It may just be an airlock. Run it up with the rad cap off until the thermostat opens, if there is an airlock you will see the air come out then.
You could also have a bad thermostat or a blown head gasket.

 

I did run it up to operating temp with radiator cap off. I had plenty of heat out of dash and then installed the radiator cap. Shouldn't have an air-lock.

I did not touch the thermostat because it seems to be working fine. Engine warms up normally (quickly) and I get plenty of heat out of the dash.

I'm willing to try anything so I'll pick up a t-stat tonight and try it.

Head gasket? I don't have the usual signs of a bad head gasket, no coolant in oil and no white smoke out the tailipipe.

 

First - it's not really an "overflow bottle" - it's an expansion reservoir, and this is not just a matter of semantics - it's actually a description of what the container actually does - when the coolant heats up, it expands and the excess is transferred to the reservoir where it is stored until the system cools and it should return.

Second - you need to be sure that you're not "over filling" the system - there should be a full line on the reservoir. If you over fill it, there will be less room for expanding coolant and it will overflow and that coolant will be lost, although I would think that a quart a day is somewhat excessive.

Third - you say ...

a) After every drive coolant comes out of the overflow bottle vent tube. Have you actually seen this?

b) Coolant inside the bottle is hot but only filled half way - are you opening the system whilst it's still warm?

c) You're adding coolant to the radiator, not to the system.

All little things - but again important ...

a) the expansion reservoir would have to be full before coolant can flow out of the vent tube - unless someone has flipped the tubes on the cap - which is not possible on all vehicles. Alternatively a crack or leak in the bottle would allow coolant to escape and it would be possible to mistakenly assume that coolant below the reservoir came from the vent tube.

b) the cooling system is designed to function as a "sealed system" - and as the system cools, a vacuum builds that draws the coolant from the reservoir back into the system - if you open the system during this time, you allow air into it and disrupt the process.

c) following on from the paragraph (b) above - ANY leak within the system can disrupt the vacuum - so if you're adding a quart of coolant a day to the system, it's going somewhere - either through a leak - or through to the expansion reservoir and not being drawn back.

Check the system for leaks - some leaks will only show up under pressure - pinhole leaks in the radiator core are a possibility and the coolant is dispersed in a mist that makes them very hard to find, leaks in the heater core are another hard to find possibility - the coolant is first trapped in the heater plenum and then dribbles out into the carpet and is absorbed - and yes - coolant being transferred to the expansion reservoir and lost from there is also possible.

Look for the leaks and find out where they are coming from - make sure that your "new" radiator cap is the correct cap, both in terms of pressure rating and size - it must seal at two points, the base of the filler neck and the top of the filler neck - or it won;t work properly.

 

I just had the same issue with my X90. I never saw where the gauge got "hot". I put a thermostat, water pump, and rad. cap on it.
My head was warped and I had to get a new one. It was a pain in the a55 but the next time I bet I could do it in a day.
I had the same symptoms as you. No water in the oil, no oil in the water, no white smoke, ran fine other than the coolant...............that was the only problem for 3 weeks. Then it started showing signs.
I would say go ahead and get one of these and either rule it out, or stop before you do more damage.
NAPA AUTO PARTS

If you do need a head I used Clear Water Cylinder Head and have no complaints.
Best of luck

 

Thanks for the very detailed response.

If I fill both the radiator and then the expansion reservoir to the "full" line and drive till I hit operating tempature it WILL expel coolant out the expansion reservoir vent tube on engine shutdown, into the right front wheelwell and onto the ground behind the RH front tire.

I'll remove the radiator cap later on after engine cools and find the radiator coolant level is very low. I'll then add about a quart of coolant to bring it all the way up to the radiator cap. The level in the expansion reservoir will also be low, between the full line and fill line.

I do not have to add coolant to the expansion reservoir for the above to occur. I can leave the expansion reservoir coolant at the fill line and fill only the radiator. After normal driving (5-10 miles) that day it will expel coolant out the vent line onto the ground on engine shutdown. Very frustrating.

