redpathtribe

So Bill, you were just testing your original theory then, right?...

WallyP

If you put the cap on only to the first notch, it will keep the coolant from splashing, slow evaporation, but still not build pressure...

Bill Ball

That would be my preference, but something is not quite right with either the cap or the relatively new reservoir tank as I cannot get the lip on the cap to clear the prongs on the tank lip despite full force compression. It hangs enough that my full hand strength can't budge it and I have to knock it around by hammering against the cap ears to get it off. So, leaving the cap loose will not allow me to easily get the cap off to check and replenish the level. I need to remedy that I guess by bending the cap lip a smidge or slightly filing off the leading edge of the tank lip prongs so I can get it past that notch by hand. [FIXED: Dremeled down a tiny bit of the prongs the cap must clear on the tank lip. Doesn't affect sealing at all. Cleaned up some corrosion there and on the cap too. Simple things that you don't think about.]

Some more experience with no radiator cap... I've been driving around a few more days with temps in the mid to high 90sF. Yesterday it was 100F and I used the AC driving 40 miles. Ran perfectly normal with the water temp gauge not even getting close to the second white line. This involved limited uphill driving, so at least for around town, if the cooling system is otherwise working, no radiator cap, which lowers the cooling system boiling point about 40F, has not presented any practical problems (yet).

FredR

Bill,

Do not forget when you were at altitude the boiling point will be lower and if boiling starts, the temperature you see on the gauge will not increase [if it is at boiling point]. That does not mean the engine temperature is constant. As long as you did not see steam heaving out of the expansion tank you should be OK I suspect.

I recently had a failure of the pressure switch thing on the expansion line returning from the radiator but did not know it. My oil pressure coincidentally seemed to be dropping when the engine was fully warmed up [in our heat]. I reasoned that the motor might not be cooling properly so immediately eased off and poked around under the hood until I fathomed out the problem.

The "problem" dissapeared after removing that thing. Whether the oil was over heating before the fix not really sure.

Regards

Fred

Bill Ball

Hi Fred: Good point about altitude and boiling. So far the water temp gauge has stayed quite low without the cap with no temps even near the second white line even at 7000 feet over Donner Summit. I thought perhaps I might get some boil over if I stopped the motor when somewhat hot, as the stored heat in the block would locally boil the uncirculating coolant, but so far I have not seen the slightest indication of that and have not lost more than a few ounces of coolant over the past week.

I fixed the cap so it no longer is so hard to remove so I can leave it in the loose potion further reducing that chance of lost coolant from sloshing out of the reservoir.

I'll continue to drive the car this way at least here at near sea level until I get a few days where I don't need to car and can pull and take the radiator to the shop.

Anyway, I think I'm satisfied the car is over all safe to drive with careful monitoring without a cap should anyone develop a pressure related coolant leak like I did. I expected it would be OK, but had no experience with doing so until last week.

dr bob

I'm usually the last person to identify thermostat as problem, but... For a while I had some similar temp gauge symptoms, gauge creeping up on hottest days to a little over the second white line. Mine went higher as speed went higher on the freeway, leading me to look at both radiator and water pump as possible causes. Both are fine though. After all that for a year or more, I replaced the thermostat for no better reason than I had a new one and the coolant was out. Suddenly all was correct again. Speculating that thermostat was not opening all the way. Warmup had been normal I thought, but now is much quicker. Running gauge reading is higher but much less load and ambient temp sensitive. Turns out thermostats are cheap, easy to change, etc. whoooda thunk?

Bill Ball

Dr. Bob, thanks for that reminder. The T-stat is over 10 years old. I changed it back then due to the same issue but saw no significant improvement. Maybe I should try again. With the radiator out it's the perfect time to do that. Actually I've been meaning to refurbish the waterbridge. It's never been out and the seals are original. No leaks but that big o-ring around the center tube going into the block must be on its last legs. and that would give me a chance to service the oil filler neck base seal which again is leaking a bit.

Kevin in Atlanta

Thermostat maybe, but a leaking thermostat seal can leak giving rise to overheating, too.

FredR

Bob,

Been there, done that & got the teeshirt! A chap in the UK had the same problem- told him to check the thermostat- eventually he got round to it and la voila! It can take a surprisingly long time to warm up once that thing is partially open from cold-even in our part of the world which I found surprising. Thermostat in good working order- the S4 motor warms up within about a km.

Another point of interest- I used to run with a 75C thermostat [an alleged recommendaiton]- I changed back to the 83 C stock model earlier this year and the car seems to run noticeably better- placebo effect-who knows?

Regards

Fred

AngelP

For at least two years now I've been running with a 7lb cap I got from Carl F. After my last waterpump change I had a small leak and I didn't feel like doing it over again so I gave the lower pressure cap a go. Still on the original radiator now at 134K miles.

AngelP

To clarify the leak was not from the rad. The bonus is that I'm still on the original radiator also. Mine's a daily driver 70mile round trip commute and I'd say I have to top off no more than once every 1.5months from increased evap from the lower pressure cap.

Tony

I suppose if you drive around in a cool enough climate having the cap "off" isnt a big deal. Less internal pressure in thecooling system...head gaskets...water pump...heater hose...etc.

just weld on some cooling fins to the block and ditch the radiator all together...whats old is new again!

dr bob

So the cooling system depends on having the ability to hold some pressure, if only to prevent localized hot-spot boiling in the block. That can cause erosion of the aluminum, plus some other fringe benefits. Bottom line is I wouldn't recommend running without the cap on tight as a long-term solution.

77tony

www.evanscooling.com

danglerb

The issue as I understand it is all about a hot spot, usually I think in the heads. Once the coolant boils you have a steam layer and cooling drops significantly in the hot spot. I don't think this is especially weather related as opposed to engine generated heat. Keep the load on the engine below some point and nothing happens, exceed it and crack or warp the heads or blow a head gasket.

What might be interesting is to put a pressure gauge on the cooling system and watch the pressure rise, but to track boiling point it might be necessary to get the motor fully up to temp and then relieve the pressure due to expansion of the coolant.