I pulled the upper & lower intakes off my Bronco last year to replace all the upper-end gaskets. While I was in there, I got rid of the throttle-body coolant hoses. I live in California, and it never gets cold here. I figured it was just one more spot for coolant to leak from. I capped off the lines with vacuum caps.

Don't do that. Vacuum caps are not designed for coolant and they will degrade and eventually leak (badly). I discovered this while I was parked alongside US 50 yesterday trying to figure out why there was steam coming from under my hood, why my engine was running so hot, and where all my coolant went.

Fortunately I had my tool box with me, and I sacrificed my reservoir hose to rig up something to plug the leak (a short 3/8" ratchet extension fits very snugly into the end of the reservoir hose, BTW.) For some reason, being able cobble this together on the side of the road impressed my wife though - so some good came of it.

For some reason, I just never stumbled across anything that said "Never use vacuum caps on any part of your coolant system!"

Now you know. And knowing is half the battle.

 

so after taking apart most every vehicle you own, or anyone else brings over, and it took this to impress your wife? hahahahaha

glad you had tools with you.

guess all those fires keep california pretty warm.

 

I'm lucky I ever got a second date with her, never mind talked her into marrying me!

Somehow I knew you'd be the first one to respond to this.

 

Andym, the normal pressure in a coolant system is about 0 on a gauge with the engine running and up to temp. when the engine is shut off the pressure may climb to a pound or 2. So keep an eye on your temperature, water pump, and thermostat. or were those caps close to the exhaust manifold or close to some area over about 250 F? were they melted or just hard and brittle? rubber or pvc or other?

 

I'll throw 5 in the pot.
'the normal pressure in a coolant system is about 0 on a gauge with the engine running and up to temp'
??? Last I tried to take the cap off when it was hot, it wanted to take my head off.

Then...
'when the engine is shut off the pressure may climb to a pound or 2'

Someone school me here. I can understand that when the engine is shut off, the temp rises, but...?
My cap and all the others were rated at 12# or more. 14# mabe.

Also, vacuum caps for coolant lines? Man, I need a drink.

 

only a fraction of a psi will cause the water to flash to steam when the cap is removed. the boiling point of coolant depends on the pressure. A vacuum aint nothing but PSI in reverse. you are basically swapping the pressure from the inside to the outside with a vacuum. normal 0 gauge pressure or atmosphere is about 14.69 psia. a 10 inch vacuum is about -5 psi. i made the mistake of taking the lid off a pressure cooker ONE time. I had let it cool for a while, noted that the rubber blow out plug dropped to cold seat, then carefully removed the weight and noticed a little flow of steam about 6 inches long, waited til it was about 3 or 4 inches long and unfastened the lid. took me an hour to clean those pinto beans off the wall and stove. I was lucky as i removed the lid, it was between me and those scalding hot beans. it made a spray pattern about 4 feet wide and 3 feet high and one or two hot beans hit me. from then on i let it sit and cool til no steam with the weight off and the pop out plug dropped or ran cold water on it if i was in a hurry. a cubic inch of water will make about 1700 cubic inches of steam. when you turn off your engine no cooling flow so coolant next to the engine block may start to boil as heat moves from hot engine block to cooler areas. like the water in a boiling pot. the bubbles on the bottom are steam about 212 but the water on top is about 190. I always leave the engine running to take the cap off. or let it sit or spray water on the radiator if it was shut off. its that 1700 to one ratio that gets you. the water in the radiator may be 180 but that closest to the block may be 212 or over. if it can hold a vacuum it is basically holding 14 psig but dont try that for every situation. the bracing may not function that way. my radiator cap is 16 psi some are more some are less. the correct answer is always depends and that involves how much you know about what they are made of and the exact environment you are using them. there in lies the catch. good grade rubber or nylon can handle heat more than some other stuff. please note that it takes about 30 inches of vacuum to make -15 psi. water boils at about 200 F in denver. at about 100 deg F at about 28 inch vacuum. All of this and Andym will win this bet many more times than lose it. it will always be safe to use the proper hose. I started to say or metal plug. but there are exceptions as some metal alloys can melt in you hand. Ive seen water heaters run for years on 14 gauge wire with no problem. but you wont find an inspector to pass it. And no i have 10 gauge to my water heater and i like Gates or Goodyear rubber hose in my truck thank you. changem every 5 or 6 years wether they need it or not. I have AAA but i still dont want to be like Andym on the side of the road.

 

you both sound like you lucked out choosing each other.

it is because i care so much

 

The rated pressure of the cap is gauge pressure, not absolute. Running the system above ambient pressure increases the boiling point throughout the system to control localized boiling, so at zero psi gauge, there might as well not even be a cap. Unless the cap is messed up, there shouldn't be any vacuum inside the cooling system because the cap has a second valve to allow purged coolant to re-enter the cooling system from the overflow tank as the system cools and relieves pressure. Any increase in pressure after the engine is shut off is a result of heat soak increasing the coolant temperature.

