What would it mean if my coolant light comes on while driving, but ONLY when the thermostat is closed? (it was very cold here tonight).

I have the Pettit Alum AST and a brand new thermostat.

Assume nothing about the condition of the hoses or the filler and AST caps, what could cause this to happen?

edit- there's no obvious coolant leaks, making me think it's probably one of the caps?

 

UP!

 

Prior to the thermostat opening, the coolant hasn't reached temp and expanded to fill the system. This probably means that you are not drawing coolant back from the overflow tank when the engine cools. Check the AST cap and tubing leading from there to the overflow tank for leakage, allowing air to be drawn back, rather than coolant.

 

Any second now I expect someone to say the usual...

your coolant seals are gone, you're fucked and need a new engine, sorry!!

 

Its drawing air into the system. Check your caps and resiviour return line.

 

AST cap didn't look so good, all crusted up. Which led me to drain the overflow tank and find about a cup of SAND/DIRT at the bottom!!! I don't live near any beaches.... Anyway, I drained that sucker really well, flushed it numerous times, and replaced the AST cap. Seems ok now, but I'll take it out again tonight when it's cold to see if i can reproduce the coolant light.

 

OK, still need some help here, cuz it leaked all weekend at the revolution.

I talked to the Pettit guys, and they told me that the system will never draw fluid back from the overflow reservoir, and that makes sense. The pressure cap only lets air out when pressure is >13psi. Otherwise it's closed. So when the system cools, there is a vacuum. It would make SENSE that it should suck it back from the overflow somehow, but it doesn't. Comments?

 

The Pettit response is just dumb.

The engine definitely draws coolant back from the overflow tank every time the coolant contracts when the engine cools, unless there is a leak in the return hoses/caps allowing air to be drawn back. There is a check valve that lets this occur with very little vacuum.

If the coolant were not drawn back, the engine would always be somewhat low on coolant, giving the symptoms you originally listed!

 

Where is the return line? It obviously can't be the same as the overflow.

 

Another vote for the Pettit guy being an idiot or not understanding your question.

If the system is fine while running but then after cooling overnight always seems a little short on coolant it's not drawing coolant back in from the overflow due to a very slight leak like DaveW says. Check both caps and the line from the AST to the overflow. Normally a cap wears out.

 

yo?

 

They're the same. It's the small one that goes from the AST to the plastic bottle in the passenger side fender.

 

How is that possible? The cap on the AST is pressure rated to 13lbs, so it's only open when pressure exceeds that. Why would it be open at vacuum?

When the coolant system heats up, coolant sprays out the AST cap. It does this with three brand new caps. The line to the overflow is not kinked, and when i remove it, I can blow and suck through it freely.

 

Pressure caps are more complex than people realize The cap will hold 13 pounds of pressure before it lets any coolant vent into the overflow bottle. As coolant gets hot it expands and takes up more room, that's why the overflow is there to catch it. As the coolant cools when the engine is off it will "shrink" and in order to not leave the coolant system partially empty it needs to retreive some coolant from the overflow. As the coolant cools down and contracts it creates a vacuum inside the coolant system. That vacuum has only one means to equalize itself with the outside air: suck coolant back out of the overflow through the pressure cap and back into the system. The cap actually has two seals; one at the top of the neck and another between the overflow and the bottom of the neck. The one at the top of the neck is sealed at all times. The one at the bottom of the neck is sealed until it's pressure limit is exceeded and coolant can then travel past it and out the overflow tube. When cooling the bottom seal also has a one way valve that will in fact let coolant be sucked back in from the overflow and through the bottom seal. If the top seal on the pressure cap is worn this will in effect make a vacuum leak and the system will never recover the coolant.

That ain't right. Aftermarket AST? Check it closely. It could have bad nicks in the neck which don't allow the cap to seal or the neck itself on the AST could be leaking. Look very closely and see if the leak is actually coming out where the bottom of the neck and top of the can meet.

 

There is a lightly spring-loaded check valve in the cap. It prevents free flow of coolant when the system is under positive pressure. At 13 psi, the cap main spring allows coolant to flow out into the open area leading to the outlet nipple. When the system is under vacuum, this check valve opens and allows coolant to return.

If the coolant is spewing from around the cap, either the cap outer seal is not sealing, or the nipple and/or hose to the overflow tank are blocked.

 

Huh, interesting, that makes sense.

Yes, it is a Pettit AST (that's why I asked them first), and here's the thing. The AST is fine, no kinks or nicks. It's the caps I'm worried about. CST brand, 13lbs, and I have a cooling cap pressure tester. The first stage venting happens at 13psi, like it should, but with the bleeding hole blocked, it comes out the topmost seal at 13.5psi on all of the caps.

 

Oh, and i just checked the overflow this morning, after running it hot last night, and it's dry, so i guess it IS getting sucked back in. But ALSO, when I pulled off the filler cap, there was a lot of vacuum (pop), and the coolant level dropped down (guessing it went into the AST, since that's where the gushing sound went).

 

Sounds like the hose to the overflow tank is collapsing or the return-flow check valve is not opening to allow the return of coolant to the engine.

 

The hose is fine, I promise. And the check valve is opening because otherwise how would it have pulled everything from the overflow back into the system?

 

Well..

Now things get more serious. If you really are losing coolant even though the caps are working properly, and the overflow coolant is being sucked back in, you need to find the source of the leak somewhere else in the system (leaky fitting, bad hose, or maybe even the dreaded engine O-ring failure).

There are a huge number of threads on coolant leaks and o-rings, so do a search.

 

Do the champaign bubble test.

 

Champagne test = negative
Exhaust Gas Test w/ that blue bubbling liquid = negative

My o-rings are apparently ok.

Here's the skinny: The line to the overflow was very very small, small enough that it was outflowed by the system expansion when the new thermostat opened. Apparently the old one was leaky enough that everything was all gradual and such. Not so anymore.

Well, what I did was relocate the AST to where the airpump was (thus making it the highest point in the system), and replaced the lines to the overflow and filler neck with clear reinforced hoses so I could see exactly what was going on. So, it heats up, shoots coolant into the overflow correctly with the bigger hoses, but on vacuum, it sucked air from somewhere. The connection between the new clear hose and the old rubber one that goes into the overflow and a tiny crack in the plastic connector, so i could see bubbles going up the hose. Replaced, works great.

Now, here's the thing. I hooked up a pressure guage to the system. When theres >4 inHg of vacuum, coolant will be sucked back into the AST. Thus, there will ALWAYS be a vacuum in the system after shutdown, and if i ever open the caps, the coolant will get sucked right down past the caps.

Anyway, I guess it's fine now Still runs pretty hot though.

 

The same thing happens to me since i bought the car...
It runs a little hotter than i would consider comfortable...

My ast was broken once but repaired.. could that be the problem??

What is the champagne test and the exhuast gas test? how do i do them??

I have not changed the water hoses.. My car has done more than 40,000 miles should i change the hoses??

I am worried now more than ever because i just rebuild my engine and have ran it 400 miles and yesterday when i turned the car off and left the key in the ignition so the fans would come on... The add coolant buzzer came on and it leaked some coolant... very little.. but it did leak... all the seals and o rings inside the engine are new....

I have some cooling system problem since i bought the car... the coolant keeps getting sucked in whenever i open the cap...Please help..Ohh and both my caps are without springs? that is how they were when i bought the car..should i change the caps?? Also i am at work right now but i am sure that a hose from the AST is just unplugged... is that normal?

Thanks

 

Hay, I was going to say that.