Rod Underwood

Does anyone know the radiator cap pressure rating off the top of their head. I'm sure it's in the Workshop Manual somewhere, but I'm just being lazy tonight.

I blew the heater valve a few weeks ago, and just blew the top radiator hose a few days ago. I'm wondering if the radiator cap may be holding too much pressure in the system.

Thanks,
Rod

928 obsession

I do not know the cap pressure rating off hand, but if it hasn't been replaced I wouldn't hesitate to replace the cap. They tend to hold too much pressure once they're old.

hans14914

Rod,

I think stock is 13psi.

Rod Underwood

Thanks, it's already been replaced by the PO and that's why I think it may not be correct and has contributed to the blow outs.

I'll replace it and see what happens.

Rod

ammonman

IIRC it is 1 Bar (14.7 psig).

Mike

dr bob

The rating is 1 bar, or about 1 atmosphere, or about 14.69 PSI at sea level. The genuine Behr caps are so cheap at the 928 vendors that it's a crime not to put a new one on whenever you change coolant, and for sure every five years or so when you change the t-belt and all the coolant hoses. JMHO anyway.

Landseer

Keep an old one. You can drill-out the rivet and replace it with a port. Then, to detect leaks, you can put just a little pressure into the system via the port (and a hand-held pressure wand). 5 or 6 psi is all it took to find two tiny hose leaks on mine.

Rod Underwood

Great idea!!!

Thanks
Rod

dr bob

I like the idea, but... The cap is the high-pressure relief valve in the cooling system. If you use the cap for a pressure port, especially with no guage or regulator, you risk discovering the weakest link in the system, and that may be found at a pressure higher than the normal relief pressure of the cap.

Landseer

Absolutely right. It shouldn't have the quick connect. The hose should never by connected to the cap directly.

I'm setting the pressure regulator on the compressor to sub-10 psi.
Then, manually pressing the rubber end of the hand-activated nozzle against the cap shown and gently applying a little bit of pressure. Very little bit is all it takes.

littleball_s4

One bar!

You sure?

It's quite a lot for a coolant system. I'm used to 0.2 - 0.6 bar range. Even in huge systems like 16 litres trucks.

1 bar is quite a lot. For the radiator cap, which is like 5x30cm, 600cm2, it would require a clamping force of 600kg!

Apart from that, what's the need for so high pressure? at 1.3, water is not going to boil before engine overheats badly.

Not saying your data is wrong, I have no idea, just saying it seems too high.

Best regards,

littleball_s4

Mine has "100" on it, so yes, it's one bar.

No wonder why we (me included) keep on blowing up coolers and hoses.

In my own experience, the only drawback when using less pressure rating is when you don't bleed the system properly and/or set the level too high or too low. When bubbles are trapped somewhere not below the cap, or the whole thing boils, after the race the cap start "venting" the excess pressure. But if the coolant go all the way up to the cap, it takes ages to "vent" (or should I say "bleed") the pressure, the cap is throwing coolant out for 5 minutes. If this happen to, say, 0.7bars, the cap make a big mess, your car seems on fire, etc. If you have a higher rating then you will not see anything, but everything will be under big stress for a while.

If I don't forget, I'll try a lower pressure rating and let you know the results.

M. Requin

Interesting! Looks like I'm guilty of blindly following the PO's bad example, by replacing the old 13psi cap with a new 13psi cap. I haven't had any problems in 30K miles, so at least with my old fart driving style there doesn't seem to be a significant downside. The WSM does say .9 - 1.15 bar are the limits for the cap, so it is definitely a 1 bar item. Comments, anyone?

R.Pires

Hi,

I've just ordered one of these for my car. Is it an ok replacement?

http://cgi.ebay.co.uk/PORSCHE-928-77...es_CarParts_SM

Thanks for your opinions!

Regards

dr bob

Coincidentally, I started gathering parts for a little manifold for pressurising the system using air from the compressor. It could just as easily come from a bicycle pump for that matter, but what's the fun in that?

I have a brass T with two hose fittings screwed in. Intent is to place in the end of the vent hose on the radiator with a short hose stub, and connect air to it through a small low-pressure instrument-size regulator, followed by a low-pressure gauge (0-30 PSI is what I have) and a shut-off valve. Adjust the regulator pressure with the valve closed, connect to the system, open the valve to allow air into the system, then close the valve and see if it holds. If it doesn't hold, open the valve to re-pressurise, find the leaks and fix them. If it does hold OK, crack the reservoir cap and bleed off the pressure, put the cap back on, and restore the original hose connection to the radiator. The small size of the regulator and maintaining the reservoir cap on the reservoir means that it will be hard to over-pressure the system accidentally.

I'll post pics and a parts list when I get it all together. I'm missing the needle valve (thought I had one in my junk bin...) so it should be ready soon. Car is running a bit warmer than usual, and I can hear some hissing when I stop in the garage. Might be AC residual noises or might be cooling system. No coolant loss through. [scratches head]

It was 93º yesterday and the car was warm enough after the slow hill climb that the manifold switch had the fans running slowly for a bit on shutdown. That's unusual even on a 90º+ day, although it has happened before in 105º+ heat.