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Discussion Starter · #1 ·
I have a 351W with a basic "donor" coloing system and stock mustang burp tank. I have a CSR T-filler neck installed at the highest point in the system to help get rid of air bubbles. I have the "overflow" nozzle on the T-filler connected to the base of the burp tank and the top of the radiator has a plug sealing where the cap would normally sit.

After a long drive sitting at lights in traffic the engine was plenty warm and the fan was running and things were well within temperature and appeared to be working fine. But after I pulled in at home and things cooled off I came out again later to find a small puddle of coolant under the car.

It was hard to tell the source, but the T-filler was very damp and I’m guessing that it was seeping out from under the cap. Another key is that when I took the cap off it released a vacuum and things gurgled down in the hoses. So this led me to a few questions:

1. Why was there such a vacuum in the coolant system? Shouldn’t the burp tank connection allowed coolant to fill back up into the T-filler area preventing that vacuum? Because the cap seal is below the overflow connection in the T-filler is there some valve that is supposed to let fluid in for these vacuum situations?

2. What is the proper way to debug this kind of issue?

3. What is the basic functional model for the burp tank connection (Burp tank 101)? This would help me debug the problem.

Thanks,
Bret
 

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FFCobra Captain
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Having a vacuum in the cooling system is normal...it's what you want.

Simple explanation:
Pressure builds up in the engine until it gets higher than the cap rating.
The cap opens to release the pressure and air trapped at the high point, should go to the overflow tank.
When the engine cools off and the fluid compresses, it pulls in extra coolant from the overflow into the engine.
After a few cycles, you have no air bubbles left in the engine. You will still have a bit of a sucking sound if you pull the cap.


If you had fluid leaking from the radiator cap, that's a problem. Make sure the hose clamps are tight and that your cap is sealing properly...but since you heard the sucking sound, it probably is fine.

If you had the overflow filled up too high, then you'd have coolant spitting out from the top of that container when the coolant expanded and had nowhere to go but out (because the overflow was full). You want the overflow to be around 1/3rd full when the engine is cool (more or less depending on how much moves to the overflow tank when the engine is hot). It's a trial & error thing. Just make sure the overflow tank isn't empty...otherwise, you're sucking air back into the engine.
 

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Mark 3.1 (Sold)
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You need to be more specific about what your "basic donor cooling system" consists of. If you have routed your T filler vent to the large plastic tank that straddles the top of the radiator, you have connected the vent to a system pressure point, attempting to send fluid back to the same pressurized system, so the cap is not relieving fluid and system pressure unless it is leaking around the cap. Since you have created a sealed system, with no vent to atmosphere, air can not get back into the system when it cools, and you develop a vacuum. Normally, the cap will relieve the vacuum by drawing air and/or water back into the system thru the overflow tank which is vented to atmosphere.
 

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Sounds like everything is hooked up correctly. Is the level in the burp tank going up and down with the heat/cool cycles?? Yes there is supposed to be a small valve in the cap to let liquid return to the system. Check to see if that is free/not stuck closed. As said check tighten all your joints and clamps. I had a couple small leaks after a few heat cool cycles when the car was first put on the road. After retightening, with the exception of a water pump problem after 7000+ miles, everything has stayed leak free.
 

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FFCobra Craftsman
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I wonder if your cap is malfunctioning. Apparantly this is something new after a bunch of no problem miles right? I actually haven't removed my cap until the car sat overnight but I agree there should be very little to no vacuum in the system as the vacuum should be pulling coolant back from the tank as the car cools. How long had the car sat before you removed the cap? I am also a little concerned that you saw dampness at the T-filler. Any overpressure should be directed to the overflow tank and no leaks at the filler. As I write this it makes me think there may be a kink or blockage in the hose from the T-filler to the tank. That would explain the leakage at the filler AND the vacuum in the system.
 

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Discussion Starter · #6 ·
Thanks everyone for the ideas.