I understand the expansion reservoir should be full before coolant can come out the vent tube but not in my case. Something really strange is happening here.

Heater core isn't leaking because my carpet is dry, I checked that. Radiator cap has two seals, an "inner" and an "outer". My "outer" seal is suspect as the plastic of the radiator neck is a bit chewed up because this radiator cap design isn't the best or real easy to put on.

 

Thanks for the very detailed response.

If I fill both the radiator and then the expansion reservoir to the "full" line and drive till I hit operating tempature it WILL expel coolant out the expansion reservoir vent tube on engine shutdown, into the right front wheelwell and onto the ground behind the RH front tire.

Can I assume that you have watched this happen - and that the expansion reservoir is in fact full - during the time that coolant is being expelled? If you do not shutdown the engine - will it expel coolant?

You're describing a "boil-over" situation in which the engine is running at a temperature hot enough that when it is shut off and the coolant circulation stops, the coolant is being heated to the point where it flashes into steam and the resulting expansion is forcing the coolant out.

This can result from two causes - the engine may be running hotter than it should, lean mixture, incorrect (retarded) valve timing, incorrect (advanced) igntion timing; unusual engine loading - overloaded vehicle, trailer towing, steel hill; a partially functional cooling system - radiator partially blocked, a defective fan clutch; - OR - the temperature at which the coolant flashes into steam may be lower than it should - IF - you have a defective or incorrect radiator cap (one with a lower pressure rating).

 

A blown head gasket does not always mix oil/coolant. If a path is opened between cylinder and coolant passage, the pressure from combustion can pressurize the coolant system. Normally the coolant system pressure is less than 8psi. Squeeze your radiator hose cold engine off (should be very soft), then again with the engine running. If it is stiff/bloated when running, well, call Houston cuz you have a problem.

 

My upper hose is soft (squeezeable) with engine off. I believe it is very stiff with engine running but I'll have to double check to be sure.

 

It doesn't expel coolant from the vent tube with enigine running, only on shut-down. It doesn't matter if the expansion reservoir is to the full mark or not.

Engine coolant temperature is normal on the gauge (no numbers). I have a non-contact laser thermometer I use for troubleshooting. If I directly point it at the thermostat housing the temp never goes above 205°F with system closed and idling for extended duration.

When I shut the engine off you can hear bubbling sounds from the radiator within seconds and moments later it will start leaking out the vent tube. It's very repeatable.

Both new radiator caps I tried had the mark 1.1 stamped on them. I'm assuming this means 1.1 bar which is about 15-16 psi?

 

Engine coolant temperature is normal on the gauge (no numbers). I have a non-contact laser thermometer I use for troubleshooting. If I directly point it at the thermostat housing the temp never goes above 205°F with system closed and idling for extended duration.

I am running a 195F thermostat, and when I IR the thermometer housing, it NEVER is above 180F. What kind of thermostat is in your car? It would sound to me that your system is running much hotter than it should.

 

Problem is solved.

I went looking for a block tester to see if I had a bad head gasket. Autozone had the tester (free loan-a-tool) but no test fluid. Duh. Counter guy didn't think I needed the test fluid and still tried loaning me the tester. I corrected him politely and asked for a thermostat while I was there. Nope, their special order at my local Autozone.

I went next door at Advance Auto and picked up a 180°F t-stat (all they had in stock). Went home, installed the t-stat and burped the system. Closed the radiator cap and ran it for another 20 mins. No coolant out the expansion reservoir vent tube at all on shutdown. IR thermometer only read 163°F max on the t-stat housing. That's alot cooler folks. Alot. I read 205°F yesterday on the housing with the old (195°F) t-stat.

For fun I put a heat gun and my IR thermometer on the old t-stat. That t-stat didn't even begin opening till 245°F and didn't fully open till 285°F. Wow.

Thanks for the help forum. Cheers!

 

Glad you got it sorted and all is well.