91 Civic cooling system pressure

My 91 Civic presurrizes the cooling system upon startup. The overflow
tank is also under pressure and there is a slight leak under the cap.
There was also a spare cap under the hood when I bought the thing not
long ago. Is this pressure normal or is the head gasket going? There is
no oil in coolant, no coolant in the oil, and it doesn't smoke. It runs
good and doesn't overheat. But if you start it with the cap off it
sprays coolant. Should the overflow tank be vented? I only see one hose.
THanks in advance.

 

Look carefully at the tank itself, there should be some sort of overflow
tube built into it. I've never encountered a Honda with a pressurized
overflow tank. My guess is that your radiator cap is bad and the overflow
hole on the overflow bottle is plugged.

 

Something is seriously wrong here. Are you saying the expansion tank has a
pressure cap on it?

On your car there should be an 0.9-bar cap (13lb) on the radiator. This cap
should allow pressure to bleed off into a tube in the rad filler nack that
goes to a non-pressurized expansion tank.

Is the rad cap aftermarket or OEM?

 

I see no overflow tube. I pulled the tank off (it just slides into a
groove built on the motor mount) and there's no vent tube. The cap to
the overflow tank screws onto the tank and I can see no sign of
tampering. I'll get a raditator cap and see what happens. BTW, the cap
that's leaking is the radiator cap. Sorry I wasn't clear about that.

 

Well, it has a screw on type cap with the single hose from the radiator
attached to it.

The cap is a 13 lb. I can't see how the expansion tank can be
non-pressurized though. There's only the single hose going to it, and it
screws onto the expansion tank unlike most snap on type ones that I'm
accustomed to seeing. I see no apparent way for the tank to vent. The
expansion tank is original...it slides right onto the motor mount
bracket. Nevermind about the expansion tank...I found a hole drilled
into a flat spot of the threads.

It's an aftermarket. Does this *really* matter? I'm thinking about a
thermostat, but it doesn't overheat and it does warm up as it should.

What I'm thinking of doing is screwing my compression testing onto each
cylinder and attaching to the air compressor and looking for bubbles in
the coolant. I guess that would tell me if it is a head gasket.

 

That's normal.

The tank /has/ to vent. As your coolant heats up, it expands. As it expands
it pushes into the reservoir, so the reservoir level will rise. When you
shut down and the car cools off, the reservoir level will go back down
again as coolant is sucked back into the rad.

Yes. If the cap is the wrong design for the car, or is defective, this will
affect cooling system operation. Change it just in case. It's cheap enough.

If it doesn't overheat, then the thermostat is fine.

Sure would. But if you're not losing coolant, then your head gasket is
certainly fine.


Other than your report of "spraying", it doesn't sound as if there's
anything wrong with your car.

 

I'm curious what effect a thermostat with the air bleed valve pointed
downward would have on a system.

This car also has hot/warm intermittant start issues and I'm wondering
if these could be related to air (that I can't seem to get rid of)
hitting the ECU temp semsor.

It doesn't lose coolant and doesn't pressurize the expansion tank when
running, but when cranking the engine and only when cranking the
warm/hot engine, I hear air escaping from the vent hole in expansion
tank. THe thing runs great except for the intermittant start issues.

BTW, I've already resoldered the main relay and can hear the fuel pump
at 'key on'.

The next no start I guess I need to check for spark.

I do get an Oxygen sensor code. From what I've read this wouldn't keep
it from starting...would it?
escaping from the vent in the