when i turn on heater sometimes there is heat, sometimes there is not any heat. though when there is it will usually cool down engine. it overheats after about 45 minutes. any ideas?

 

Not sure if I'm having the same problem as the OP but my '95 Civic EX coupe D16Z6 SOHC VTEC 230,000+ miles has been doing a mystery overheating thing occasionally since I bought it for over 4 years and 70,000+ miles.

It often pegs the water temp gauge with other indications of running hot; no pinging, radiator relatively cool, cooling fan off.

Most of the time, the gauge is @ 25-30% of its sweep and I take this to be normal operating temperature.

For other reasons, I've replaced the water pump and radiator.

After it actually for real overheated a couple of months ago, I replaced the radiator cap.

I bleed the cooling system every time I top off or check the coolant in the radiator.

I considered replacing the water temperature sender but realized that when the gauge was indicating hot @ 75% sweep, the rev limiter kicked in well before redline on the tach so unless the engine management system's temperature sensor is giving bad information as well, the engine truly is running hot. I might still install a mechanical water temp gauge and be 100% certain.

Yesterday, it overheated again for real. After 40 minutes of freeway driving at seemingly normal temperature (25-30% of sweep) the gauge shot up as it often does but 10-15 minutes later I smelled coolant and pulled off. The pressure relief on the cap had popped and steam was dumping into the reservoir / catch tank / overflow. The radiator fan was not running even though I've heard it run recently. I let the steaming radiator vent for a few minutes then quickly removed the cap (turned it 1/4 turn and jumped away from the car) and 10 minutes later dumped what little water I had with me in. Less than two quarts of water didn't fill it completely but the temperature was steady and normal for the 10-15 minutes of driving that I had left.

Today, I'm removing and/or modifying the thermostat just to guarantee that nothing mechanical is preventing the coolant from circulating and to see if it has a positive effect on what seemingly has to be an air lock problem. The split o-ring that seals the thermostat housing attaches to the thermostat so I probably can't just remove the t-stat. I also know that the cooling system relies on the restriction through an open thermostat to pressurize the coolant above the cap pressure (the "side" between the outlet of water pump and thermostat is under higher pressure than the "side" between the thermostat and inlet of the water pump) I just would prefer to not destroy a newish oem Honda t-stat.

I'll also try bleeding with the front of the car elevated, something that I've not done previously.

I'll post again next week.

 

Blown head gasket confirmed by leakdown test today, coolant bubbling from radiator when I pressurized cylinder #2.

Last weekend, I drove about an hour on the freeway until the coolant temperature shot up and boiled over. Same circumstance on my return trip.

Before the temp shot up, it was steady at 25-30% of the temp gauge's sweep

Plymouth Fire Arrow gets returned to smog legal and registered then the Civic gets head work.

 

Warm the car up to operating temps and have someone rev the engine to about 4K while you're looking at the upper and lower rad hoses. Possible for them to collapse as the water pump starts pulling fluid through.