Coolant pressure cap failure or something else?

[99ways]

I have noticed several times now that the pressure cap on my reservoir tank (pressure tank) has vented, spewing small amounts of fluid and causing my temp to rise from it's normal position (on the middle of the gauge) up to the 6th or even 7th position (of 8 total).

When I get out of the car and look under the hood I can see that coolant has been sprayed from this cap and is burning on hot surfaces of the engine. Also, the small line leading to the upper part of the tank seems to be making a bubbling sound...

Has this happened to anyone else? Does this mean that my pressure cap needs to be replaced, end of story?

Thanks

[Ranger]

The cap could be bad. It should hold 16 psi. If not that might explain your problem. Have it tested at a radiator shop. If it is good, it might be head gaskets. Have them do a test for exhaust gases in the coolant.

[99ways]

If it was a head gasket problem wouldn't I be losing a notable amount of coolant also? Are you suggesting that this could be causing excessive pressure in the system?

[99ways]

UPDATE:
Drove the car to work and back today (40 min. each way), including a WOT that surprisingly took me just over 100MPH for maybe 30 seconds or so and then a gradual ramp back down to about 80, then 65.

During both trips the car warmed up to the center line and didn't budge in either direction no matter if I was stopped in tight traffic, or WOT.

Ranger, I certainly don't mean to question your expertise, but if I had a head-gasket issue wouldn't it be more consistent?

I did have a similar problem before, but I replaced the thermostat and it went away for a good while.

Thoughts?

[Ranger]

Head gaskets can be somewhat intermittent in the early stages. Don't mean to alarm you. Sounds like it may just have been a stuck thermostat or clogged purge line. Check those if it happens again along with the cap.

[rfishing]

I need to pipe in here since all the problems I had with head gaskets. First of all, make sure you wipe the area around the radiator cap clean before installing the cap, if it had debree in there it will not seal properly, and will allow coolant to overflow. You mentioned air bubbles, so I can understand why Ranger would suggest Head Gasket right off. My experience with blown head gaskets on my 94sts makes me suspicious of your engine as well. I too was able to drive it high speeds and high RPMs sometimes, then other times it would just blow coolant out the overflow cap. Eventually it will be undrivable. You may have had an air pocket releasing? Personally, I would be watch it closely for coolant loss, and if you still are losing coolant, go get it tested for combustion gases in the coolant. Check coolant cold, in morning to see if level has changed. Best to identify a blown gasket early on, before other things are damaged, right now sounds like things might be normal, but keep an eye on it.

[99ways]

To be clear, the bubbling sound came from the highest narrow-diameter tube leading from the tank. This could be the purge tube. How can I be sure that it is not clogged?

[JimD]

Originally Posted by 99ways

To be clear, the bubbling sound came from the highest narrow-diameter tube leading from the tank. This could be the purge tube. How can I be sure that it is not clogged?

Pull that rubber hose off the reservoir and point the end into a small container to catch the coolant. Run the engine at idle speed and you should see a small stream of coolant flowing from the rubber hose. But if you can hear the air-coolant mix flowing through that hose, the hose circuit is not likely plugged.

Be certain the rubber gasket in the pressure cap is clean and not ruptured. And the gasket seating surface at the top of the reservoir should be straight and flat with no knicks or other damage.

If your cooling system is constantly purging air, your next step is to figure out how the air is getting into the (closed) cooling system. And failed headgaskets is only one of the possible sources of air; there are several others. Small leaks at hoses, the radiator, or water pump will allow air to enter the system as the engine cools.

[rfishing]

While engine is cold, take off radiator cap and start engine. Watch for bubbles in the overflow tank. If you got bubbling its blown head gasket. Sometimes with these norstars you will not see anything wrong with the head gasket until the engine is under load, like going up hills. Depends on how badly it is blown, if it is blown. I did not see bubbling for almost a year after it was blown.

On mine, even the test for gases in coolant gave a negative result 2 out 3 times.

[99ways]

Well I haven't checked the hose output to a container yet, and I can't seem to find a place to test my radiator cap, but with those things on my to-do list here is an update that may help.

The temp has remained top-dead-center every day through one-hour long drives in sometimes stop and go traffic. Today I was driving home and got stuck in some massive traffic with 0-50MPH then stopping off and on traffic. Under these heavy strain conditions the temp started to increase a bit, eventually getting up to the highest safe point.

Once I was off the highway and onto city streets I noticed that my temps would rise after acceleration nd fall a bit after braking and sitting.

When I got home and popped the hood there was not an obvious loss of coolant outside of a wet area around the pressure cap. There was also a bubbling, or rather boiling sound coming from the pressure tank and the small narrow line...

Previously, when I inspected the cap and pressure tank opening for damage, I did notice a small indention on the top of the pressure tank opening (on the highest ring that appears to make primary contact with the cap.

I guess I jjust don't understand why the temp won't level out once it's 'popped' under pressure. Could a tiny indention cause this?

I will pursue the other suggestions you have given me as well. Thanks for all your help.

[JimD]

Originally Posted by 99ways

Well I haven't checked the hose output to a container yet, and I can't seem to find a place to test my radiator cap

Certainly a Cadillac dealer would be able to test your cap.

