Hi, all.

My first post here and am hoping for a little help with coolant issue that has me stumped. The truck is an 03’ Dakota. It has a 4.7L with 263K on it.

For the past few months, I seem to be losing a little coolant every few days. So far I’ve replaced the coolant filler neck and radiator cap. The radiator is only a few years old. I ran a pressure test which found a minor drip in a heater bypass hose, which I’ve fixed. It seems the drip only showed up with the pressure test because the area always dry before that. Current pressure tests show less than 1 PSI drop, after leaving it pressured for 20 minutes at 21 psi. I’m guessing that’s acceptable.

Now here is the thing. I can fill up to the neck with coolant while cold and leave the overflow at half capacity. After a day of driving, and full cool down, the overflow bottle ends up full and the neck is empty. In other words, it seems to be pushing coolant out to the overflow tank, but not sucking it back in when cool, as it’s supposed to. Overall, I’m still losing coolant somewhere though, as I end up adding here and there. I found no further leaks, none in the heater core either. If this was a head gasket issue, I guess I’d have coolant in the oil and vice versa… I see nothing, plus again, it passes pressure tests.

Maybe you guys have come across this before. It’s a minor annoyance now, but I want to figure it out before it gets worse.

Thanks.

Oil and coolant clean.




Overflow started out with half capacity, now full after a day or two


Coolant neck shows no coolant after day or two. Takes about an ouce or two to refill.


20 min pressure test start

20 min pressure test end

Any air bubbles in the radiator when running? how old is the water pump? any wetness at the seep hole.

Water pump is two years old. If I run with cap off, I may get a single air bubble every now and then. No leaks anywhere that I can see. I've spent time under truck with strong light while running and while pressure testing. Saw no leak.

first I'd solve why the coolant ain't being drawn back into the rad: bad recovery cap or leak in the hose or container (above coolant level being burped out) that is keeping vacuum from drawing the coolant back to the rad when the system cools off. IF (I ain't sure) recovery tanks are vented (very well might be as the hose comes in thru the bottom) then the hot coolant could simply be evaporating out of it as it sits.

New cap. I checked the tank and hose... no leaks. Indeed, the tank is vented. I closed off vent and applied air pressure with my mouth, finger over end of hose. It held air tight. I feel like I've covered all the bases, but no dice.

My '02 Durango 4.7 does the exact same thing. I also can't find a cause. My thoughts are possible headgasket. Although there are no signs of that. My system holds pressure in hoses well after it cools.

Radiator level should be about 1" down rather than full to the top.

I've had the truck for a few years and in the past, it was always up to the neck when I checked. The issue is relatively new. I've never run it low on coolant or oil and the temp needle is always just below half. Never overheated while in my possession. I can't see the head gasket just going bad on it's own and even if it was, pressure test would fail, I think?

on a tough case with a slow leak that ain't showing anything with normal diagnostics (like yours) I would get it hot at the end of the day & pump it up to 15 lbs with the pressure tester & let it set overnight & see if it holds. A person could have an emissions testing station stick their probe into the air space above the coolant under the rad cap & see if the machine shows any hydrocarbons present (power brake it to stress it) or they have that blok check kit with the yellow fluid & you suck some of that same air up into one of those mini antifreeze testers with the balls in it & if it turns blue then you know what comes next. (dealing with that myself with the cooling system from Hell in the stock car)

The rad cap on these engines are inline with upper hose. Rad doesn't have a cap. They use a large res mounted on shroud. You can't see rad level.

True... this is how it looks

That pipe plug on housing by clamps is to bleed engine. Should be removed while filling till a/f comes out of it.

That bleeder was tightened by the devil himself. I've never been able to crack it. I put the front end on ramps and run with the cap off till bubbles stop.

compression test may reveal whether it is a head gasket or not.

Your not losing coolant..more like pushing ot out..most likely a start of a head gasket

There is no real reason to remove that plug because rad cap itself is at same level., and thermostat is at engine lower hose. I as well, would think a head gasket may be starting to fail, especially at that mileage.

A slow coolant leak can be sneaky. My/my daughter's Neon was loosing coolant and I couldn't find it. Had to top it up every week for several months. Then as she was pulling into the garage one day, the radiator just let go. There was a small leak in the corner of the core hidden by the shroud. It evaporated as is leaked so it was not dripping and I couldn't see the white telltale residue from it evaporating. New radiator fixed it.

In your case, suggest you get it good and hot running, shut down and inspect carefully. Also, may be worth trying another cap... I know you replaced it but it sure sounds like the cap may not be doing its job.

If coolant is getting into a combustion chamber, it will tend to steam clean the plug. It would be worth pulling and inspecting the plugs... if one looks really clean good chance it is a leaking head gasket or crack.

See pics of oil and coolant in first post. No smoke and exhaust smells clean. Been driving it a few years with no overheat, so if the gasket failed, it would be for no real reason. If it was the head gasket, I think it would lose pressure during the pressure test, right?

Yes but you need to get it hot (duplicates actual conditions when it acts up) and let the tester set on it/pressurized overnight cuz it is a slow enough leak that it needs time to show itself

Must say, wow! The rate of response in this forum is incredible. I'd have to wait 5 days in the other forums for all these responses. I'm impressed and humbled. Thanks, guys.

I tried this last night. I ran the truck with the pressure tester attached. From cold, the pressure started slowly from zero and rose steadily as it heated up. I killed the engine when it hit 23 PSI because the tester's cap was not going to vent into the overflow tank and it was rising rapidly at that point. Since the pressure was from thermal expansion and not me pumping air pressure, it dropped slowly as it cooled, back to zero. Saw no leaks. I'm not ruling out head gasket, but I saw nothing unstable as I watched it. It was all very linear. I will be pulling plugs next.

Thanks again.


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RTH, my understanding is that it shouldn't hold pressure when it cools. It should actually be a vacuum, which ends up being zeroed by return flow from the expansion tank. So, there should be no hiss when you open the cap, perfectly cold.