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Radiator cap pressure

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bluemiata90

bluemiata90

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I got my newly purchased TR4A running and noticed small amounts of water dripping from the waterpump. No problem, but I also noticed that the pressure cap reads 14 to 16 psi. Seems high to me, so I went to the local (non british) autoparts store for a new cap. They say it should be 7 psi, my Haynes manual mentions something about 3.5 to 4.5 psi. What is the correct cap pressure for a TR4A. Funny thing is my temp guage never got hot and read about 170 degrees the whole time I was driving it (about 45 min.) /ubbthreads/images/graemlins/confused.gif
 
I think the 4 pounds is correct if I recall from when I first got my TR4A on the road. Mine also dripped from the water pump a bit, but I ended up changing the pump to fix that. 16 pounds seems high, I would replace the cap first.
 
I believe that 7# is correct... the TR3s were a 4# cap (different cap, longer reach).

Using a higher pressure cap would not effect running temps but could aagravate leaks and bother the heater core.

I assume your 170 degrees is a conversion from the Celsius reading on your gauge... that could be normal as these gauges are not totally accurate (big surprise).
 
I'm sure your correct about the gauge reading, all I know is that it was in the normal range and didn't boil over. I purchased the 7 pound cap and will run the car tomorrow to see if it helps stop the leak and hopefully not effect the temp. Thanks for the input.
This site has been a savior since I got back into british cars about 1 year ago. This winter I'm going to continue my restoration of my TR3 and put the TR4 on hold until next summer. I'm sure I'll be utilizing this site on a regular basis this winter. Anyways, thanks again for the input.
 
Hi,

Your TR4 definitely uses 7 psi in the cooling system. This is dictated by the heater core more than anything else. Early TR4 had a long neck radiator just like TR3, but use a 7 psi cap. Earlier cars also need 1" reach cap, later use .75" reach. However, measure yours to be sure. The radiator cap neck is often replaced in the course of a rebuild.

TR3 use a 4 psi cap, again because of the heater core.

Higher pressure will actually act to prevent boiling and might help the motor cool more efficiently. Many modern cars use over 20 psi cooling systems. Racers increase this in old and new alike, to improve cooling efficiency.

The leak at the water pump might go away with a proper cap on the radiator. Or, it might not. Often a few drops are the first sign that a new pump will be needed soon, I'm sorry to say.

The newer pumps have better seals than the old ones. The old ones required a "dry break-in" of the seals, actually running the pump without coolant for about 45 seconds, to seat the seal. This often wasn't done when the pumps were replaced and the seal never really seats, the seal fails early. Newer pumps usually have seals that don't require this break-in. If you end up replacing it, be sure to ask any pump supplier for the proper procedure for that particular pump.

Regarding caps, there are very original looking ones available from TRF, perhaps some of the other LBC suppliers. I've got a few of these in the garage but ended up using a Prestone cap from the auto parts place down the street. The Prestone cap has double seals, one at the smaller, inner flange, another just inside the cap to seal on the upper rim of the neck. I figure that's a better arrangement if the car is fitted with an overflow recovery bottle. The original appearance caps *might* not draw the fluid back in as well, *might* let the slight vacuum from the cooling system leak past.

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L
 
For what it is worth....

I just finished battling an overheating problem with a 2000 Dodge. At cruising speeds and ambient temps over 80 degrees I'd blow all the coolant out the overflow bottle. I flushed out everything and patched a hole in the bottom of the bottle. Still losing coolant. So I replaced the 16lb cap with one of these "Prestone" jobs from "Twilight/Auto-zone".

The cap was noticably cheaper than the original I pulled off. It appeared to be made from stamping out an old swanson TV dinner tray. I still was losing coolant. I could find no other leaks, thermostat was working fine, core held pressure. So finally I went down to Napa and bought a genuine "Hecho in Mexico" original cap for the same price as the Prestone replacement. No more overheating and it's been working fine in 95+ degree weather for a month now.

But... the double seal cap is definately the way to go if you want a "closed" recovery system. I'd just be wary of the cheap replacement ones.
 
[ QUOTE ]
Regarding caps, there are very original looking ones available from TRF... The original appearance caps *might* not draw the fluid back in as well...

[/ QUOTE ]

Alan -- FWIW, I added a TR4A overflow bottle to my TR4 using the original cap and it works fine (also looks better than some aftermarket overflow). This doesn't address the quality of the replacement caps from TRF but they may well serve that purpose too.
 
Hi Geo,

Glad to hear the repro caps work fine.

I really had no reason to believe the more accurate looking caps wouldn't work, and they would especially be okay on cars not using an overflow recovery bottle. I just feel more secure with the double-seal type even though it looks a little more modern due to the red plastic "locking/safety" lever on top.

By the way, that plastic lever makes the cap taller and it slightly interfered with the hood on my TR4 until I moved the radiator forward a little and then adjusted the braces to lean it back a bit more. This happens to be the "long neck" early TR4 radiator that has a 3.25" thick electric fan installed, which fits very close to the cross brace.

There probably would be plenty of adjustability or no cap clearance issues at all with later, "no-neck" radiator and/or a stock fan setup.

Yes, I've got a TR4A rad overflow tank too, but not installed on the car. I hope to do some vintage racing with the car eventually and most organizations require a larger overflow catch tank on the radiator. Many of these are not "return" tanks, just catch tanks. Moroso makes one exception, an aluminum tank that I plan to use with the radiator cap now installed. If I have any cooling problems, I might experiment with slightly higher pressure caps, but I have no heater installed to worry about.

Incidentally, my car really shouldn't have the long-neck rad, based upon the chassis number. I suspect it was a replacement from another car, along with a few other things. I know the car was hit and repaired on the right/front sometime before I bought it.

Finally, the radiator cap that *was* on my car when I pulled it out of storage happened to be the 4 psi/TR3 type. Even so, the car never had any overheating problems during it's ten years as a daily driver at high Colorado altitudes (water boils at a lower temp), with high compression, headers, Webers, etc. So, I'd guess the cap rate doesn't make a huge difference, or maybe I was just plain dumb-lucky!

Probably a bigger issue is to match whether the cap is 3/4" or 1" deep type. It's easy to check by measuring the neck on the radiator, top of top flange to sealing area of bottom flange.

Still, 7 psi is correct for the TR4/4A and recommended to get the best possible cooling performance, since the heater core is improved over the TR3's and can handle the slightly higher pressure.

Cheers!

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L

[ QUOTE ]
[ QUOTE ]
Regarding caps, there are very original looking ones available from TRF... The original appearance caps *might* not draw the fluid back in as well...

[/ QUOTE ]

Alan -- FWIW, I added a TR4A overflow bottle to my TR4 using the original cap and it works fine (also looks better than some aftermarket overflow

[/ QUOTE ]
 
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