Need help with a chronic overheat

1991 Subaru 2.2L, rear-mounted in a 1972 VW Beetle. This swap is about 5 years old, and the current issue didn't rear its ugly head until about two years ago. Don't specifically remember what or if anything changed to precipitate the change in performance.

First, a quick run-down of the system.

Radiator is in the front, an aluminum two-core job. I've used this style of radiator since I built the car; have broken several by user error. This one is a couple years old. Cap is something like 22psi. Cap overflows to a coolant tank in the front, and has a return valve for when the system cools off and goes back under vacuum.

Rubber coolant hoses run back to the engine under the floorpan. There are a couple of galvanized steel hose barb connectors to patch together pre-bent hoses to the off-the-roll hose that runs under the length of the car. Hoses are 5 years old, but all appear to be in good condition (no frays, chafes, rubs, splits, cracks, don't feel collapsed).

Heater core and "header tank" are located above and in front of the engine, in the rear parcel shelf. Header tank is a vertical unit with an inlet and outlet and a radiator cap on top. Cap is something like 29psi (intentionally higher pressure than the cap at the radiator).

Water pump was replaced a couple of years ago, I THINK since the overheating issue started.

Currently running without a thermostat; had one of the fail-safe units fail not-safely 3000 miles from home in 2015, have run without ever since (did so for several months without the current problem).

High points are the heater "header tank" and the radiator (as the hoses have to run down low under the car). As such you have to burp the system to make sure you get all the air out.

Now, some symptoms.

At idle, the system works fine. Once the system finds its equilibrium volume (once the coolant overflows adequately after initial filling), it doesn't overflow or overheat. Fan comes on around 200, shuts off around 185. Radiator gets hot (pump is moving the coolant), hoses get hot and hard with pressure. I let it run for several hours and had no issues with overheating, overflowing, or leaking.

I mostly use the car for short trips (10-15 minute drives). After a few such short trips, the overflow tank will fill to capacity and start to spill, and after the system cools off, there will be a considerable amount of air in the header tank and the radiator. No sign of oil in the water or water in the oil. If you don't catch the symptoms early, it will overheat. Longer trips will prompt the same symptoms.

Since it started doing this, I've replaced the head gaskets, replaced the engine entirely (junkyard replacement was cheaper than resurfacing), and replaced the water pump. Not sure when I replaced the radiator, as I've had to replace it several times.

Please, someone show me the error in my ways. This is driving me mad, and as much as I love driving this car, I can't have this kind of negativity in my life.

Fans???

maybe they're not coming on when you start driving.  Maybe there is a sketchy ground that gets overwhelmed as the alternator output rises??

I'd start by seeing if the return hose is collapsing at high RPM.  That's a really long suction hose to be made out of rubber.

In reply to APEowner :

That's an interesting thought. It wasn't an issue for several years, but as those long stretches of rubber hose get older, I could see a collapse being a real problem... how would I go about checking that? Presumably would have to be while under load.

In reply to jfryjfry :

There is so much that qualifies as "sketchy" with this car. It would really be easier to list what isn't sketchy.

In reply to JohnInKansas :

You probably don't need to be under load just at temp with the RPMs up.

what kind of air flow are you getting to the radiator while driving? without a thermostat maybe the coolant is flowing too quickly through the radiator and if it isn't getting decent airflow then maybe that's enough to cause the issue, 2 smaller problems that cause a bigger issue when happening at the same time

Any possibility the positive and negative wires for the fan got reversed in one of the radiator swaps? If so fan would run backward and could cool enough to keep temp down at idle parked but not while moving.

APEowner said:

In reply to JohnInKansas :

You probably don't need to be under load just at temp with the RPMs up.

My first thought also. Does that hose have a spring in it? If not, it's probably collapsing. Get one and put it in and then try.

Replace the caps? With a 22psi cap you'd be seeing engine damaging temps before it should start to boil over like that.

So an update, for posterity.

Head gaskets. I know, I said I had just done them on the old engine to no avail, and had swapped with a different junkyard engine, but I had a minor leak from one cylinder to the water jacket. Runs like a champ again. On a positive note, I have full-length aluminum pipes from radiator to engine now, so no more threat of collapsing hoses.

If it looks like a duck, and quacks like a duck...

Thanks for the update.  I can't tell you how many times I've fixed weird problems by going back to basics and rechecking stuff that I thought I'd ruled out.  I do think you're better off with the new plumbing setup regardless.