The vacuum caps on my unused vacuum ports are degrading without the help of engine coolant . Glad to hear you got it fixed and back on the road.

 

Eric, in my truck it takes a vacuum or negative pressure reference to pull the coolant from the over flow tank back into the radiator (the vacuum relief you mention). As coolant heats it expands (not boiling) pushing past the lower seal (16 psig) on the radiator cap and over flows into the overflow tank. Since the coolant is about zero compressible and below boiling it wont flash to steam and scald you and the pressure in the system will drop to the pound or so the pump needs to push the coolant to the tank and engine. (remember when you fill your coolant system you leave some out, leave the cap off, then let engine get up to temp and wait until you see flow in the radiator when the tstat opens. you then fill to over flow level and put the cap on,you dont get sprayed like putting a closed spray nozzle on a garden hose. if the system pressure was 16 psig you would have coolant squirting. 16 psig can push a column of water about 34 feet straight up) The boiling point of 50/50 coolant is about 225F. As the engine cools down the coolant shrinks making it occupy less volume thus causing the pressure in engine to be less than the outside barometric pressure. In my truck the coolant level in the overflow tank is a few inches below the over flow hose at the radiator cap. My over flow tank is open to the atmosphere. The thermostat is 185 so unless you are on Pike's Peak the water wont boil. The only way coolant can get back in the radiator is for atmospheric or barometric pressure to be greater than that in the radiator and push the coolant from the tank up the few inches to the radiator cap. every time i take my radiator cap off, i see it ready to go out the over flow hole. if it doesnt, i look for a leak or a empty over flow tank. its usually the tank. Even on my 97 sable with the over flow tank cap 16 psi and engine hot a 60 psi guage will show about 0 or a pound. it hits a pound or 2 a bit after shutting off the engine. I even pressure tested the system after it cooled enough to get the cap off ( I aint forgot that pressure cooker ordeal) and it held 12 psi on the same gauge that showed about 0 running.

 

The system pressure has to build to more than the cap's rated pressure in order to push coolant past the cap into the overflow tank, right? Squeeze the top radiator hose with the engine off and cool, then squeeze it again with the engine running at operating temperature. It's harder to squeeze at operating temperature because the system is pressurized. When bleeding the cooling system, no pressure builds when the cap is off since at that time it is not a closed system - the coolant can expand freely. Where are you measuring the system pressure?

It's not so much about the boiling point, but the thermal expansion of the coolant. Even at 185 degrees, the difference in volume from cold coolant due to thermal expansion of the coolant will cause the pressure to rise in the closed (constant volume) system, allowing the cap to regulate the system pressure to its rated value. The level in the over flow tank doesn't really matter as long as the end of the tube is below the fluid surface in the tank and the tube doesn't leak.

 

