on August 08, 2012
I just picked up a 1992 Mazda B2200 that had the same problem. Temp was fine while idling and around town, but 10 min at highway speed and it got quite hot, but has never overheated. Turns out my radiator had a pin-hole leak that was preventing the system from pressurizing correctly, which seemed to only matter when the engine was under a substantive load at high speed. Try a pressure test of the system
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