Can i start and run car with radiator cap off ?

[Thraxeh]

I'm calibrating my Temp gauge, and i need to stick a thermometer into the water to get the most accurate reading possible. Is it fine to run the engine while the radiator cap is off ? or will i get coolant spilled everywhere ?

[Mr_Toyota]

Ive started and ran my hilux with no cap for about 2mins ideling..

so probably, but then again i dont know if the hilux was at its running temp.

[Ghetto-AW11]

When the coolant gets hot, it will expand ie... flow out the radiator cap...

[BigWorm]

Yeah that's fine, it'll take quite a while to get it up to operating temp with it just idling though, both my cars usually take around 10 mins at least, if you're prepared to have to give things a bit of a rinse with plain water afterwards it's no big drama.

[clubagreenie]

When the coolant gets hot, it will expand ie... flow out the radiator cap...

o'rly?

i thnk youll find its more to do with the fact that the water spot boils and the generated gas forces water out

in answer to the question - its perfectly fine to start, and run, and rive the car withou a rad cap - however the collant will boil at a lower temp, and will eventually overflow due to this boiling - so carry a bottle of fresh water to top up the system as you go along....

[Ghetto-AW11]

so it doesnt expand at all?

[bionikal]

lol, in my laser i had the cap off while running a few times.

but then again its got a radiator leak somewhere (assuming badly clamped hoses) and it dumps water whenever its under pressure.

engine wise i dont think it has any bad side effects, it didnt die or explode after doing it. i cant think of why it would..

worse case scenario is you get an some air bubles or some dirt flowing through.

- edit YES it does expand, but being water, it also evaporates you will probably get a little overflow.

[Screamn_Sleeka]

Takeing the cap off and running it is fine, thats one of the ways u check for a blown head gasket, bubbles and a lot of pressure.

[bionikal]

Takeing the cap off and running it is fine, thats one of the ways u check for a blown head gasket, bubbles and a lot of pressure.

i thaught it was white under the oil cap

[MRMOPARMAN]

if there is something wrong with your engine and it is overheating, then yes it would be boiling water blowing out of the radiator. otherwise its just natural expansion of the coolant that will flow out of the radiator.

just be careful that the expanding water doesnt hit a moving fan and splash up into your eyes or something.

[Starfire]

The water does expand a little, which is whtat the overflow bottle is for. The cap maintains pressure in the system which raises the boiling temperature.

Be careful if running your car without a radiator cap not to overheat it - it is quite easy to warp an alloy head.

[Screamn_Sleeka]

i thaught it was white under the oil cap

thats one of the ways

There are many ways to diagnose a BHG.

And Starfire is right, the over flow bottle will be the first place "expanded" water will go, not that it will happen under normal circumstances.

[Thraxeh]

what is the highest normal temp the car can run at ?

[ed]

so it doesnt expand at all?

The thermal coefficient of expansion of water is 0.00021 per 1° Celsius at 20° Celsius

you dont have a spare 80cc of volume in your cooling system?

the reason coolant spews is spontaneous boiling within the fluid (in a closed space) which gets forced out the open rad cap. thermal expansion is slow and boring, and nearly irrelevant to the 'safety' of running an engine with the cap off

[lojik]

Would it be difficult to have a container of just boiled water, drop sensor in and calibrate from that, rather than messing around with the radiator cap off?
just a suggestion...

[Ghetto-AW11]

I never said it was dangerous, i said water would come out of the radaitor cap...

[ajh]

Thraxeh,

If you're the person I'm thinking of, you have an SV21 correct?? (I think I've replied to you re: temperature gauge)

I've run ours with the cap off plenty of times.

Mainly to bleed air out. When the thermostat opens, you'll see the coolant start to flow through the radiator instead of sitting there.

The level will rise slightly too and the coolant gets BLOODY HOT!!!

Oh and by the way, anytime now, the thermo fans will start so mind the fingers!!! I had a near miss........Also be careful when a little coolant runs out of the opening, it will run down and get drawn into the fans (if they are running) mind your eyes, as it will spray up and hit you in the face.

Cheers.

[Mos]

The thermal coefficient of expansion of water is 0.00021 per 1° Celsius at 20° Celsius

That coefficient of expansion applies only at 20°C. The coefficient increases at the temperature increases.
Overall expansion from freezing to boiling is about 4.3%, which is somewhat significant.

If a 1GZ-FE takes about 15L of coolant (and it does), the expansion to boiling would be over 0.6L, which is significant and will expand out into the overflow bottle.

That said, as everyone said, it's fine to run an engine without the rad cap, as long as you keep in mind the boiling temp is reduced. In a normally operating engine this should not be a problem for testing purposes.

Mos.

[thechuckster]

to calibrate a temp sensor/guage:
preparation
-get a cooking themometer (digital or old-style)
-fill up kettle and put onto boil
-fill up a metal jug of water and leave in the sun to warm up to ambient temps
-fill up a plastic container and put into the freezer to cool down
-obtain some human help to carry stuff around or read the dash guage
-remove the temp sensor from the head, leave connected and earth the case with another length of wire

measuring
-get helper in the car ready to turn the ignition on and look at the dash guage
-you get to read out the thermometer
-get freshly boiled jug from kitchen and dunk sensor and thermometer in it - take readings after 30 seconds
-get metal jug from sunny spot, dunk sensor and thermometer in it - take readings after 60 seconds
-get plastic jug from freezer, dunk sensor and thermometer in it and take readings after 60-90 seconds

you now have actual temp readings at approx 0, 25 and 100 degrees C relating to what your guage says...

note: don't use the hot water mentioned above to make coffee - tastes horrible. Also, don't dunk the sensor into the the hot water kettle - makes future cups of coffee taste strange.

[MRMOPARMAN]

most cars run in between 95-115 celcius. most jap cars run on the hotter side for efficiency. it really depends on the engine. i know the old clunker VN buick engines operate at 105!