lexus-is2

Last year I had an overheating issue on my IS250 as I was driving up steep hilly mountain roads. After letting car cool down decided to go down hill with very minimal acceleration but still started to overheat. So took it into a nearby Toyota garage who diagnosed it as a broken valve on the pressure cap. Coolant was boiling in expansion bottle. Had 50% coolant refill and a new cap installed and has been fine since.

From what I understand, the purpose of the pressure cap is to allow the system to pressurize to a set psi and hence increasing the boiling point temperature. But what I didn't understand is if the role of the pressure cap is to increase boiling point temp, then why was car overheating?

I performed a coolant drain & refill today. I bought a cheap radiator cap for few bucks and broke off the valves to convert it to a zero pressure cap putting it directly in contact with the expansion bottle to easily bleed out air. Got this idea from a member on another forum in order to bleed air without making a mess. With the car front raised the air was expelled through the expansion bottle with the help of the zero pressure cap.

I let the car get to operating temperature as per Lexus tech docs and left A/C off and heating on full, periodically pressing on the lower and upper radiator hoses. But then when temp got to operating temp, i thought i'd try and see if temp goes above the half way line a little as it did back when my car overheated due to the faulty cap. But i waited about 10mins but the temp didn't rise above the operating temp even though I was using a zero pressure cap. So i'm a little confused as to why the pressure cap that was faulty caused overheat and with a zero pressure cap temp didn't rise. Only difference is this time car was idling this time at 2k - 2.5k rpm

chenderson

Yep. Amazing how sophisticated and important radiator caps can be!
Remember "temperature and pressure" from physics class in school?

Here's a Lexus owner in the UK who diagnosed and explains it well:
http://www.lexusownersclub.co.uk/for...ansion-bottle/

And check out this cool nifty page & video that explains it so easy:
http://www.truckradiators.com/defective-radiator-cap/

Also, you may want to replace the cap on the expansion tank (AKA "reservoir" or "spill over") too if it's not well either.

Cheers!