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Coolant Boiling out


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After about a week of trouble free operation, my STS is beginning to act up on me again.

I got a low coolant level message last friday, so I took the car home and popped open the cap. Indeed the coolant was low. I topped off the level and started the car with the cap off to see how if the level was going to be sucked down - or if there were going to be bubbles.

Shortly thereafter, coolant started boiling out of the tank and my blood began to boil as well (for those of you who know, I just had my engine replaced). My first thought was head gasket. I turned the car off and filled the level up a bit more. I let it sit a while and started it again and watched the coolant temp on the DIC. Once the temp got to 212 - 213 all hell broke loose again. I repeated this a few times and it always boiled around that temp.

I decided to pull the water pump and thermostat. I replaced both even though they looked fine. I vacuumed out and drained most of the old coolant I could. I stuck a drill bit in the "hollow bolt" to make sure it was open. I added 3 tubes of bars leak and put new coolant in.

I repeated the test, it now boiled at a slightly higher temp but seemed less intense. I put the cap back on and went out for a test drive - the temp got up to 225 at idle and with no overflow.

Well today I got the message again. I repeated the test and I get a significant boil over at 215f.

I read the coolant bottle and it says the boil over temp was 260+. I am assuming that there must be a hotspot somewhere in the cooling system. I grabbed all the hoses I could reach to see if one was in fact cool. They were all pretty cool.

So caddy experts, what do you think? Could I have a clogged cooling system and how do I go about testing it and ultimately fixing it?

Could it be as simple as a bad radiator cap? It seemed to work fine on the previous engine.

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When you squeeze the upper hose is it hard or soft when the car is at operating temp. If you are boiling at 215 degrees you must have no pressure. What kind of coolant are you using? Premix? If you are NOT getting any pressure in your upper hose, I would think you have a bad cap.

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50/50 premix

Sorry I wasn't clear. I was running the car with the cap off. Is 215 to be expected without pressure in the system? I thought it would be a bit higher.

Based on the unpressurized boiling point, I assumed that it was boiling out. I guess it could be disappearing some other way - but I see

no leaks.

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The boiling point of Dexcool at 1 atmosphere is 228 degrees, with the cap on at a 50/50 mix it rises to 265 degrees and with a 70/30 mix 276 degrees


Read this regarding the CAP, especially the part about losing coolant


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I had this happen to me after an engine swap. (I installed a used N*). The temp would go out of sight and boil over. The reason was that the small return line that originates at the hollow bolt was plugged at the surge tank end. If that line is plugged, the system cannot purge it's self of trapped air and it will boil over & get HOT. :o

I don't know if this is your problem or not, but I would remove the small hose at the top front of the surge tank & check it and the surge tank nipple for obstruction. With the surge tank at operating level, place a small container under the open end of the small hose and have someone start the engine & let it idle. there should be a continuous flow from the hose end.

Be sure that you don't let anything touch the serpentine belt while doing this. :(

I would eliminate this area of concern before getting to the more expensive items.

If all the lines are open & you don't get any flow (or very little), I would check the water pump next. Check to see if the belt tensioner for the water pump belt is working.

Good Luck,


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I found this portion of Scotty's link quite interesting as I have never heard of this before, What say you Guru?

As for deciding when to change your antifreeze, don't go by miles or you WILL certainly have seal and mechanical failures. One interesting spec I found is to use a multimeter. You put your negative probe to the negative post on your battery. You then place the positive probe in the neck of your radiator, making sure that the positive probe touches nothing but the antifreeze. Make sure the coolant is warm but not HOT (this is for SAFETY reasons as well as accuracy of your readings. Always be careful when opening the radiator cap on a warm engine). Your readings (regardless of negative symbol on readout) should be:

* 0.2 V to 0.5 V - antifreeze is still good

* 0.5 V to 0.7 V - antifreeze is borderline

* 0.7 V or greater - antifreeze is unacceptable.

You can also use test strips (available at a quality auto parts store for $5 or less), they work on both green and red types too. But if you already have a multimeter, why go buy test strips? The multimeter is the more technically accurate method anyway.

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Thanks for the replies - but please note that I said that I had already replaced the water pump, thermostat and coolant - no problems with those items. Also note that I am not overheating, as far as I know 225 is still normal operating temp at idle - it does not go higher.

