2005 Civic LX (D17A1) recurring overheating, intermittent loss of cabin heat, coolant disappearing. New radiator, thermostat, and cap already replaced
I’ve been chasing a cooling system issue for a while now and could use some opinions before I keep throwing parts at it.
A few months ago I replaced the radiator (OEM-style), thermostat (OEM), and recently the radiator cap after chasing a coolant leak. I also found and fixed one leaking hose clamp. Ever since fixing that clamp, the time between needing to bleed the cooling system increased significantly, so I know I made progress.
The new radiator cap also seemed to help a lot. For the past week the car has been perfect. I’ve driven it hard, let it idle after spirited driving, sat in traffic, and even in 100°F weather the temperature never moved from normal.
Today it started raining, so I turned the heat on because it got chilly. The blower worked fine, but the heater never got warm. Shortly after that, the temp gauge climbed to about 3/4.
I got home and started bleeding the system. At first, once it got up to temp, the coolant in the funnel started rising. It rose more with slight revs, and after shutting the engine off it rose to the brim and overflowed a little. I let it sit for a minute or two, started it back up, and after that the bleeding process became completely normal. The coolant level stayed stable, even when revving, and didn’t continue rising.
One thing that’s been consistent through all of this is the heater. Before this past week when the cooling system has been performing good because of the new radiator cap, it usually takes around 15 minutes of bleeding before the cabin heat finally starts getting hot. Before that, one heater hose at the firewall will stay much cooler than the other before it eventually warms up.
Another thing I’ve noticed is that if I use the pressure-release lever on my new radiator cap while the engine is hot, the pressure is released through the overflow reservoir, and coolant will come out around the reservoir cap if I don’t release it slowly.
So far I’ve replaced:
Radiator
OEM thermostat
Thermostat housing
Coolant hoses & clamps
Radiator cap
Fixed one confirmed external coolant leak
I still think I have at least one more external coolant leak somewhere because I’ve found fresh coolant in the engine bay before, but I’m trying to figure out if something else is going on too.
I don't have any of the usual head gasket symptoms. No white smoke from the exhaust, no milky oil, no sweet-smelling exhaust, and the car has gone through plenty of hard driving and long idling sessions where the temperature stayed perfectly normal after replacing the radiator cap. Because of that, I’m hoping it’s not a head gasket, although I’ll admit I’m probably a little biased because I really don’t want it to be.
Has anyone experienced something similar on a D17?
Would you be looking more toward trapped air, another external leak, a heater core flow issue, water pump, or should I be doing a pressure test and block test before replacing anything else?