'96 Miata.
Sunday one of my heater core hoses failed. I limped the car to work filling it up with water while letting it cool several times. It DID get hot at least once, top of the gauge. It's sat there until today when I clamped off the hose on either side of the hole and ran it. (Hopefully I'll see the parts next week..)
Started it up and let it idle for a while and it was fine. Gauge never got above normal. Drove it and it was fine until the stop sign next to my work parking lot. Then the gauge started moving upwards... at about the 3/4 mark I moved about a carlength ahead and the gauge dropped back down to normal?
In the parking lot I checked the coolant (straight water) level and it was where it was when I topped it off, everything seemed to be ok.
Drove it to/from the grocery store nearby and it behaved fine most of the time. Coming back it rapidly got hot and cooled off the same way once or twice. Upon stopping both times there was under-hood steam, seemed to be coming from the area of the broken hose (which is right above the exhaust).
What say you?
Woody
UltimaDork
6/14/12 7:17 p.m.
I think you have air in the system. The thermostatically controlled fan won't kick on until it's in hot water. Then the car cools off. Maybe
Test the compression. If the compression is fine, the gasket is fine.
Woody wrote:
I think you have air in the system. The thermostatically controlled fan won't kick on until it's in hot water. Then the car cools off. Maybe
This sounds right. When its cool, boil the radiator. Take the cap off, run it and keep adding water until its full for sure. Recap. Drive.
Stop driving it while its broken. Fix the hose properly, then see if the coolant flowing properly has solved your problem.
And next time you blow a hose, stop driving it.
I like the thought of air.
The only reason I drove it tonight is because I need to buy food and it's my only car. Grocery shopping on my bicycle is kind of a pain. Replacing everything and then re-evaluating is the plan.
Don49
Reader
6/14/12 8:19 p.m.
If you got it hot enough to peg the gauge, your thermostat could be toast. It would be cheap insurance to replace it at the same time you fix the hose.
Oh, and water bites as a long-term coolant. Temp fix? yeah.
Ojala
Reader
6/14/12 9:48 p.m.
In reply to DoctorBlade:
Water is a great coolant....till the first freeze 
I agree with the air - doesn't sound like a head gasket. Also agree a compression check is a quick and easy way to find out. Don't agree that you shouldn't drive it with a bypassed heater hose. I drove a the Merkur like that for years. Thermostat is a good plane. Water is bad, um-kay? Don't use straight water. Not only does antifreeze cool better, it also lubricates your water pump seals and inhibits corrosion.
There you go. All my opinions on the subject.
carzan
HalfDork
6/14/12 11:06 p.m.
Don49 wrote:
If you got it hot enough to peg the gauge, your thermostat could be toast. It would be cheap insurance to replace it at the same time you fix the hose.
^^^This. I've had thermostats go wonky after an overheat. Air in the system can cause erratic cooling issues, too, but it doesn't sound like a head gasket issue, so far, to me.
BTW, water transfers heat better than typical antifreeze/coolant. It just freezes at warmer temps, boils at cooler temps, doesn't lubricate well and promotes corrosion. Other than that, it's great!
A quick way to check headgaskets is to pull the plugs and compare, one or more really white ones usually indicates a blown gasket, as does mixing coolant/oil.
The water is as temporary as the clamps.
Current list of parts that I'm waiting for includes all new hoses, aluminum radiator, timing belt kit and water pump. Guess I'll add a t-stat to the list.
Thanks for the advice, all.