The temp has remained top-dead-center every day through one-hour long drives in sometimes stop and go traffic. Today I was driving home and got stuck in some massive traffic with 0-50MPH then stopping off and on traffic. Under these heavy strain conditions the temp started to increase a bit, eventually getting up to the highest safe point.

If your A/C is OFF, you can expect coolant temperature fluctuations when the car is not moving. The fans are cycled ON/OFF according to the coolant temp. sensor and the PCM makes the decision. On my '98, the fans come ON at 223 F and go OFF at 212 F. You will not be able to hear the fans cycling inside the car with the windows and hood closed. In fact, you would not hear the fans if you were standing in front of the car with the hood open.

When I got home and popped the hood there was not an obvious loss of coolant outside of a wet area around the pressure cap. There was also a bubbling, or rather boiling sound coming from the pressure tank and the small narrow line...

The cap is leaking and therefore not holding pressure. The vapor point (boiling) of a 50/50 mix of Dex-Cool / water is 223 F at atmospheric pressure at 950 ft above mean sea level. With a defective cap, you can expect to "hear" boiling at about the time the fans would come ON.

Previously, when I inspected the cap and pressure tank opening for damage, I did notice a small indention on the top of the pressure tank opening (on the highest ring that appears to make primary contact with the cap.

If the damage is located on the top surface where the cap gasket makes contact, that could be your problem. But I think you are referring to the threaded portion of the reservoir?

Without numbers on your temperature gauge, it is hard to be sure what temperature your engine is running. The file below shows the range of temperatures my '98 displays. And temps up to 240 - 250 are not dangerous. You just don't expect to see them very often.

http://i70.photobucket.com/albums/i1...ithnumbers.jpg

[99ways]

JimD, thanks for the reply!

Everything you wrote sounds like you have a strong understanding of what I am expereincing here. The highest operating temp that my car has been reaching, according to your graphic, is the highest white bar past the 250 point.

Here is the line that I mentioned appears to be bubbling:
http://www.flickr.com/photos/57784241@N00/417780919/

Here is a picture of the filler neck on the pressure tank with two indentions where pressure may be leaking:
http://www.flickr.com/photos/5778424...n/photostream/

Finally, here is the underside of the cap:
http://www.flickr.com/photos/5778424...n/photostream/

Becuase the system has been losing fluid, I'm sure the mixture is not exactly 50/50 anymore... I sometimes add dexcool and other times water in effort to even this out. This comes from my old-school mentality that as long as it's not freezing outside, you could run straight water if need be.

This could theoritically lower my boiling point if more water exists than dexcool, correct? I have not added any tablets or barrs stop-leak to the system since I replaced the thermostat and replaced most of the fluid. Is this teribly important?

I can't imagine why there would be this sort of damage on the neck of the pressure tank, but do you think I will have to replace the pressure tank entirely to remedy this problem?

Thanks for all your help!

[JimD]

Originally Posted by 99ways

....Here is the line that I mentioned appears to be bubbling:
http://www.flickr.com/photos/57784241@N00/417780919/

That is the purge line and it is doing exactly what it is designed to do; provide an escape path for air (or vapor).

Here is a picture of the filler neck on the pressure tank with two indentions where pressure may be leaking:
http://www.flickr.com/photos/5778424...n/photostream/

I cannot tell how deep those grooves are, but I suspect they would prevent a seal.

Finally, here is the underside of the cap:
http://www.flickr.com/photos/5778424...n/photostream/

I cannot see the entire rubber gasket surface but what I can see does not show any damage.

Becuase the system has been losing fluid, I'm sure the mixture is not exactly 50/50 anymore... I sometimes add dexcool and other times water in effort to even this out. This comes from my old-school mentality that as long as it's not freezing outside, you could run straight water if need be.

Two reasons to use anti-freeze. Freeze protection and elevated vapor point protection of the mixture.

This could theoritically lower my boiling point if more water exists than dexcool, correct?

Correct

I have not added any tablets or barrs stop-leak to the system since I replaced the thermostat and replaced most of the fluid. Is this teribly important?

Important, yes. But it has nothing to do with your current situation.

I can't imagine why there would be this sort of damage on the neck of the pressure tank, but do you think I will have to replace the pressure tank entirely to remedy this problem?

Replacing the reservoir might be the ultimate solution. But if it was my car, I would remove the reservoir and attempt to remove material from the top surface and therefore eliminate the two grooves. This will require some patience and developing some draw-fileing skills as you need to end up with a gasket mating surface that is square/perpendicular to the threads. But if it does not work, you were faced with replacing the reservoir anyway, right?

Keep in mind the possibility that this little seal problem might not be the only place you are losing coolant; it could be the one place where you can see coolant escaping.

[Ranger]

I agree with Jim, but rather than filing the surge tank neck, I would try applying some silicone or RTV to fill the grooves and see it that helps. I think that might be easier with less accuracy required. Just another way to skin the cat.

[Mountie]

I just replaced the heads on my GMC truck ( 4-bolt main- 350 Chevy replaced 5 years ago & never overheated) My radiator cap started to leak a bit, then about 3 weeks later there was some steam from my exhaust for about 3 minutes.....once in a while. Cracked head. It never overheated. Hardly lost water, because it didn't loose much....the crack wasn't very big.

At low idle ( first thing in the morning) is there any sort of a miss or rough idle.....

.........Good luck, and hopefully it is a simpler problem than that.