Weird honda radiator problem

2003 civic, 1.6l, autotrans.

Water from radiator goes into over flow tank.

When engine cools coolent does not draw back into radiator. Tank eventually overfills and sprays into compartment.

There is no indication of gasket leak.

Before I found out about overflow I replaced, radiator and cap, 2 caps, thermostat, and hoses.

Also drilled hole in reservoir cap to be sure of pressure balance.

Mechanic can’t figure it out. Has never seen this before.

I had the same problem with a 1993 Oldsmobile. The hose between the overflow tank and radiator would collapse when the radiator tried to suck the coolant back from the overflow tank. The hose acted as a check valve. A new hose was about $2 and the repair took 5 minutes.

I sure hope that works for you! I leap to the conclusion the air was introduced into the system on cooling and contraction by the radiator cap. Keep an eye on the coolant level for a bit in case there does happen to be a coolant leak somewhere.

I did replace the overflow tubing with a heavier hose.
Fluid is definitely going to reservoir as reservoir overflows after several days.

The cap might be cracked on the reservoir bottle. Did you replace the hose, attached to the reservoir bottle cap, that goes into the reservoir bottle. If it’s leaking, it won’t suck.

If the cap was cracked that would release whatever vacuum that was in the reservoir.

A couple of days ago I left the cap extra loose to see if that would help. Chk’g this morning I found the radiator level to be at top. I’m thinking I didn’t drill a large enough hole in the cap.

Honda uses a neat little trick to supply atmospheric pressure to the reservoir. There are two holes, about 3/32" drilled right at the top of the threads of the reservoir. I’m guessing to keep contaminants out of the reservoir.

I did replace the hoses. see above.

Check inside the filler neck of the radiator very carefully. There must be a perfect seal between the radiator neck and the radiator cap. Check that the radiator cap seal with spring is long enough to reach the bottom of the filler neck. If not, there will be no vacuum to draw the coolant back into the radiator.

I seem to have this same problem. 2001 Honda Civic EX manual trans. Fluid spray in compartment when I drive say 10 miles or more. Also, pressure and leak tests are OK. Dealer says probably partially blow head gasket if that makes sense. Does not to me. Let me know if you hit upon the answer.

Still have the same problem.

Sorry it has taken so long to update,

It turned out to be the head gasket/ warped head. The auto shop analyzer apparently was not sensitive enough to detect the carbon monoxide in the radiator water. The dealers was.

It cost $100. for the dealer check for the problem. They wanted about $1200, to fix. The shop did it for about $750. + the dealers $100.

My guess is that it was hard to detect because of the low compression. If it had been about 11:1 I’m sure it would have been more obvious.

Been about 3 months now and every thing is fine.