For the past year or more, I had an inconsistent and disturbing coolant
leak in my '91 Integra. The engine would lose coolant at differing rates
which appeared to coincide with warmer temperatures, but still exhibited
considerable variability within temperature ranges.
I went over the entire system with a strong flashlight, many many times,
looking for the tell-tale blue crud that results when genuine-Honda coolant
escapes the confines of the system.
There was no air in the system. The reservoir level dropped as the system
sucked in its contents as loss occurred. All I could do was to monitor the
reservoir and rad level every day, and top up as necessary.
Some online (in rec.autos.tech) suggested a cracked head. But that did not
explain the lack of air.
Well, one day this past January, I got careless. I skipped a day checking
the coolant, since it had been stable for several days prior. I drove about
3 miles, and wondered why there was no interior heat. An investigation
revealed a dry rad and empty reservoir. I topped them up, then idled the
engine to allow pressure to build. Coolant began pouring out from under the
front bumper. Uh-oh.
Further investigation led me to strongly suspect the rad, so I ordered a
new one. The new rad did not leak, but would not build pressure. I
discovered a deep gouge in the plastic filler neck's sealing ring. The rad
shop took back the rad and gave me a new one. This one worked fine, but the
defect in the original meant that I ended up having to pull the rad twice,
on a zero-Fahrenheit day! NOT fun!
Once I had the old rad out, it became clear what had happened: The gasket
between the core and the bottom tank was leaking, right at the hose stub
for the bottom rad hose. This area is virtually impossible to see on my car
with the rad and hose installed. And since the car is treated against rust
yearly and the coolant leak was inconsistent, the oily rustproofing residue
prevented the coolant from crusting over and becoming visible. When the
gasket finally allowed a Niagara-type leak, I could then see the streaks on
the tank where the coolant flow washed the tank clean.
Engines's been fine since. Haven't had to top it up since February.