Eric, I’m no mechanic or licensed anything other than EPA universal and automotive refrigerant cards and a BSEE from turkey tech. I can let some smoke out of wires and make 6 foot long 500,000 volt sparks and that is about it. I no longer need or remember much about the EPA stuff in my present job. The EPA hasn’t put SF6 gas on the bad stuff list yet like we expected 10 years ago when I got the licenses. I just got the automotive on my own as I had R-12 at the house. I am not a licensed engineer. I just get blamed if the smoke comes out. Thermo was one of my worst subjects at turkey tech. That was 40 years ago. Vacuum tubes were still king. I was taught by my mama and in thermo class you don’t ever lift the lid on a pressure cooker. Mama just told me it will explode and scald you. Had been in hot water before and wanted nothing to do with it. Took cold showers for many years cause I wanted nothing to do with hot water. Turkey tech told me why pressure cookers flash to steam when you lift the lid. It really hit home when I bought my first pressure cooker about 30 years ago. I still have it and it still works. Remember the beans from the above post. I love beans. I knew the danger and thought I had cooled the pot enough to lift the lid. It wasn’t nothing but a tiny wimpy looking flow of steam from a 6 quart pot through a tiny hole, blow out plug dropped, weight off, the lid moved easy like no pressure and a dry gasket, and it still blew most all them beans out. Remember the 1700 to one ratio above. Squeezing the hoses is what I have been doing for years. Even on a running engine before I open the cap I squeeze the hoses 3 or 4 times, look for steam in the overflow tank or just a wisp of steam anywhere. Look at that 16 psi cap remember the beans. Then put a folded up towel over the cap, position myself to turn and run, and slowly turn the cap listening for any kind of gurgle or sound that might say something ain’t right , SCRAM. Remember that cap says 16 PSI. On a running hot engine with no heating problems the hoses don’t seem to squeeze any harder for me than when cold. But then again feel ain’t always accurate. If they did feel solid I wouldn’t open the cap. But mama always told me I was twice as strong as any ox and didn’t smell any better. Like you I still see that 16 PSI on the cap and remember them beans. I thought anything with an operating pressure over 15 PSIG (30 PSIA) had to have a pressure vessel tag or at least the insurance inspector tells me that. I have seen steam and coolant blow out when the cap was lifted on an engine that has been turned off a little while but never on a running one unless there was a problem or it blew something. I just assumed that since most tstats I have seen are about 180 F, the pressure cap was there just in case something went wrong and to allow for the expansion. Yet thermo also taught me that the hotter you can run an engine the more efficient it will be or at least for steam engines. Not sure if that applies to automotive engines, but they are horribly inefficient energy conversion contraptions. Keeping the system under pressure and raising the coolant from 180 F to about 260 F or so at 16 psi should help efficiency but a hotter engine has more pollution oxides. Up until about a year ago I had never put a pressure gauge on a cooling system. I had just put the third coolant tank in sweeties 02 Sable for a leak in the same spot in all 3 tanks. Realized the 97 sable still had the original and dang near identical tank and seemed to be needing to have make up coolant added a little more often after I had just put a tstat in it cause the heater hadn’t been much more than luke warm for a couple of years. It doesn’t get much below chilly a couple hundred miles south of the Mason and Dixon and that tstat under the coil pack and wire jungle looked like Ford had a momentary lost presence of mind idea not to mention having to pull the front end off and jack the car up to get the radiator out. Cheapest radiator test set I could find was $85, so I modified the old cap from the 02 with a schrader valve and put a tee and 60 psi gauge in the small top hose to the 97 tank. It was a 16 psi cap, so I pressured it to 20 psi (25% over rating) and it held. I decided to take it for a spin to see if it was a pin hole tank hot leak like the 02. Removed the test cap and put the regular one back on. Left the gage in place and fired it up thinking it would pressurize to about 8 to 12 psi after I drove it a few miles. 25% to 50% under cap rating and this is a closed system with air in expansion tank so I am expecting to see pressure but not lift the 16 psi relief. I pulled into a parking lot after about 15 minutes driving, left the engine running, and popped the hood. Gauge read dang near zero. So I thumped it a few times and still zero. Scratched head and rear and thought and thumped some more and turned the engine off to see if it would go up. Gauge climbed to about a pound or 2. And basically that put me where I am now with my show and tell. I still remember the beans and I am still scratching my head and rear. Cause like you I think there should be pressure at least in the 02 and 97 sables. When I got to work this morning, I popped the hood on the 87 f150 with engine running, squeezed the hoses, looked for steam and realized I didn’t have my towel. Turned off the engine, got key to get towel from tool box, restarted engine, waited a few minutes. Then I squeezed the hose again, pulled the over flow hose off, squeezed the hose again to make sure I was collapsing the hose far enough to spit a little coolant out the over flow connection because it should have little or no air and only a small amount of expansion room in the hoses when I squeeze. If the rubber would squeeze in but had no expansion I would expect it to have felt much stiffer than it did. I had to use both hands and squeeze the hose at two places to get the volume reduced enough to spit a little past the cap and out the hose connection. I put the hose back on, got positioned to run if needed, then put the towel over the cap and turned it slowly. Cap turned like cold check ,heard no gurgle, eased cap off and the coolant level was just almost at the over flow hole as I expected for a full system. No bubbles just a little roll or slosh like flow swirl waves sloshing coolant down the over flow. Nothing violent like a 260 F boiling point dropping to a 225 F boiling point with cap off. There is an engine coolant sensor so maybe one day I will get curious and try to figure out how to find the specs and ohms versus temp or measure voltage and current at it to get temp of hot engine. Or modify a 3/8 IPS plug with my trusty bimetal dial thermo stuck in it at the temp gage sender. I checked again this evening with the truck cold and it seemed easier than this morning the old one hand thumb and finger pushed coolant out the over flow. For about 25 minutes I thought you were right I have been discombobulated and lucky as all get out. Pulled in driveway turned off engine, and tried again. Pretty much same ease as cold at work. Cranked it back up ran it some more, still easy. And I noticed my f150 cap was 13 psi not 16psi. the sables are 16. I still think you are right. I just keep getting it wrong somewhere. Anyway I’m off on vacation next 2 weeks. Maybe I can cobble up something and do some more checking. Keep it between the ditches til I get back

 

This made me chuckle. Have a good vacation!

 

Eric, I hope those who read through this understand my main point that releasing the pressure on a coolant system or boiler can have serious consequences. The burns are no fun. One could be blinded or lose their pot of beans. So far I haven't been hurt but you see the song and dance I go to just to be sure before I lossen that cap. Remember the little wisp of steam? On my last trip to florida same time last year, i noticed the small coolant leak in th 02 sable expansion tank or pressurizer in steam plant terms, and it had a wisp of steam blowing out it about 6 inches long. Remember the beans? No way I was lossening the cap on that. I knew it was pressurized and that is why I was surprised about the 97 sable showing 0 after passing a pressure test.

 

Dude, your beans in the pressure cooker story is the funniest thing I've read all week. Your getting some reps for that one.

Tim

 

not too late for you to go to paragraph school.

your post is impossible to read.