Coolant overflows from water reservoir, bubbles

I have a 1998 4.0 SOHC that's been getting low on coolant every week or so. I've never seen any water under the vehicle so today I finally ran it around the block and immediately checked under the hood when I got home. I found the coolant reservoir completely full and water seemed to be leaking out from the reservoir cap (the outside of the tank was wet and dripping). The water was bubbling like it was boiling but it was just air bubbles (the coolant was cold). I could hear a hissing that sounded like it was coming from the radiator cap, but I couldn't pin it down...when I slowly relieved the pressure by turning the radiator cap, the hissing stopped, but the reservoir remained completely full.

Then I noticed the hose going to the passenger side valve cover had oil around it on the valve cover. I pulled the hose and it had very watery oil in it, However my oil isn't milky at all at the dipstick, and no white exhaust. I have noticed a few times in the morning when I get a few miles down the road the red "Check Gauge" light comes on briefly and the temp gauge is pegged to HOT, then immediately drops to normal...clearly the engine isn't overheating so I don't know what is going on. There are no oil leaks that I can see and no oil is being used excessively. I recently replaced my thermostat and when I did that I completely flushed my water heater (tested it to make sure it wasn't plugged too) and radiator and the entire coolant system.

1) Could a bad radiator cap cause the water reservoir to overflow and the temp gauge problem?

2) Does the hose that goes to the passenger valve cover normally have oil in it, and should that be clamped onto the valve cover (mine is just pushed on, no clamp)?

3) What should I do first?

I would test the thermostat. They have been known to come bad right out of the box.
Boil some water and drop the thermostat in, watch for it to open. Check to make sure it is the 195* or whatever yours calls for.

Then if that checks out, make sure you "Burp" the system of air. These are the two main reasons for your problem.

The water pump would be the next item to check. Your not getting proper circulation.

The temp gauge is most likely working, and the red check gauge comes on when one of the gauges goes to an extreme, (either high or low). The temp needle comes back down after the coolant has started flowing. The thermostat had opened (even partially) and let the coolant rush in.

Repeated operation of this can lead to bad things to come, so a fix asap is in order.

Most likely it's a blown head gasket. You have combustion chamber gasses leaking into your cooling system, that's why it's bubbling. A blown head gasket doesn't necessarally mean milky oil, it depends where the gasket fails. In your case, it's failed between the combustion chamber and the water jacket.

Good catch on the head gasket.

Look for signs of coolant leaking under the heads on the engine block. If the leak is in between spark plugs, you might be able to seal it with some Bars leak or something similar.

I too would suspect the head gasket... But in thinking about it, even if the gasket isn't failed between the oil and water jackets, wouldn't the coolant (under pressure when warm) be pumped into the combustion chamber when the engine is shut down? Wouldn't that show up as either a puff of smoke when the engine starts, or coolant (or coolant smell) in the oil? (from what leaks past the rings with the engine off)

If the engine was, in fact, overheating, you'd also experience an elevated idle speed, and the cooling system would be under a significant amount of pressure.... I wouldn't suspect the thermostat personally... Not without an associated overheating condition (high idle) and a gauge registering hot (assuming it's working properly).

I had a similar problem with a blown head gasket (twice) on a Subaru. As combustion pressure builds and pumps into the coolant chambers, it forces the coolant back into the reservoir. Eventually there is little coolant left in the engine and the temp gauge climbs. Then the engine essentially burps, the gases exit the reservoir, and coolant flows back into the block to start the process over again. And at first when the leak is small, you will not get a noticeable puff on starting, but it doesn't take long after that. Sorry.

I had a thermostat go bad, it wouldn't open till much higher than 195*. This in turn would cause my temp to go up to where the temp gauge needle would climb to the top position. Once the thermostat did open, it would cool off real fast.

This caused (over time) me to have a head gasket leak. It leaked between the the cylinders, and did not leak into the cylinders whatsoever. It just dribbled out and down the block. I simply ran some Heavy duty Bars Leak thru it, changed the thermostat, and changed the fluids a couple times after awhile. No more leaks. No more high temps.

This is just my experience with a problem I had, it sounded similar to the OP's problem.

I was scared it might be related to the head gasket. When I purchased this Explorer the heater wouldn't heat up, which is why I replaced the thermostat in the first place. When I removed the old thermostat the bridge on the thermostat was crushed like someone put a damaged thermostat in, or maybe purposely did it so the thermostat wouldn't close. The vehicle always ran really cold and the heater didn't work, until I put in this new thermostat and everything has been great until now.

I've had other vehicles where the head gasket blew somewhere but what's puzzling with this is i'm not seeing any symptons other than this coolant thing. My idle is actually low, even when hot. Sometimes it dies when it's warmed up and I go from park to drive.

I probably should have also mentioned that there was a significant amount of air in the system after I purchased the vehicle. I would hear it gurgling while driving even. After I flushed it and replaced the thermostat that went away, but maybe there is still air in the system.

Thanks for all the responses and suggestions.

I think I solved the bubbling/hissing problem. I was checking all the hoses and the hose going from the reservoir to the radiator cap was completely loose. I put a zip tie on it temporarily just to see if that was the problem and there is no more bubbling/overflow of coolant...so far.

I also forgot to mention in my first post that the "check engine" light came on yesterday. Might be unrelated but I haven't looked into yet. I know I do need an oil change.

i had a 90 ranger with the 2.9 engine, it had a crack in one of the heads that caused the same problem. the coolant in the reservoir smelled like exhaust gases. never got in the oil. the 2.9 engine of that year were notorious for bad heads.

I have the same problem

Hi,
I am driving a Mercury Mountaineer-2005-V6-4.0 Ltrs and I am having the same problem... coolant level drops... no leak... would briefly see check gauge.. temperature goes very high... I stop and by the time I pull over, temperature is back to normal.... water overflows from the tank.... this happened like 4 times...

What I have done so far is replacing the tank's cap.... replaced thermostat... flushed the coolant system... filled 50/50 antifreeze coolant/distilled water.... removed air (or I guess so) by running the heater and watching air bubbles flow out of the tank... What I'm suffering now is that coolant level goes down around half a liter everyday... I commute to work 110KM each way daily... I reach home... let the car cool down till next day... check water level... down a little, around half a liter as mentioned above and the big note is that I always find coolant tank is under a lot of pressure during removing the cap... I am trying to see whether I had air in the system and will ultimately burb completely and water level stabilizes.... would love to know whether you ultimately got that solved by tightening the hose between the radiator cap and the coolant tank??!!!

Would love to have help from all

Thanks

badcrc said:

I think I solved the bubbling/hissing problem. I was checking all the hoses and the hose going from the reservoir to the radiator cap was completely loose. I put a zip tie on it temporarily just to see if that was the problem and there is no more bubbling/overflow of coolant...so far.

I also forgot to mention in my first post that the "check engine" light came on yesterday. Might be unrelated but I haven't looked into yet. I know I do need an oil change.

Click to expand...

Certainly sounds like a bad head gasket leaking pressure into the cooling system. You need a mechanic to test for the presence of exhaust gas byproducts in the coolant.Also, compression test/leakdown test might also be instructive. Good luck.