Well after extensive tests with non pressure relief and pressure relief lever style radiator caps, I have decided it is not the radiator or the caps or the overflow bottle or the hose.
I have noted that if I relieve the cap pressure slowly using the lever release, shortly after turning off the engine, that it releases lots of gas (and some coolant) from the radiator. Then if I close the lever once the pressure is gone and wait over night about 75 to 85% of the coolant returns to the radiator. Other wise only 10 to 30% returns, if any overnight.
I am begining to think that the water pump is drawing air in at freeway top end operation, (lets call it WOT with the A/C running and the engine good and hot). I think Al was right, the water pump seal is sucking air, but it acts like it is sucking the air at top end operation only max highway speed, 5th gear & WOT, with A/C on high, based on my test results. This seems even more bizzare than a pump seal leaking during the cool down stage, but perhaps one of you has seen this before? This has been going on for years now, and I have changed all the hoses, etc, everything but the water pump, so I am leaning towards Al's conclusion, except that it seems to be doing it during cool down and while the engine is running and at higher RPMs.
I ran it at Idle in the driveway with the A/C on max and the doors wide open for 3 hours yesterday, and a new 16 lb lever cap (Been using 13 lb. caps) held the pressure and nothing flowed into the overflow bottle the whole time. Engine never got over 170 F.
I was working on the A/C and discovered the prior owner never hooked up the A/C blend door vacuum valve controls to the SD22 vacuum pump! I found and fixed that and now on max A/C it will freeze your Bxxxs off (with R-134a!), and the engine-radiator-condensor with the added pusher fan now never gets over 190 F, at 95 F ambient!
One other item I am noticing. Seems that I have little power or reserve power between 50% and 100% WOT, once I am up to about 65 to 70 mphs, with the A/C running, 3 occupants on board a 4500 lb jeep + say 600 lb of people and tools). I have power-response in fourth gear all the way to WOT with all that load but not in fifth gear past 50% WOT. I also notice with all that load, that I can run a few MPHs faster in fourth gear than in fifth gear with all that load, but the engine and transmission RPMs are a bit higher than I like. If I switch to 5th gear and floor it, I really don't get a signifcant response between 50% and 100% WOT, but the engine does run hotter if I maintain WOT for say 5 minutes. I am thinking it is running leaner and making it run hotter and thinking I may need to readjust the smoke screw setting slightly. Just thinking outload. MY SD22 currently does not smoke even at WOT under heavy load.
With A/C off, and only me on board fifth gear has plenty of response all the way to 100% WOT and I think I am gettijng up to about 75 mphs with the Jeep rear end I have.
I am thinking the fifth gear is just too much with all that load.
I bought all the goodies to replace the water pump and intake and exhaust manifold gaskets, but have not had an opportunity to do it yet. Got to get the A/C up and running on the other DD first and the hot rainy...flooding season has already arrived, which is not helping my schedule any.
One last note, I discovered that Stant has two style caps that look exactly alike, but the suttle difference is the lack of a return spring on the vent return valve plate. Part number 11330 has the spring and part number 11331 dies not have the spring. Other than that they both are the same size (exactly) 16 lb caps. It finally dawned on me why they have the one with no spring return. It is for closed cooling systems that have a pressure inspection cap on the radiator and that also have a closed system pressure bottle! Got me to thinking if I just grabbed a good pressure bottle from the JY, I could use one of those caps (closed system) and forget my weird water pump.............problem. I must admit it is temping to just try it for a week to see what it does.
1985 Jeep Cherokee Pioneer, 2WD, retrofitted with SD-22 & 5 spd manual trans, a 4X4 Gas Wagoneer ltd. (XJ) Jeep, 4.0 L w/ AW4 auto, and now 2 spare 2wd Jeeps, 87 & 89.