Thanks for the tips.

From previous questions:

Spark plugs are relatively new dry as a bone but does seem that one or two were “cleaner” than the others which I took to mean had been steamed.

14 lb pressure holds overnight. That’s in fact one of my symptoms. After a short drive the coolant hoses are very tight and stay tight sometimes even several days. I even went to a 1.4 bar tank cap to keep from destroying another tank. At this point I can only drive it on short grocery trips before it get over pressure.

Would love for it not to be a headgasket but I believe it is.

Btw- new waterpump, thermostat, radiator and coolant tank.

Oh, and the CCV is clogged with “mayo”. Some under the valve cover also but not as much as I expected to see.

Bought a cylinder leakdown kit on sale at a Harbor Freight on a 20% off. (Reviews are terrible but I’ll give it a try)

Questions: currently have valve cover off can I run a Loews down without it being on? Does the engine need to be at operating temp to do the test? Obviously not possible with the valve cover off.

Yes, you can still do a leakdown test with the valve cover / cylinder head cover off. Measurements will not be as good with the engine cold, but may still tell you what you need to know.

On the lack of a positive test result from the combustion gases ... I agree that based on your symptoms, you probably have some sort of HG / head issue that requires disassembly. But still, I would not tear anything apart until you can actually measure combustion gases in the coolant. It's a pretty huge job and you'd probably hate to find out you did it unnecessarily, or that you do it all and still have the problem.

Thanks. If it’s not pouring rain tomorrow will do the leakdown test. I assume that if I get any kind of leakage it will confirm a leaky gasket.

I don’t know of any other way that builds up pressure in the cooling system that high but I’m all ears if somebody has some way that I overlooked.

The only time I had a similar issue was a clogged radiator on an e30. Got to the point it blew a radiator hose. Coolant couldn't circulate. Of course my daughter had the car at school 200 miles away...
Sounds like your car thinks it's a retiree, passes bubbles when it heats up too.

Hah. Maybe.

I've heard that the transmission cooler thermostat failing can kill the coolant reservoir. Do you have coolant mixed properly or with problem solving is it weighted to more distilled water ?

Actually, you almost certainly will get some leakage, it is just a matter of how much that matters.

All bets may be off with Harbor Freight, but I believe a standard leakdown setup will have a 0.04" diameter orifice between the two gauges. You adjust the supply flow/pressure so you get 100 psi on the upstream gauge. Then you've got that orifice, which will drop pressure according to the flow rate. Then you measure pressure on the second gauge. Air will be leaking from various options in the cylinder - head cracks, valves burned, valves not sealing, ring problems, cylinder scoring, etc.

Typical is to look at the pressure on the second gauge and see what percentage below the first gauge it is and then say something like it's got a 7% leakdown measurement. But realize, it's really a flow measurement.

So one thing that will be a clue is a big variation from one cylinder to the next. In theory, the flow rate will be proportional to pressure drop squared, so consider that when thinking about how the bad cylinders compare to the good ones; and if you've got a failure mode like it sounds like you do, I expect you may still have some cylinders that are tight, but one or more will be way off.

If the flow is significant enough, the other thing is that you can listen for where the air is leaking from. E.g., remove the expansion tank cap and see if air is coming out of that - if you do, that's pretty definitive, but still it's surprising you don't get a positive read on combustion gases there.

Agreed. As far as I know, nothing would be able to create pressure which would then remain after the system cooled down, other than a leak of combustion gases into the coolant. But I'm always learning new stuff, so ...

Thanks. As for the transmission thermostat failure having replaced the coolant tank now for a fourth time I have used 4 different thermostats. Must admit I was tempted to try it with no thermostat installed whatsoever. Wish that were the problem, don’t think so.

I see tiny bubbles in the reservoir with the cap off at idle. I have installed a pressure gauge in place of the cap and idling, not much pressure. However in gear and slight revving with my foot on the brake the pressure rises almost dramatically to the point it would fail if I kept it up.

I have seen the reviews of the HarborFreight leak tester and don’t expect scientific results. What I expect is too be able to isolate a leak (if it exists) by a sudden whoosh of air. From there i hope to be able to isolate the problem area. Degree of a leak is inconsequential. If it leaks at all, needs a head gasket.

I noted earlier that my CCV connection at the valve cover was gummed “mayo” and the whole system (oil separator, etc) will need replacing. Any chance a clogged CCV would increase pressure?