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My 1999 Mazda Miata is having some problems I can't seem to diagnose....


It is real rough to start, it will crank hard and strong forever, but won't start unless I push the throttle in and out a couple times at the right moments. Then after it starts, it will idle correctly then after a while it will drop to 400 revs and if I don't touch the throttle it will stall. If I drive it around, and let off the gas, the revs will most of the time drop to 400 and stall. However.... if I keep on the gas, and do a steady speed, the car runs like normal. If there's any drop in performance it's extremely slight, I can't tell a difference.


I was running the heater everywhere I went and managed to do 100 miles on the car at night and the car held steady. Any time i turned the AC on it would overheat after 30 seconds.

I took the car into a cheap mechanic and he found the heater core hose was blown, he rerouted the heater core.... and still it has the same problems. I have not used the A/C though since this change.



Thoughts? Does this sound like a blown head gasket, a lot of people have been saying this is not the case?


After looking around the options I've found could potentially be:

Head Gasket
Catalytic converter clogged
Crankshaft sensor
Camshaft position sensor
Main Relay


The problem is.... why would it overheat if it was anything except the head gasket? But if it were a head gasket.... wouldn't there be more problems?

Are all of these potential options I should check?
 

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Not sure what reroute the heater core is, but if the heater core is bypassed you must connect the 2 hose that went to the heater core together to maintain the coolant flow in the head.
I would check for vacuum leak too good luck

Edit to add
Check the radiator cap that it is the correct temperature, a pin hole leak in the coolant system will cause it to over heat too especially when the AC is been used
 

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several things could be wrong.. When ur driving around do u feel a tremendous loss in power and excessive noise from the engine? if u were to keep the car idling, would it overheat? and i dont understand what u mean by rerouting the heater core. for one if the hose was "blown" it would have been pumping coolant out of there. When was a service done to the car? (plugs,wires, NEW COOLANT, etc). I know its not the crank sensor and im pretty sure its not the cam sensor, u can check the coolant for oil to see if it is your headgasket,
 

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I don't think it's sensors either. This sounds similar to an issue I had on my 7mge mk3 supra.. turned out to be clogged cat was backing up system, caused engine to heat up really quick, just a 3 min idle would have the car up to temp and a 3 min drive would have the engine soo hot, you could use it as a heating source for a tailgating party in Chicago.

Eventually, the clogged cat backed up soo much it blew my exhaust gasket and then began running rough. Could not get over 1,000 rpm if I floored it, wouldn't idle higher than 400-500 rpm, seconds later rattling and rocking to a stall.

Also, if the heater core was blown and bypassed, you won't have coolant flowing through your heater core any more... hope your winters aren't terribly cold..
 
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