I have searchs for hours...anyway in-a-way this is an unique issue so thought it would help if got posted.

Last night with a new NAPA 13 psi pressure cap, I decided to burb/fill the cooling system (engine was cool 24 hrs). The following steps were done in order:

1. Disconnect upper coolant hose to TB
2. Fill the alum AST, put new cap on
3. Fill the filler neck

While doing step # 3, the overflow tank start to leak coolant. WTF...checked the OF tank it was past F. Siped the tank dry (almost); repeat step # 3. As soon as the filler neck full with coolant, AST release coolant into the OF tank.

Question:

1. Bad AST?
2. Another bad NAPA brand pressure cap? Cap was pressure tested. It held 13 psi for 30 sec then slowly drop pressure. With a cold engine the capped AST shouldn't release coolant to overflow tank at all...correct?

Thanks.

 

Sounds like your AST isn't sealing with the cap. If the cap works correctly on the pressure tester, I'd suspect the AST. Are you absolutely sure you've got the hoses routed correctly? The very top hose (closest to the cap) should be going to the expansion tank.

-s-


PS, you can't beat a filler funnel for removing air from a freshly-filled cooling system. From the FAQ:

Coolant Burping Tool/ Radiator Cap Funnel
https://www.rx7club.com/3rd-generation-specific-1993-2002-16/radiator-cap-funnel-eliminate-air-bubbles-342309/

 

ditto, sounds like the hoses are not on the AST correctly.

 

The only thing I would buy at NAPA for my FD is NGK plugs & maybe some air freshener strips.

I made the MAJOR mistake of buying a NAPA thermostat when I purchased my car (7 years ago).

The motor blew a coolant seal 2 months later.

No coolant low light or temp out of the normal range (I know the stock gauge only reads "cold-ok-too late")

Anyway, talked to Atkins rotary & was told they get 2-3 motor rebuilds a month due to non oem thermostats. Told me the parts book list ALL Mazda cars & trucks with the same part #.

They went on to say they even when the engine is cold some coolant need to circulate through the motor to prevent localized hot spots.

So my advice is to pitch that POS rad cap & get a real one. I bet your problems will be solved.

I pitched a bitch at NAPA & told them they blew my motor. They said no problem, all their parts are covered by a warranty. IM thinking they might spring for my rebuild.

NOPE......Bring in the old thermostat & they would give me another FOR FREE.

 

All hosing are on correctly...

Top - OF tank
Mid - Filler neck
Bottom - radiator

I am getting another cap, if this doesn't work then the AST is bad.

Everything was working perfectly fine with the stock plastic AST.

 

Thursday's afternoon I went back to NAPA got another cap, the Stant unit wasn't avail so I settled for a Balkamp unit...it was $2 more so it must be good (yea).

After put the cap onto the AST...look around...and hey the Pettit AST sat 2" lower than the stock AST and stock intercooler. A home fabricated bracket to elevate the AST back up to 1.5".

While filling the filler neck I notice a different of coolant level...hey maybe I am on the right track.

Today test Drive @ 9:30 am, ambient temperature = ~90*
It wasn't an intensive drive but sp still averaging 80 mph, with AC set to 2, water temp never went past 90*C. Best of all no spillage from the OF tank.

With the Pettit AST coolant temp drop about 2*C from stock AST. However after examing the diff between the two I am curious which one is better.

Looking to buy a stock AST, low mile, good condition.

Thanks.

 

I have a stock AST Low milage and still the dark green color. I had a Pettit AST and wound up getiing the Tri-power AST that uses the stock rad. cap, had much better results with the other AST and don't care for the Pettit one as it thin and flimsy. With the proper pressure cap on the AST coolant should never flow freely from the AST to the recovery tank if it's sealing properly no matter what level it's installed at.

 

All of this hassel is caused by you not removing the AST.
You can lead a horse to water, but can't make it drink!

 

not removing the AST...it's there for a reason...frankly I doubt none of us here know more than the engineers designed this motor.

1995 Ford Mustang Cobra R has an AST...I am sure American engineers thought it was necessary since the R was built for racing.

 

^ Yes, I agree completely.