Pressure testing an FD radiator
 

My "add coolant" alarm starting coming on this week and I've read oodles of "add coolant" threads with some good suggestions but I have a specific question about pressure testing the radiator.

A friend of mine connected his pressure tester to the fill cap, when he reaches some high level (I forgot to note the pressure), the coolant is heard bubbling out somewhere (not the ground). I'm assuming this is bubbling into the overflow tank, is that what is expected to happen?

There was definitely some air in the system because after sitting for a couple hours the coolant drops down the filler neck. I'm just curious about what to expect while pressure testing. The shop manual says not to exceed 14.9psi, maybe we went just over that?

Is it normal for the pressure to drop some if the engine is rev'd? Neither of us can explain that.

do you still have the stock ast? thats probably where your leak is. but where is the bubbling coming from note the location also what was the pressure you used try it again and note both...

The bubbling noise is probably due to the 0.9 bar (13psi) pressure-relief cap on the AST. When your system pressure exceeds 13psi, the AST cap should open, and vent excess air/fluid to the overflow tank. You can watch this by disconnecting the coolant line from the top of the AST (near the cap). Connect a clear piece of hose (be sure it's air-tight) to that nipple, and put the other end of the hose into a water bottle. I'd also like to know what pressure your cap is venting at.

-s-

replace caps?
 

It's venting at ~11psi instead of 13psi(.9); I just ordered new caps.
Yes, stock AST.. ..I'll keep an eye on the coolant level and see if it remains stable after I replace the caps.

Thanks,
Dan

blocked hoses somewhere?
 

Today I topped off the filler and checked the overflow and headed to work. The water tempurature stabilized at ~94-c and I parked the car while at the office.

At the end of the work day, I checked the filler neck and it needed filling again (not sure how much; 1/2-cup?). The overflow seems to be higher. Interesting that there was a slight vacuum when I released the filler cap.

My random guess is that the recover/return hose may be clogged? I'll see if this repeats with each heat cycle. Any other suggestions?

rebuild time
 

I replaced both caps. Had the coolant flushed. Same symptoms. I replaced the AST and the overflow tank. I have another radiator, but I think that's a waste of time:

I saw the coolant turn dark in the filler funnel I was using (the kind that stays connected to the filler while the engine idles) ...also saw a small stream of bubbles which I've heard it also a very bad sign. I didn't think much of it at first, I thought it was just air from the parts I had replaced, until the coolant turned dirty.

Looks like symptoms of a coolant leak.. ..time for a rebuild? Not much else can explain dark coolant (the old overflow tank was full of dark fluid also).

Dark coolant?? I would pressure test again and maybe have a shop check the coolant for hydrocarbons before you start talkin' rebuild. Unless you fully burped the system, you'll get a few bubbles and even then, the champagne test isn't the greatest test for coolant seal failure.

It sounds like you've got a coolant seal failure. The coolant o-rings are what keep the combustion chamber sealed off from the coolant jackets. When they fail (from overheating or from boosting when the car is too cold) it will allow exhaust gases to mix with the coolant, which is causing the bubbles and discoloration that you're seeing.


If you want to be absolutely sure, you can take your car to a mechanic and have them test the coolant for exhaust gases. Another thing you can do is use your filler funnel and let the car idle for a while. When the car is warmed up, open the throttle a bit and watch the stream of bubbles. With blown coolant seals, you should be able to see more bubbles when you open the throttle.

-s-