Criag, I was thinking along the same lines. If the hose from the overflow on the T-filler to the burp tank is clogged somehow, that would explain this.

When I said "donor coolant" I mean the setup from a mustang. So the radiator, shroud and the burp tank. The overflow hose used to go from the top of the radiator by the cap do the bottomr of the tank. It now goes from the similar location on the T-filler to the bottom of the tank.

I just rolled over 800 miles, but this was really the first good run in almost a year (paint and then many distractions and other issues). Very early on I had a leak around the thermostat housing, but this location is different. It's clearly not at the burp tank or radiator area, it's right under the T-filler and some issue with the capture tank matches most of the behavior.

In the expansion and contraction phases, I assume that the top of the radiator cap seals as much as the "pressure" spring part of the cap. Otherwise it would compromise the expansion tank system operation. But I assume if the expansion line was cogged or kinked it pressure would build up and the expanding coolant would work its way out past the top part of the cap. Then on contraction it would build up the kind of vacuum I witnesed.

Tonight I'll give it a good inspection and see if I can find a way to blow air through it or something.

Let me know if anyone has any other debug ideas. Thanks. Bret
 

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FFCobra Fanatic
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What is the psi rateing on the radiator cap?
Is the radiator cap a pressure/vacum cap, similar to what would be on the original donor set up?
The radiator cap has to be able to allow fluid/coolant travel back and forth between the over flow bottle and cooling system depending on temp/pressure.
Bad cap or restriction in hose between T filler and averflow are likely cause.
Does level in overflow tank rise and fall depending on engine temps?
Few checks to do:)
 

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Discussion Starter · #8 ·
OK, I've done some checking and I'm afraid it won't be as obvious as I'd like. The overflow tube doesn't seem to be obstructed. I was able to connect a tube to the overflow spot and blow bubbles in the expansion tank.

I have a CSR T filler and matching cap:
Inline Radiator Hose Filler - 1.25 to 1.25 Inches - Cooling Products - CSR
Polished Chrome Radiator Cap - Cooling Products - CSR

The cap description says 14-18 PSI. I took the chrome piece off but the main cap had no writing, so I couldn't confirm anythin. The two rubber rings looked OK and it seemed like it should be working. I can see how on expansion the coolant should ooze past the main seal and be pressured down the overflow tube and then on contraction the center metal piece should pull down with the vacuum to allow coolant to ooze back into the system.

What is the next suggestion for debugging this? Fill it up and let it run until hot to see what happens? See if I can get a replacement cap (that will fit under the chrome piece) and see if that solves the issue? Anything else?

Thanks,
Bret
 

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NAPA has hexagon shaped thermostats. You want a Vented cap…it vents to the recirculating tank. You shouldn’t have fluid escaping from around the cap I would think it is either the cap or the mating surface on the filler has a bugger on it…for some reason the cap is not sealing properly. Is the smaller diameter seal able to move freely up and down in the bore of the filler neck?

Jack
 

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FFCobra Craftsman
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Does your cap look like mine?

Click to enlarge.
The small brass disc in the center is loose. It needs to move so it seals when psuhed against the rubber but can move about 1/8 inch away from the rubber so coolant can flow back into the filler as the engine cools.
 

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Discussion Starter · #13 ·
Yes, it's the donor mustang tank. So it's the translucent plastic off-white tank with an air vent at the top and the expansion hose connected at the bottom.

The cap looks pretty much like the one Craig pictured. The lower rubber seal near the bottom has the smaller metal disc that can spring down like would happen under a vacuum during the cooling phase. The whole rubber seal area springs up and I would think during the expansion phase when the pressure is high it would let coolant seep out. On the top there is another rubber seal ring (can't see it on this picture, but it's probably there too). This should seal around the top of the filler area so the coolant goes down the overflow tube instead of being squished out the top of the filler port.

Mine does have that small brass disc in the center. It's not always loose, but when I compress the spring as it would be when installed the disc is loose and can move away from the seal.