I did remove the hose to the hollow bolt, but I did not actually start the engine to look for flow, it was hot to the touch. I will follow BBobs advice in the morning and check out the return path for problems.

I also think that I will go ahead and replace the cap, since the part looks pretty affordable. I may try to find an 18psi cap if possible.

I will let you all know what I find out.

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Good luck with the boil-over problems (both you and the Caddy). You have the attention of the right guy (Guru - but not me in any case).

But regading the coolant test method using voltage... The test method did not make a lot of sense (to me anyway) because there should not be any significant voltage potential between the negative post and the coolant. There migt be some tiny, measurable (millivolts) produced by certain types of coolant flows, but it really depends on the over all systems (not really applicable here...I don't think)

Regardless, I immediately thought of conductivity (the opposite of resistance), which is a very general indicator of "how much other stuff is in water" (typically salts and perhaps some metals).

There is some real interesting info about recycling (cleaning and re-using) used coolant with "High Efficiency Electrodialysis" (HEED) from EET Corp. Conductivity is simply one of the parameters that indicate less salts and metals as the coolant gets cleaner with HEED. pH is also lowered as the coolant is cleaned up (as it is approaching pure water and ethelylene glycol). 2,000-3,000 micromhos conductivity is getting a bit "soupy" for water and EG (and that seems similar for other cooling solutions).

OK then. here is some info from "Penray" specific to conductivity and coolants

Category: Coolants

Bulletin No. 01.005

Date: 01/12/01

Replaces: 96.010


It is has been common practice for this and other companies to promote the use of conductivity meters and conductivity measurements as a measure of the concentration of inhibitors and contaminants (total dissolved solids or TDS) in engine coolant. This coolant property is of interest because excessive TDS is generally acknowledged as negative, having been associated with premature water pump failures and/or other engine damage.


The meters and procedures used for these field and laboratory evaluations are borrowed from wastewater analytical techniques. Typical concentrations of TDS in wastewater are well below 100 ppm. Concentrations of TDS in coolant have been gravimetrically measured to be in the range of well over ten thousand ppm. Further complicating the issue, the varying concentration of glycol dramatically affects the conductivity of engine coolant, even if a 10:1 aqueous dilution is prepared, per the Penray method.

In the past, a measurement by the Penray method of 2,000 µmhos has been used as a condemning limit resulting in a recommendation that the coolant be drained and replaced. Various observations have suggested weaknesses and inconsistencies with the practice of using coolant conductivity as a method of evaluating the current condition of a coolant.

Penray's Recommendation:

Penray recommends that coolants that contain any mixture of technologies from different companies not be condemned based on a conductivity reading. Research reported in an ASTM paper in November, 1997 demonstrated that the TDS concentration of mixed coolants cannot reliably be directly measured by conductivity. Indeed, coolants with high conductivities have been found, by gravimetric analysis, to contain TDS concentrations well within levels generally accepted as safe. Therefore, conductivity may prove useful only as a screening tool for some coolants and customers.

Fall asleep yet? :)

A coolant engineer (with EG in his veins) should be able to shed light on this subject (coolant and conductivity).

Add power to leave problems behind. Most braking is just - poor planning.
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I have had several cars including two 4.9 Cadillacs that at idle after driving, you could hear the cooling system boiling into the overflow tank. In each case, replacing the pressure cap completely cured the problem. They just seem to go bad over time and don't hold the pressure on the system they should. Without the correct pressure, it's not difficult at all to boil your coolant.

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I checked the return tubes today at startup there is a pretty steady flow of coolant into the tank.

To be sure that I am losing coolant through the overflow, I jammed some wadded up paper towlels in the overflow tube. I figure that they will be blown out if the coolant pours out through it, or at least they will get wet.

If this isn't the case, I may have to hunt down other leaks.

In regards to the coolant voltage, I would expect the reading is affected by acidity of the solution. A lower voltage would mean that there is still buffering available in the coolant and a higher reading would indicate that there is less acid protection. I would think that this voltage reading would also be affected by the type of metal in the probe tips (copper or aluminum or steel). It may be good to establish a relative measure of coolant life, if you monitor the value at several points - but a specific range for all multimeter probes, coolant types and engines is probably not as reliable.

