Radiator cap OEM style vs aftermarket

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05-14-2022, 12:30 PM
82DMC12

3 Attachment(s)

Radiator cap OEM style vs aftermarket

Working on my engine temperature issue today and I rented a cooling system pressure tester. I first tried to test my new DMCMW radiator cap (Made in England, 15 PSI). I could not get any pressure at all to show on the gauge. Trying to decide if I should rent a different tester, or even buy one, but I won't bother if these testers don't work on the OEM cap. Note that the OEM cap does not have a rubber gasket on the big metal disc but an aftermarket 16 psi cap I have does (and does hold pressure on the gauge. Can anyone confirm that these caps can't be tested, or do I just have to find a different tester with a different adapter design?

By the way, I know the 15 psi cap certainly holds some pressure because I don't have coolant boiling out everywhere and after 90 minutes of the car being parked there was still pressure behind the cap when I removed it to check header bottle level. It's just not helping my diagnosis if I can't be sure the cap works on a tester.

This tester also shows I have 11 PSI in the system after 15 minutes (originally pumped to 15 PSI), but I think any loss is just around the pump because I can hear a tiny sizzle coming from around there and there are no visual leaks on the car. Perhaps a different tester would fit or hold better.

Original style cap - 15 PSI made in England
Attachment 68460 Attachment 68461

Aftermarket 16 PSI cap
Attachment 68459
05-14-2022, 12:56 PM
WHO1DMC

I only have experience with the stainless header bottle.
I found that the 15psi GM radiator cap works great. From what I can see that's what it's designed for.

Dave B
05-14-2022, 12:58 PM
82DMC12

Right, I also have the SS bottle. Honestly I've never seen a radiator cap that didn't have a second rubber gasket around the spring but I've chatted a couple other people who looked at theirs and said that's how their cap is also. I'm having a hard time testing it so I might just go back to the 16 PSI aftermarket cap. I wasn't able to find a 15 psi anywhere locally.

Looking at the header bottle you can see that the main seal is the smaller gasket so I'm not sure how critical this is at all.

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05-14-2022, 02:29 PM
Rich

The OE caps can be tested. Rent or buy a tester with the correct (Size A) rad cap adapter. The adapter you used was for a different cap design if it held no pressure on the new DMCMW/UK cap.

The upper (larger dia.) gasket on your aftermarket cap isn't necessary for a D with the correct coolant header bottle. The seal between bottle and cap is made against the small diameter flange at the bottom of the filler neck. Coolant or vapor that leaks past that seal flows into the overflow/dump hose.

A proper coolant pressure test kit (pump, gauge together with a cap test adapter) is a good investment. A 40+ year old cooling system with dozens of connections/fittings calls for it. The pump also helps with cold-bleeding the system. This linked test kit has an adapter that fits a D.

Quote:

Originally Posted by 82DMC12

Working on my engine temperature issue today and I rented a cooling system pressure tester. I first tried to test my new DMCMW radiator cap (Made in England, 15 PSI). I could not get any pressure at all to show on the gauge. Trying to decide if I should rent a different tester, or even buy one, but I won't bother if these testers don't work on the OEM cap. Note that the OEM cap does not have a rubber gasket on the big metal disc but an aftermarket 16 psi cap I have does (and does hold pressure on the gauge. Can anyone confirm that these caps can't be tested, or do I just have to find a different tester with a different adapter design?

By the way, I know the 15 psi cap certainly holds some pressure because I don't have coolant boiling out everywhere and after 90 minutes of the car being parked there was still pressure behind the cap when I removed it to check header bottle level. It's just not helping my diagnosis if I can't be sure the cap works on a tester.

This tester also shows I have 11 PSI in the system after 15 minutes (originally pumped to 15 PSI), but I think any loss is just around the pump because I can hear a tiny sizzle coming from around there and there are no visual leaks on the car. Perhaps a different tester would fit or hold better.
05-14-2022, 02:39 PM
82DMC12

Quote:

Originally Posted by Rich

The OE caps can be tested. Rent or buy a tester with the correct (Size A) rad cap adapter. The adapter you used was for a different cap design if it held no pressure on the new DMCMW/UK cap.

The upper (larger dia.) gasket on your aftermarket cap isn't necessary for a D with the correct coolant header bottle. The seal between bottle and cap is made against the small diameter flange at the bottom of the filler neck. Coolant or vapor that leaks past that seal flows into the overflow/dump hose.

A proper coolant pressure test kit (pump, gauge together with a cap test adapter) is a good investment. A 40+ year old cooling system with dozens of connections/fittings calls for it. The pump also helps with cold-bleeding the system. This linked test kit has an adapter that fits a D.

Thanks - I'm going to buy a kit and you're right, with so many connections it would be wise to have my own so I can test annually anyway.

Will report back soon....

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05-14-2022, 05:58 PM
82DMC12

So I ended up buying a pressure tester at NAPA, part # 700-3025. It's working for both radiator caps, and I had no trouble pumping up my SS header bottle to 15 psi. No leaks and still 15 psi after 20 minutes. I was getting false negatives from crappy rental pumps.

I have no problem at all buying tools that I might use more than once, and I guess a good quality cooling system tester would qualify.

No idea why the gasket-less 15 psi OEM cap would not test properly with the rental pump.https://uploads.tapatalk-cdn.com/202...8ff382b90e.jpg

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05-14-2022, 07:11 PM
Rich

Good move.

That NAPA kit looks identical to the Stant kit aside from the logo/ID on the pump and gauge. It will last a long time.

Quote:

Originally Posted by 82DMC12

So I ended up buying a pressure tester at NAPA, part # 700-3025.

I have no problem at all buying tools that I might use more than once, and I guess a good quality cooling system tester would qualify.
05-14-2022, 07:15 PM
82DMC12

Quote:

Originally Posted by Rich

Good move.

That NAPA kit looks identical to the Stant kit aside from the logo/ID on the pump and gauge. It will last a long time.

Still at 15 psi after 75 minutes so I'm going to say my system is plenty tight enough. On to the next task....

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