I wished I'd taken a picture down the filler neck last night so I could post this. There is the top area seal point and then the seal area for the compression seal. The opening under the compression seal point is round for maybe 1/8" of depth and then it switches to an oval opening for the rest of the way down to the main passage. The width of this oval is 7/8" which exactly matches the diameter of the brass disc being discussed. At first I thougth maybe the disc was hanging up on this oval section and therefore not allowing fluid to return. But I think the 1/8" depth at the wider circle would be sufficient. And that would explain the vacuum, but not the coolant leak near the filler area (if it was near the expansion tank it would've made sense, but it wasn't). So I don't think that's the issue.

The best next step for debug I can come up with is to refill the system and then let it get nice and warm while watching it to see what's happening.

It's possible there is a leak somewhere in the filler neck as well which would explain this behavior or as Jack said that the mating surface of the cap or filler could have issues and not be sealing. Is there an easy way to hook up the air compressor to the system and test for leaks under pressure?

Any other debug ideas?

Thanks,
Bret
 

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Snake Farmer
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There is a hand pump type pressure tester designed for testing cooling systems. I don't think I would want to use an air compressor to test it..

Occasionally some of the cast T fillers have had issues, from what I recall. Could be a pinhole in the casting. Cast units also can be a little rough around the sealing point.
You might just want to try a new cap to start with. A fresh seal, and a new spring may solve the problem. Cheap too..
 

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FFCobra Captain
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Autozone has a 'loan a tool' program. Get the cooling system pressure tester and see if your system holds pressure. It's easy to check.
 

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At this stage maybe buy a replacement cap as they are not expensive and that will rule out a bad cap.
Don't forget everytime you remove the filler cap you will have to re burp the cooling system.
Allow engine to come up to operating temp then allowed to cool back down.
This may need to happen twice to get all air out.
Watch the coolant level in overflow bottle when cool then when hot and note the change(pencil mark on bottle):)
 

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Discussion Starter · #17 ·
Thanks for the suggestions guys. I won't have time to do any testing the next couple of days but I should be able to play with it on the weekend.

I think I'll start by filling it up and getting things nice and warm to see if I can find the leak point. That may answer a lot of questions. I may pick up a new cap anyway because I really think there's somethinb about the top seal on this cap (I just can't find the evidence). Depending on the results of those tests I'll see if I can borrow the pressure tester and pinpoint any other issues.

Thanks for all the help. I couldn't have gotten this far on the car without the help of the great folks on this forum.
-Bret
 

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FFCobra Craftsman
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When you are filling the system make sure to remove a temp sensor or heater hose from the manifold. This lets air escape as you fill the T-filler. Once coolant comes out of the hose or sender port, cap it off and fill a little more at the filler and install the pressure cap. From now on never remove the cap. All topping up as you cycle from cold to hot should be done at the overflow tank.

Click to enlarge.
In this pic you can see my heater hose to the right of the top rad hose. Just to the right of the rad hose you can see the small brass nut that holds the wire on the temp sensor. Either is a good air bleed.
 

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Discussion Starter · #19 ·
Thanks Craig. The air bleed is a great idea. I did drill a small hole in the thermostat to help get the air out so I can see how having an air bleed on the manifold will help. I used the fancy shrink wrap-like hose clamps so it's a little hard to remove the heater hose. I think I can pull the temp sensor though so I'll give this a shot. If I can't get an air bleed on the manifold, do you have any suggestions?

Also, do I need to do anything to get the long overflow hose primed? For the short stock setup I can see that it would just self prime. But for a hose this long (probably 8') that could cause a problem. I could "suck" fluid up through the hose, but the thought of coolant in my mouth is very unpleasant. Maybe I could blow into the overflow tank somehow after blocking other hole at the top. Ideas? Is this needed?\

This is very sad because I just got it back on the road after paint and we're having the driest fall on record but my cobra is stuck in the garage :-(

Thanks,
Bret
 
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