Overheating despite new parts

My black 1994 miata has an overheating problem that I cannot diagnose. Most of my driving is on the highway, where the car does not overheat. But if I get caught in stop and go traffic or city red lights after driving across town on the highway, the needle begins to rise above the halfway mark towards H. Turning off the A/C and blasting the heat lessens the problem. Resuming highway speeds completely returns the engine to normal temp (the needle is just left of the halfway mark).

When I stop the car I can hear the coolant boiling. Under the hood I can see that I've been losing coolant directly out of the overflow tank (the area around the left headlight is wet with coolant directly below the hose that vents from the overflow tank into the engine bay). When I check the radiator afterwards (after I let the car cool down). The coolant level is low by about a quarter gallon even though ample coolant is left in the overflow container.

Mysteriously, the car does not always overheat. It happens about 50% of the time when I drive in the conditions mentioned above. It does seem to happen more often on hot days. I live in Florida and want to fix the problem before the summer arrives.

Since this problem began, I have replaced the following parts:

Water Pump - New

Radiator - New (but installed under questionable circumstances - I took the car in for a leaking water pump, the shop said the pump was fine and I needed a radiator. After they installed the radiator, they "noticed" that my pump was leaking (like I had told them) and needed replaced. Beware of the Daytona area Mazda dealership. I feel they unethically inflate their bills).

Thermostat - New - replaced twice (once without the dual valve (my mistake) once with)

Radiator cap - New - The one that came with my new radiator didn't hold pressure (grumble, grumble)

I'd greatly appreciate any further suggestions or speculation as to what the problem may be. The car has 118,000 miles, but runs fine otherwise. Does anyone know how I can check if the fan is functioning properly? How about the fan relay switch?

Reply to
Garrett Gman
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Reply to
Mal Osborne

Maybe. But I'd replace the lower radiator hose first (mainly because it's cheaper). There is a sprin in there, that keeps the hose from sucking in. It rusts out eventually. When it does, you over heat. Probably a good idea to replace both though. it's cheap insurance against a hose failure at a most annoying time.

How to check the fan is easy. When it starts to overheat, get out, pop the hood and see if the thing is spinning. You could also do the same thing at home, by just not driving anywhere.

Reply to
<SnowGhost>

"Garrett Gman" wrote

Classic symptoms of a bad primary fan. Plenty of air moving through the radiator at speed, *NO* air moving through the radiator when not moving.

wrote

A spring in the lower hose? I did not see this when I replaced all the coolant hoses with OEM on our '90. Is this a later year change? Not that it would be a bad idea to replace all the hoses on a 13 year old overheating car......

Reply to
Ken Lyons

My original fan motor had a dead spot on its commutator. Depending on where it coasted to a stop, it sometimes wouldn't spin up again without a nudge. The result was unpredictable, intermittent overheating. Replacing the fan motor fixed the problem. I bought a used one from FM.

Reply to
Lanny Chambers

My '90 tends to overheat if I run a 50/50 mixture. I cut it back to about

Reply to
BK

Do you remember what they charged you? I have one that I will let go cheap.

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Also, did you swap the fan blade as well or just the motor? I was thinking that taking the fan blade off to save on shipping costs would make sense if I sold it, or at least give the customer the option.

Pat

Reply to
pws

IIRC, it was $40 for the entire assembly--bracket, fan, motor.

Reply to
Lanny Chambers

That's not normal. My armchair diagnosis: a partially-clogged radiator core.

Reply to
Lanny Chambers

Thank you for the suggestions. I had a shop look at the electronics, and the wiring to my cooling fan relay switch needed replaced. It had corroded. The cooling fan now appears to work correctly, but the corrosion seemed to be a side effect of the overheating (coolant fluid getting everywhere) and not a cause.

Unfortunately the car still overheats in the same pattern. Last night I looked under the hood while it was overheating and the coolant fan was spinning away just as it is supposed to.

When I replaced coolant this morning, I left the cap off and started the car to see if the level would drop when the thermostat kicked in. As soon as the car started (it had been sitting in the driveway all night), the coolant began to leak out of the open cap at a steady trickle. I also noticed that when I step on the gas the level immediately drops, but then immediately overflows again as soon as I take my foot off the gas.

I think for some reason, my cooling system is not circulating when I am not hitting the gas. This would explain why it only overheats at redlights - even when the fan is working. I'm going to start looking for a blockage, starting with the lower radiator hose as suggested above. Thanks for the advice. I thought I'd keep y'all updated.

Reply to
Garrett Gman

Thank you for the suggestions. I had a shop look at the electronics, and the wiring to my cooling fan relay switch needed replaced. It had corroded. The cooling fan now appears to work correctly, but the corrosion seemed to be a side effect of the overheating (coolant fluid getting everywhere) and not a cause.

Unfortunately the car still overheats in the same pattern. Last night I looked under the hood while it was overheating and the coolant fan was spinning away just as it is supposed to.

When I replaced coolant this morning, I left the cap off and started the car to see if the level would drop when the thermostat kicked in. As soon as the car started (it had been sitting in the driveway all night), the coolant began to leak out of the open cap at a steady trickle. I also noticed that when I step on the gas the level immediately drops, but then immediately overflows again as soon as I take my foot off the gas.

I think for some reason, my cooling system is not circulating when I am not hitting the gas. This would explain why it only overheats at redlights - even when the fan is working. I'm going to start looking for a blockage, starting with the lower radiator hose as suggested above. Thanks for the advice. I thought I'd keep y'all updated.

Reply to
Garrett Gman

That's normal. If the top tank of your radiator has turned from black to olive green, it's probably pretty well clogged and in need of replacement.

Reply to
Lanny Chambers

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