overflow bottle magically full

[dynamite44]

i havn't touched the cooling system on my XJ in about 6 months. i recently went off roading. after the trip, i popped the hood to re-oil my K&N, and i noticed the overflow bottle on my 2000 XJ is so full its almost spilling out of the top. no one else has touched the vehicle. the coolant was at an acceptable level the day before the trip.
it isn't overheating, it stays at 210, and the aux. fan isn't coming on any more frequently than it used to.
any ideas?
the radiator cap is a levervent that's about a year old.

 

[Osprey413]

If you went up a steep incline the coolant may have flowed back into the tank due to gravity. I know my windshield washer nozzles have leaked on my when I went up a steep incline. If it isn't overheating then you probably don't have a clog in the system, so gravity is my only thought.

 

[JEEPZZ]

Coolant gremlins. I bet by morning, after the engine cools back down, it mysteriously goes back to it's previous level or even lower if the temp outside is colder than it was 6 month ago when you last check it. Damn gremlins.

Don't be surprised if the "air in your tires" gremlins show up to adjust your tire pressure by a few pounds after a long drive. :viking:

 

[ifixit8]

try poping your radiator cap and make sure that it's full. It should be since you aren't overheating. but it couldn't hurt to check. sounds like a gravity thing though.

 

[Dr. Dyno]

If you went up a steep incline the coolant may have flowed back into the tank due to gravity. I know my windshield washer nozzles have leaked on my when I went up a steep incline. If it isn't overheating then you probably don't have a clog in the system, so gravity is my only thought.

Gravity would make sense except that the coolant overflow bottle would only fill up if you were going DOWN a steep hill, not up.
I'm sure the bottle will go down again after you run the engine warm for a few miles and shut it off.

 

[xjbubba]

If you checked the level at the end of the trial--engine at its highest temp, you are probably observing normal action. Not sure about open cooling system XJ's, but normal, modern radiator systems have a radiator cap that allows excess coolant to exit the radiator as the coolant normally expands as it heats up, then allows it to be sucked back into the radiator when the coolant contracts as it cools. That's why the "overflow" bottle normally has the "full" mark somewhere near the middle of the container. JEEPZZ is on to it!

 

[Rick Anderson]

Gravity does NOT make sense to me. The pressure caps seals the system from the overflow bottle, you can hang the vehicle upside down and and all the fluid would flow out of the overflow bottle, but it would still stay inside the cooling system.

The only way for the fluid to leave the cooling system and go into overflow bottle is if it creates more than 16lbs of pressure and forces the seal back against the spring. If the pressure cap is bad, than it would take less or even no pressure to flow past the seal. Normal expansion of the fluid from increased temp will force past the seal in the pressure cap, but that should be constant, i.e. the overflow tank raises to the same level everytime the motor warms up, unless the motor ran hotter. Overheating and especially overheating to the point of boiling fluid would push more past the pressure cap, but dynamite44 said it never overheated.

When you noticed the overflow tank was at higher level, the motor was cooled off right?? If not, I'd just suspect the cooling system ran hotter, even if the motor didn't show higher temp from the extra load of thrashing around off road. But the overflow bottle should go back to the normal level after the system cools off. If not, something is wrong.

As well, something has to replace that fluid, it doesn't magically get bigger/greater volume at the same temp? Whats in the cooling system now that caused that? Air? Oil? Exhaust Gas? Could you have a BLOWN HEADGASKET forcing those things into the cooling system?

I'd open the pressure cap, when the system is COLD, and pear into the radiator. Look for air/oil/exhaust gas in the system. Exhaust gas and oil dirty ups the fluid, often looks like milky clouds in the fluid. The coolant gets lighter in color as it wears out, if its darker than original fluid, than exhaust gas/oil could be darkening it. Oil can also be spots floating on top.

 

[langer1]

And being a 2000 you could have a cracked head.
Also a small coolent leak can also cause it same thing if it don't go back down.
As the engine cools down it pulls vacuum to draw coolent out of the overflow, if theres a leak it will not draw coolent back water pump seals are good for that.

 

[Rick Anderson]

Theres a valve in the pressure cap that allows the fluid to be drawn back in when the fluid in the cooling system cools down and contracts.

The little metal hat in the center of the seal on the pressure cap, you can pull it back with your fingure nail. That could be bad, or there could be a leak in the tube or the channel in the overflow tank that leaks the vacuum that draws the fluid back in, that could be a reason why the tank is getting higher and higher.

 

[dynamite44]

i know there's something wrong with this engine. the XJ has been a lemon since day one. i won't list everything, but under warranty i had 2 transfer cases, front axle, fuel pump, PCM, TCU, airbag module, wire harness, alternator, at least 8 oxygen sensors... so i'm not suprised that this is happening. it has 65k on it. i checked my fluid levels the day before going off roading, the levels were fine.
i talked to the other people who were driving cherokees that day we went off roading, none of them reported any abnormal levels.
now, i think i have a bad head gasket or something. the coolant in the overflow tank turns black within about 300 miles of me driving it. i used to change the oil and coolant every thousand miles, now i just don't care. i've been changing the plugs every oil change since 60k, when it went out of extended warranty. owned it since new, i've got a pool set up with my friends as to when/what mileage it'll finally blow up.
i didn't think about this until now, but it could be a ton of oil in the coolant, like a quart or so, causing the overflow bottle to be full. it isn't knocking any louder than usual, i guess i'll have to check the plugs and see how bad they are.
i've been running the 1.28$ autolite plugs because i buy them so often, gapped at .035.
when i checked the coolant level, the thing was cooled down, both times. before and after the off roading trip, on level ground both times.
it smells like coolant all the time, but its been doing that since about 40k. the 5 star dealer even wrote on my service sheet that the smell is "normal for this vehicle".
if i'm smelling it, its probably getting out somewhere.
any more ideas?

 

[JEEPZZ]

i know there's something wrong with this engine...
now, i think i have a bad head gasket or something. the coolant in the overflow tank turns black within about 300 miles of me driving it.

These are things you failed to mention in your first post at the top of this thread. Sound like a head or head gasket problem. No gremlins.

 

[dynamite44]

my bad.
i had some people convince me that it wasn't a bad head gasket on another online forum. i thought i wouldn't mention it just to see what people had to say.
thanks for all the input!

 

[Dr. Dyno]

now, i think i have a bad head gasket or something. the coolant in the overflow tank turns black within about 300 miles of me driving it.....

i didn't think about this until now, but it could be a ton of oil in the coolant, like a quart or so, causing the overflow bottle to be full.

I wish you'd told us these details earlier because now we're in a whole new ballgame. Since yours is a 2000 model, I fear that your engine has been stricken by the notorious head cracking problem. Remove the oil filler cap and take a look inside the crankcase between the no.3 and 4 cylinders. If you see any coolant stains or crusts, you've found the problem.
If there is indeed oil in the coolant bottle, the engine oil level should be low on the dipstick. The oil is probably also contaminated with coolant so it could have a chocolate milkshake appearance.