Coolant reducing in radiator

[devilcena2]

Salaam everyone
There is an issue in my old cultus. The coolant in radiator decreases but increases in reservoir bottle. First thought is definitely radiator cap, i changed it with 2 different kabli caps but same issue
Then someone suggested to bleed air out of cooling system, i did but same issue.
Now i changed radiator cap to new one which they sell as Japanese. Same issue even worse because now reservoir bottle is overflowing.
There are no visible leaks.

Can anyone guide me, thanks in advance

@Xulfiqar

[pwuser162557941033]

The coolant in radiator decreases but increases in reservoir bottle

This is normal. It is called an overflow reservoir or an expansion tank for that reason. When the system is hot, coolant will expand and overflow into the reservoir, moderated by the radiator cap, which tries to maintain a constant pressure in the system.

As the system cools down after the engine is off, the reduction in pressure will pull coolant out of the overflow reservoir into the radiator.

Do not constantly open the radiator cap. Top the radiator off once, then install the cap. Only check coolant level at the reservoir. The level will be higher when hot, and lower when cold.

You only need to investigate if your reservoir is completely empty regularly.

[Xulfiqar]

Can you verity that the tube from the radiator cap neck to the expansion bottle cap is not loose, and actually leads all the way down to the bottom of the expansion bottle.

There are two outlets on the expansion bottle cap, one is the air vent and the other is for the pipe/tube, if the pipe is loose or leaky, use a thin cable tie to secure it as it maybe sucking air.

Air is easier to suck than liquid, if you see your radiator empty/low but coolant keeps on filling in the tank then you may have a leak too somewhere which is causing air to get in and replacing the lost coolant

The way its doing it is

When hot the coolant is expanding and lifting the radiator cap spring to eject it and get deposited in the overflow tan

When its cooling down a negative pressure/vacuum is created in the system which pulls the radiator cap hat and pulls the coolant in.

Now if you have a leak that negative pressure pulls in air from the leak site, which can be anywhere and all the way down to the expansion tank tube. Thats why I asked to check that too.

If topical naked eye issues dont exist then do a pressure test on it.

Expected points:
I hope you are not opening a warm/hot radiator cap and actually allowing the system to actually do a heat/cooldown cycle itself.

[devilcena2]

actually i am talking about checking the level the next day. and i even checked after driving for 2-3 days and the level in radiator goes lower and lower

[devilcena2]

thanks for your reply sir
yes i am checking levels the next day, the tube from radiator to expansion bottle didnt seemed loose. and the tube inside expansion bottle looked maybe around an inch above the bottom surface of expansion bottle(not touching the absolute bottom surface). as its relatively new issue i dont think the length of tube may be issue. also didnt find any visible leaks also.
should i change the both tubes first or do you think any leak might be the culprit ? @Xulfiqar

[brownrose78]

@Xulfiqar
Sir in my Cultus, although coolant quantity is not decreasing but Engine is always running too hot. In few seconds, engine reaches to more than half and even 3-5 minutes after starting car, if you check radiator body or reservoir water by hand, it is burning too hot and also same is reflecting in temperature needle.
Although engine is recently newly built just 6 months ago, water body is new, Radiator is new but even now engine is running too hot.
What checks i am missing, please guide me as i am exhausted by this

[Xulfiqar]

You have a leak somewhere, find it.

[miqdadkanani]

This is surprising. If you recall, I had the exact same issue in my khyber before the recent rebuilt.

The old mechanic had worked upon it and right after the rebuilt, I was noticing temperature needle remaining above the normal. Despite my complaints, the mechanic ignored and only paid heed when gasket burnt. Still don't know the issue though.

Is your radiator fine? Choked tou nhi?

[Xulfiqar]

There used to be a “wrong headgasket” also being sold in the market for the suzi g10 engine.

It caused overheating of cyl1, it had some wrong holes punched in it, I believe it was for some goofy efi version, in carby model coolant flows from intake manifold to the cyl head, in efi model there is an external coolant manifold (flange/flunch/daat/paep nippal <——- choose your own.

[devilcena2]

mine is carby one, could gaskit be also culprit?
@Xulfiqar

[sardarg]

[ghafoormotors]

Wa Alaikum Assalam,

This issue is quite common in older Cultus models, and you're right to suspect the radiator cap first — but since you’ve already tried multiple caps (including a new Japanese one), and the reservoir bottle is now overflowing, here are a few deeper things to check:

Possible Causes:
Blown Head Gasket (Early Stage)
Sometimes combustion gases leak into the cooling system, building pressure and pushing coolant into the reservoir.
Look for: bubbles in radiator when engine is cold, white smoke, or milky engine oil.

Blocked Radiator Hose
A soft or partially blocked lower radiator hose can cause poor return flow.

Faulty Thermostat
If it’s stuck closed or partially open, it can trap pressure and cause overflow.

Reservoir Pipe Issue
The thin pipe from radiator neck to reservoir might be clogged or cracked.

Weak Water Pump
If coolant circulation is weak, pressure imbalance can build up.

What to Do:
With engine completely cold, open radiator cap and start engine.
If bubbles appear constantly, it may indicate head gasket trouble.

Check the pipe from radiator to bottle — clean it or replace if needed.

Get a coolant pressure test or a head gasket check if possible.

Hope this helps! Let us know what you find after checking these things.

[pakearthinfo]

It sounds like a potential head gasket problem; engine gases may be pressurizing the coolant. To be sure, get a compression or leak-down test.

[devilcena2]

Checked the gasket on cold engine + got confirmed from mechanic also. Gasket doesn't seems to be culprit or maybe its early stage. For now taken out thermostat valve, when it was taken out it was in closed position, maybe that might be culprit. But for now i am gonna test without thermostat which i think maybe the culprit @pakearthinfo

[sardarg]

before removing thermostat did you tried to purge the air from system??

[devilcena2]

Yes i did @sardarg