I have a 98 Ranger 4x4, 4litre, auto trans, with 134564 miles (it's a US truck), and a weird engine coolant loss. No engine oil or ATF in radiator. Engine oil is clean as is ATF (not milky at all) and levels are not high. No visible leak anywhere. Level in overflow tank doesn't empty. Engine runs perfectly, not even a slight miss. Engine does not overheat or run cool, and in cab heater produces a ton of heat. There is no steam coming off the engine when hot so there are no apparent hairline cracks or leaks externally. No indication of burning on spark plugs, no white exhaust, yet after driving about 50 kms, I go out the next morning when engine is cooled, the coolant level in the rad is down about .5 litre. Again, the overflow tank is still at the full line. Line from rad filler neck to the overflow is clear and unobstructed.

I don't own a cooling system pressure tester so that or rad cap hasn't been checked. Rad cap appears to be in great shape. Cooling system does pressurize when hot.

I'm still thinking head gasket but before I do the tear down, I'm wondering if anyone else has experienced this, and if so... well, what was done about it? Head gasket?

Very much appreciated

 

Welcome to the forum

Check the overflow hose from radiator to tank, make sure it holds pressure, i.e. pull it off the rad and blow into it, should bubble up inside the tank

What can happen if it has a leak is that as the coolant expands in the cooling system some coolant will ALWAYS flow over to the tank, if there is a leak the hot coolant comes out and evaporates quickly, so no sign of a leak
As system cools down it sucks air back in from the crank in the overflow hose

 

Hey Ron, thanks very much for your help. Yes I actually pulled the overflow hose off to check it. It's good. I then hooked it back up and blew into the hose at the rad end. This is how I know it is unobstructed. I noticed there were no clamps at either end and thought that could be the prob as it potentially would lose that vacuum during engine cool down. I've since put plastic tie straps on both ends to ensure a tight seal. I was sure that was the problem. Unfortunately, it wasn't. I'm still losing the same amount of coolant daily.

the only thing I can come up with is a slow head gasket leak.

 

If you were losing coolant the overflow tank should get lower, so I don't think you are losing it in the cooling system, unless there is an obvious leak

Its actually a recovery tank system
As coolant expands when heated it builds up the 14psi pressure(rad cap rating) in the system, at 15psi rad cap is pushed up and hot coolant flows out, OR any air that may be at the top of the radiator, when pressure drops to 14psi then rad cap closes again

Any air that goes out the overflow hose bubbles up inside the tank and is gone
As engine cools down the smaller valve in the center of the rad cap is sucked open and this pulls coolant back in from the recovery tank

If there is a leak then air could be sucked in from that leak point, instead of coolant from the recovery tank, but it would have to be high up leak, maybe upper rad hose or upper rad for the air to be at the top of the radiator in the morning, i.e. the air has to have a way to travel from the leak point to the top of rad without circulation

No on the head gasket leak
Cylinder pressure is 1,000PSI when a cylinder fires, so any leak will force air into the cooling system, as well as suck coolant into the cylinder, not a one-way street
It why head gasket issues are associated with overheating, because coolant in the head is displaced by air, so cooling is lost until circulation can clear out the air

 

Thanks again Ron. You make sense. I'll do more checking...

 

So just to follow up on this in case anyone else experiences it as well. I took a closer look at the rad cap, and after cleaning off the large gasket, i found 3 small nicks in the rubber. Replaced it and all seems good. I initially dismissed rad cap because it pressurized the system and there was absolutely no boiling of coolant. But, i guess the 3 nicks were enough to allow air to enter the system when cooling down. Anyway, rad cap...

 

Good work

Thanks for posting the FIX

 

Gracious thanks for your help Ron. I was heading down the wrong (and very expensive path) and your advice made me investigate further.