Can anyone tell me why my coolent system still will build pressure even though the car hasn't be running for 2-3 days. Reason for asking is just changed fluids bleeding air out so left cap off an let it run for awhile till fan came on, an still let it run, finally I shut it off, then next day went back out to see if I needed any fluid so I topped it off. Then started it back up let it run notice the fan came on an at that point 15mins has gone by an fan was still running. Which I thought was weird. So shut it off an let it cool went back out took cap off to depressureize system, it was fine. Then next day when back out an took cap off again it still had pressure( trying to figure out why it building pressure an I'm not running my car. Any help on the reason shyly. Thx

 

I think you are overthinking this. Are you having any overheating or other actual problems?

In a normally operating system, any temperature change still causes a pressure change. Even changes between day and night or hot weather vs cool weather.

 

I might be �� lol it's just never seen a fan not shut off even on a hot day for just a few. As far as I know the system is good, just trying to figure out with the car sitting parked an has not be running a few days (in hot weather) can the coolent still build pressure but not enough to flow over in over flow tank?

 

Are you saying it still had pressure after sitting (cooling off) overnight?

 

Yep even if have popped the cap night before. Without it running. Could it be due to the temp out side it has been in the high 90's making the coolent build pressure?

 

No..outside temp shouldn't keep it from cooling off over that many hours. I think the first place to start is to remove the reservoir and the hose that leads from the top of the radiator to the reservoir. Clean both out very well and ensure flow through the reservoir hose ..replace it if need be. . Also, inspect the radiator cap and replace if needed..Honda genuine (OEM) only.. aftermarket caps do not seal /fit as well.

Purchase some HVAC (a/c) coils cleaning foam and while engine is cold spray the cleaning foam on both sides of the radiator. Allow to work per directions on can then spray the radiator off with a garden hose. Use a spray nozzle on the hose that will provide good pressure but not too much that could possibly bend the radiator fins.

Secondly, do a proper coolant bleed per the service manual
1) Put front of car on jack-stands as high as possible (radiator higher than engine)
2) Set climate control to max heat and interior fan off
3) Idle engine with rad cap and reservoir cap off until engine reaches operating temp . In addition to service manual: After reaching operating temp occasionally rev engine to 2500-3000 rpm to help force air out of coolant system. Fill radiator and reservoir as needed to keep proper coolant level during bleed process.
4) After radiator fan engages twice inspect for air bubbles in coolant while engine is idling. There should be very minimal (tiny bubbles) to no bubbles.
5) Turn engine off and fill reservoir to the top
6) After dead cold (overnight) inspect radiator and coolant levels.

Optionally: If the current coolant is green then do a complete drain and fill by:
1) Put front of car on jack-stands
2) Run the engine (from cold start) until the thermostat opens (temp gauge starts to rise),
3) Unscrew the pet-**** (lol..hehe) and drain as much coolant out as possible (roughly half of coolant)
4) Tighten pet-**** and fill radiator and distilled water, repeat a drain and fill with distilled water
5) Drain radiator again and this time fill with Pentosin Pentafrost A3 (blue) 50/50 coolant mix.
6) Do one more drain and fill with Pentafrost A3
7) Do bleed as described in steps above.

Pentafrost A3 ingredients (spec sheet) is the closest (if not the same) as the original coolant used in 6th gen Civics when new.

If the above suggestions don't solve the problem then try a valves adjustment .
Other than that the coolant system may be clogged?