Coolant Vacuum Fill: System Holds pressure, but keeps losing vacuum at 5inHG (2+ Psi)

[Brian Mifsud]

Hi All,
I replaced all plastic fittings and hoses and overflow tank and caps on my 3.9 engine in my 2004 T-Bird. My first attempt to fill the system and burp the air failed so I bought a Mishimoto Coolant Vacuum Bleed and Pressure Test kit. I drained all the coolant, then pumped up the system to 10psi. I left if for 10 minutes, and it held fine with no drop off. Great!

I then hooked up the vacuum generator and started to pull air out of the system Full range of the dial is 30inHg or 76cmHg which is full vacuum. It starts pulling the vacuum, then with it hits 5 inHg, it drops back to zero vacuum. This repeats over and over as though a pressure relief valve is opening under vacuum and venting to the atmosphere. Should I be clamping off both lines to the degas bottle to prevent this air leak only under Vacuum? For those who have not heard about these systems, you pull all air out, switch a valve, and coolant fills all the voids inside with out air getting in.

 

[joegr]

You missed something, you have a leak. Which part(s) didn't you replace?
Don't clamp off the lines. That would make the vacuum fill completely useless.

 

[04_Sport_LS]

Why try to "reinvent the wheel" ?

The LS manual has a specific fill and bleed proceedure that works. If you couldn't get it to bleed properly... you weren't doing it right... or didn' t have the system filled with enough coolant.

It's the PRESSURE of the cooling system... that seals the cooling system. Much easier for a leak to happen when applying a vacuum.

Could be as simple as a hose leaking slighly under a vacuum (past a hose clamp) that will most likely seal under pressure.

The cooling system is designed to function at 16 psi... not 10 psi. Says that right on the degas fill cap. If it holds 16 psi pressure when pumped up... you should be good.

Then fill and bleed properly.

 

[joegr]

To be fair, vacuum filling is the recommend way in the factory service manual. It's not working for the same reason that the manual fill procedure didn't work. There is a leak or leaks somewhere, possible a plastic part that was not replaced. The OP didn't mention the radiator hoses, both of which have plastic sections. The lower hose lasts longer than the upper hose. It's also possible the radiator has a crack in the front of the passenger side tank. Other things to look at are the DCCV and the aux pump.
If the system leaks under vacuum, it is suck air in every time it cools down, and that is a problem.

 

[04_Sport_LS]

If the system leaks under vacuum, it is suck air in every time it cools down, and that is a problem.

Touche

 

[Brian Mifsud]

As I said in original post. ..... I replaced ALL plastic fittings and hoses and overflow tank and caps on. Also, waterpump, intake manifold gasket, and thermostat (of course as well is its housing).

Since these engines operate normally at 215+, the plastic ages prematurely. Turns out most car brands from 2000 on have these operating temps to meet emissions requirements for full burn.


I kept digging and found the culprit. The overflow tank has to be sealed. It allows air in when the coolant contracts to allow it to flow into the engine. I put a plastic bag under the cap and finally it pulled a vacuum and started to fill.

 

[04_Sport_LS]

Good sleuthing !!! I forgot about the overflow/degas cap.

I'm making a mental note of that.

 

[joegr]

...I kept digging and found the culprit. The overflow tank has to be sealed. It allows air in when the coolant contracts to allow it to flow into the engine. I put a plastic bag under the cap and finally it pulled a vacuum and started to fill.

Sorry, I assumed you were doing the normal way, which is to replace the cap with an adapter that connects to the vacuum fill. How were you connected to the car then?

 

[04_Sport_LS]

If I had to guess (could be wrong) probably the T-stat housing.

 

[Brian Mifsud]

Yes, T-Stat opening.. Ford calls it the engine fill cap. I bought the Mishimoto Vacuum Bleeder and Pressure Test kit. It has adapters for pretty well every car under the sun including "old fashioned" brass fill cap radiators. They screw on to radiators for oldest cars and T-Stat housing for the contemporary stuff. $84. The directions are slim at best. The Ford Manual is almost useless in so many areas, I want to throw it across the room. Makes sense why this used set was so cheap.

I did experiment with pulling a vacuum at the coolant tank also.. This would work if the engine was empty, but I kept pulling slugs of coolant. Ultimately, I did run the prescribed Ford test to open heater bleed valve, heater and defrost turn on, and put a stick on the accelerator pedal for 3 minutes at 2000RPM. It's still unsettling to see the coolant temp swing between 208-224F as "normal" via my OBD Scanner in Live Data mode.

I gotta say, I've never worked in a engine bay so inaccessible. The T-Bird was clearly packaged only for looks and no way for servicing. I had to have my wife lay upon plywood over the engine to fish her hand into the gap between brake lines to attach the new coolant tank hose several weeks ago. My hand is too big, and my back can't take all that reaching.. I really was tempted to drop the whole K-member and engine just to get to those crappy locations. I'm very tempted to sell the car now that everything is working (63K miles).

Thanks for the ideas though. I find most often (at least for me) that in the act of verbalizing the problem on forums, the answers may not be directly on target, but they often start the chain reaction in my mind of things I did not consider or check.

Car seems okay now, but I'll be checking the fluid level.

Oh and thanks, I redid the pressure test on the whole system today and it held at 15psi, but bled off anything above that (as a new overflow tank cap ought to... ).