90 740ti, black 75kPa cap.

My cap is venting pressure (and LOTS of coolant) after the car warms up. After losing a good portion of my coolant many times, and replacing it with distilled water, I'm sure I'm running close to pure water.

Is there any way to test that the cap HOLDS pressure up to 75 kPa before I buy a new one? I bought it just recently, and can press the vent open with my fingernail. The top rad hose does seem to hold quite a bit of pressure, but I can squeeze it fairly easily.

Yes, I'm having cooling system issues. Right now I've been dumping distilled (doesn't leave deposits) water into it. I think it may either be venting because
1) Air bubble, which I keep clearing out. (water pump may be leaking from weep hole)
2) Pure water boils at 212.... I need more antifreeze in the thing.
3) Black pressure cap is either too low a pressure, or is defective.


Any help would be appreciated. I am broke BrOkE, BROKE, so any free solutions would be good.... my financial aid is 18 days late, and I'm going to school full time...

Anyway,
Happy Bricking,
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond

Hi Will,

Water at 740 cap pressure should make it to 260ºF as a liquid. Sure, the glycol helps with that, but is not likely to be the difference. I share Steve's concern you may have a combustion leak. Pressure testing a cap is easiest done by tapping the small vent hose at the top of the expansion tank.






--
Art Benstein near Baltimore

The short fortune-teller who escaped from prison was a small medium at large.

Will, when your financial aid comes in, may I suggest that you buy a white, 100kPa cap? These are also known as 1.00 Bar caps, because 1.0 bar = 100 kPa. I use one on my 1991 745 SE Turbo. The very high pressure of the green, 150 kPa caps scared me for cooling systems this old and with the known weaknesses that they have (heater control valve, etc). But I thought that the very low pressure of the 75 kPa caps would not prevent localized boiling in the hottest parts of the engine, the parts that need constant cooling the most. Localized boiling disrupts cooling by breaking the coolant-to-metal contact at the site of the boiling. I was able to find the 100kPa caps at FCP Groton. When you input your car's make, model, and year from the drop-down menus, just tell the site that you have a Volvo, 240/260 diesel, 1981. The caps are just $4.99 + shipping. It seems that a Volvo screw-on cap will fit nearly any Volvo coolant tank: mine is on a new, genuine Volvo 960 tank in a 740, and it is identical to the old green and black caps that I have, and will fit the original 740 tank too. So, compatibility is not a problem.

If you do install a 100kPa cap, make sure that your hoses are in good enough shape to handle the slightly increased pressure. And, of course, make sure your heater control valve is in good shape. I have had new ones, of the all plastic design, begin to fail on me in just a couple of months.

--
Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

Thanks to both of you. I was looking earlier for the pressure ratings of the caps and couldn't find it.
I was debating ordering a medium pressure cap, but didn't see the listing around anywhere.

As for the cause of the "boiling"... I'm about to post about that.

Happy Bricking,
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond

Thanks to both of you. I was looking earlier for the pressure ratings of the caps and couldn't find it.
I was debating ordering a medium pressure cap, but didn't see the listing around anywhere.

As for the cause of the "boiling"... I'm about to post about that.

Happy Bricking,
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond

2) Pure water boils at 212.... I need more antifreeze in the thing.

That's the cause. The cap is also audibly venting a little pressure, so I'll upgrade, but mostly it was a lack of antifreeze.

It happened like this:

1) changed thermostat and didn't allow car to fully warm up before driving. Created an air bubble.
2) Replaced missing coolant with distilled water, which ruined the anti-boil properties.
3) Coolant repeatedly boiled and was refilled with distilled water until it was pure water.

Fixed this by letting some boil out (lazy way) and refilling with straight antifreeze.
Tested by letting it idle for 30 minutes and with some engine revving and monitored coolant expansion after shut-off.


Happy Bricking, hope this ends up helping someone.
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond

First check for combustion gases in the coolant before you buy a new cap. You can get a small test kit at NAPA (Lisle 75500) which, when mixed with coolant, will show whether you have exhaust gas in the coolant and a bad head gasket. Or have a mechanic do the test. It takes about five minutes. If you have no leak, then buy a new cap.
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See the 700/900 FAQ at the drop-down menu above right.