Hello Members,
I am just wondering what would cause my engine to overheat only while driving.

I installed a new thermostat, and overheating started. Removed thermostat and no overheating while driving, only that it takes longer to reach operating temp.

Tested thermostat by boiling in water, it opens at 82°C and closes when put in cool water. Meaning thermostat is ok.
System was bled of air, using 50/50 coolant mix, can see coolant flowing when engine gets hot.
My rad cap is 0.9, could this be a problem? Last drive, my coolant overflow started boiling.

When idling it never overheats.

​​​​​​All radiator fans work well.
the car is Honda crv 1997, 198,000km,
Any ideas?

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Radiator caps are usually somewhere around 16 psi of pressure so 0.9 sounds a tad low. That’s below 14.7 psi. Is it an OEM cap? I always go OEM for radiator caps. The boiling in the reservoir makes me think it’s the cap. I had the same issue and replacing the cap worked.

The other thing you could do is pressure test the system to its rated pressure for 30 min and see if it it holds. If not, you have a leak somewhere. Your auto parts store should have the required tools available for rent.

 

0.9 Bar is a standard cap. That's 13psi as in OEM cap. Regardless, the eng opp temp is usually 80 to 85c so that's below boiling point of 50/50 mix so you should hot overheat with the cap off anyway.
Since you said it happened after the t stat install, make sure your installing the stat in the correct orientation ( not upside down)

The air needs to be all purged out carefully to ensure proper operation of the system. Run the engine with the cap off and the HVAC on HOT and make sure it gets all the air out. As it reaches the top on the neck of the rad, secure the cap and watch the temp gauge. The fans should cycle twice and the eng should be shut down and the rad topped up again.

 

one other thing... if the coolant is boiling in the overflow, the coolant is excessively overheated.

Coolant at 50/50 mix at atmosphere (no pressure) will boil at approximately 108C... at 12 psi the vaporization point is 125C.... your t stat opens fully by 85C... so if the coolant is boiling, the engine is overheating and the excessive vapor pressure is exceeding the cap limit and the coolant is being dumped into the reservoir where its atmospheric pressure only... so the coolant will vaporize as its over 108C... it must be boiling in the closed cooling system ...so this means the coolant temp is over 258C.

 

Problem solved, thanks for the advise;
Installed new radiator with new fans old one had a leak but not noticable, got another t'stat. Bled the system, test drove in all possible conditions and no more over heating.