Can't remember if it was suggested in this thread, but a combustion leak detector is another great tool to add to the arsenal for a problem like this:

https://www.amazon.com/Lisle-75500-C.../dp/B0007ZDRUI

I have a different brand, but it is the same general principle. A combustion leak into the cooling system can be detected in the air at the radiator neck. The fluid in the tester changes color in the presence of exhaust gas.

 

Yes, it's soaked in oil/crud from when things were really overflowing.

 

More late night thoughts. Here's another easy, zero-dollar way to check for combustion leaks into the cooling system.

Remove the radiator cap. Start the engine and idle until warm. Shut off engine. Quickly remove the V-belt(s) for the water pump. Make sure the radiator is full to the neck. Keep the radiator cap off. Start the engine and quickly rev it a couple of times. Look for bubbles in the coolant. Without any coolant flow (water pump not spinning), there should be no bubbles.

Obviously, only run the engine briefly like this, because it will overheat without any coolant circulation. You should be able to go a minute or two without any problems.

This test is very similar to Chris' 'wrong' test, but is quicker to set up and tests all cylinders at the same time. Chris' 'wrong' test is much more specific and can confirm which cylinder is at fault.

 

OK, today I drove into town and went to NAPA. I got a new premium 195F thermostat and gasket. Not sure if I need it right now or not, but it doesn't hurt to have a spare. I also went to Autozone and borrowed the radiator pressure tester. I hooked it up and pumped it up to 5 psi and the gauge stayed steady for over 2 minutes. The radiator cap says 13 psi, so I pumped it up to 11 psi. I noticed some coolant dripping out where the upper hose is clamped to the radiator. The gauge had barely moved after 2 minutes. I don't think there are any big coolant leaks , so it must be condensation or rain sucked in through the dipstick tube, right?

I'm going to take it for a drive and then check the thermostat housing with an infrared thermometer to see if the current thermostat is working.

 

The gauge shouldn't move at all. And, you should pump it up to at least what the cap is rated for since your system will pressurize to that point every time it warms up.

And, you aren't looking for a big leak. Just a small one will cause the problems you are seeing.

 

The gauge moved about 1/10th of a psi. It seemed to fit with the drip from the radiator hose connection.

After driving it, the thermostat housing read 170 on my IR thermometer. After idling a bit it had dropped to 160. So it seems the wrong thermostat is installed. I didn't see any water on the dipstick either. Also, I had no coolant dripping from the hose, so I don't think it's getting up to 13 psi.

It's fixing to rain here now, so tomorrow (it's supposed to be dry) I'll put in the new thermostat and do the pressure test again up to 13 psi.

 

At 170 degrees you won't get much pressure buildup. But, you also won't get much boiling off of condensation. I'd put the new 'stat in and tighten up the hose clamp.

 

Some progress you've made, huh? In your first post, you were convinced your engine was wrecked. We've talked you off the ledge and you've since become a troubleshooting genius. Little by little, you've been ruling out some major problems and cleaning up some minor issues.

How far you want to proceed for peace of mind is your call. It seems you've got the moisture problem licked, but make sure you continue to monitor that, even after replacing the thermostat, PCV valve, and PCV breather.

I've been pulling Chris' leg about doing his 'wrong' procedure, but it's actually a very helpful test. It can find faults that won't appear with the 'right' cooling system pressure test. The only downside is the 'wrong' test is relatively time-consuming. You have to test each cylinder individually, and start by putting that piston at TDC of the compression stroke. If you still want to run that test, by all means have at it. Myself? I'm lazy. If I'm no longer getting any other indications of moisture in the oil, I'd probably pass on that for now. But at the very least, do keep a close eye on it for signs of moisture in the oil.

What are your driving habits? Lots of short trips (under 20 miles) typically allow moisture buildup. That could be a big part of the problem, especially if compounded by marginal PCV operation.

 

Yesterday I finally got the thermostat replaced. I had to wait to get an upper radiator hose since I decided to replace it too. The old one was pretty well ballooned out.

The old thermostat was stamped 195, and the new one doesn't make my gauge go any higher, but I see 170 with my IR thermometer. I think the difference is because the housing is not painted, just bare metal, looks like cast aluminum, so it's not really a good spot for shooting a temp.

But, while I had the hoses undone I backflushed the heater core, and now I have heat! I don't need heat very often around here, but it will be nice to be able to control the AC temperature later in the year.

I also replaced the fuel filter, the breather filter, air filter, and PCV valve.

So far it looks good after a 20 mile test drive.

 

It is amazing what comes out of the heater cores. Makes a huge difference. Enjoy!