This is the first I have heard of such a test.

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Pull the 3/8 line off at the surge tank.  Coolant from the surge tank should stream out of the nipple on the surge tank.  If not, find the plug.  Start the engine again.  Coolant should pour out of the 3/8 hose toward the surge tank.  If not, find out where the line from the hollow bolt is plugged.

I want to be sure I'm clear on this test.

As I recall, my surge tank has 2 hoses - a big one at the bottom and a small one at the top coming out from under the intake. Are you saying that if the top hose is pulled, coolant should come out of the nipple on the tank?

I pulled the small top hose, started the engine, and got coolant running out of the hose toward the tank, but nothing coming from the nipple. (which seemed to make sense because the nipple is above the coolant level).

If I had pulled the bottom hose, it seemed that I would have dumped the contents of the tank.

Can you clarify?


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I have never heard of trying to establish the life of the antifreeze based on a voltage reading like that.....not to say that there isn't some way that it might work somehow...but I don't know how...LOL

I have heard of using a voltage reading like that to try and pinpoint a combustion gas leakage into the cooling system but the information I saw about that indicated that it was a very inconclusive test.

Why worry about testing antifreeze for life anyway...just replace it every 2-3 years/24-25K and forget testing....

It does sound like the vapor vent line is plugged.

Pull the hose off at the hollow bolt and start the engine and make sure coolant pours out of the bolt. If not, take it out and find the plug. Reinstall the hose.

Pull the 3/8 line off at the surge tank. Coolant from the surge tank should stream out of the nipple on the surge tank. If not, find the plug. Start the engine again. Coolant should pour out of the 3/8 hose toward the surge tank. If not, find out where the line from the hollow bolt is plugged.

The cold hoses indicate that the coolant is just not circulating. If the pump is vapor locking on air in the system it cannot purge the air if the vapor vent line is plugged and it will stop pumping.

I assume you have looked at the water pump drive belt and belt tensioner...?? If the pump is not turning then it will not pump either. If the tensioner is frozen and not tensioning the belt then it will slip and cause the pump to run too slowly or not at all.

Guru, if this volt meter method works, couldn't it be incorporated into the vehicles monitoring system to let you know when the coolant needs to be refreshed? Mike

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Jackc, the top hose is the purge "supply" line. That line should flow coolant into the tank, not out of the tank. The bottom hose is the return line and if removed would drain the surge tank.

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Jackc, the top hose is the purge "supply" line. That line should flow coolant into the tank, not out of the tank. The bottom hose is the return line and if removed would drain the surge tank.

Yes - That's exactly how I saw it. I think what is confusing me is this statement in Guru's post:

"Pull the 3/8 line off at the surge tank. Coolant from the surge tank should stream out of the nipple on the surge tank."

So this refers to the bottom nipple? And could be restated to say "Take the hose off at the bottom of the surge tank. Coolant should run out of the tank. If not, the fitting at the bottom of the tank is plugged"?

Sorry if I'm being thick. Just trying to be sure I understand what to look for........

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An update for those still interested...

I have driven the car a few days with the paper towel wad in the overflow tube. It has stayed put and is dry. The weather has been cold but I did get the coolant temp up to 222f a few times by setting the AC to max cool and driving hard followed by a long stop at idle.

The coolant level has not dropped much either, I have noticed as slow drip underneath the vehicle, but I cannot tell if this is previously spilled coolant finding its way to the pavement or if it is leaking out. I have to get the car up on ramps to investigate. The drip does seem to be slowing so either it is just old pooled up coolant dripping or the Bars Leak is doing its magic.

This could just be a case of not having the radiator cap on tight enough. I found that I had to really twist to get the cap to seal well. I may not have done that previously. That coupled with my misunderstanding of the boil over protection temp made me believe that it boiled out. I will have to wait for some hotter weather I think.

This weekend, I will go on a short leak hunt; after which I need to address some other nagging issues with the car. Namely, the brakes still feel odd - but the car stops fine and I get a lot of smoke from the tailpipe on deceleration (i had the same issue with my previous engine) maybe I have to stop buying the cheap super-tech oil.


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