• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Radiator Cap Leak Problem

Share this page

NardisCNC

NardisCNC

Senior Member
Offline
This is strange.... For the first 6 or 8 months of having this car (66 midget), everything was perfect. Then all of a sudden, one day it started leaking antifreeze Badly from the front of the car! I traced it down and it's actually coming from that pressure release hose that attaches at the radiator cap. I figured the cap was faulty. So I went to Advance Auto Parts, they did not have anthing in their computer that matched my car. They did have an IDENTICAL cap though to what came off of my car. (which could also be the wrong part, but why did it work for so long?) So I went ahead and got it. Does the same thing!!!

BTW, it does not have to be hot for it to leak. Just barely warm. If it's cold, it will start leaking before you can see the temp gauge move. Definitely not a pressure release issue.

Is it critical that the correct replacement cap be used, or will these cars accept generic caps? What should be the pressure rating of the cap?
 
How do you know it's not a pressure release problem?
Have you started it without a cap to see if you have a geyser?

Head, or head gasket, can place combustion pressure directly into the colling system.

If it's leaking out the cap, that means there is enough level and pressure to do it, and if it isn't fully warmed up, that could be a problem.

Then again, could be a damaged seat.

Inspect that closely.

Could be a stuck thermostat, but generally that wont give you that issue until the gauge is ready to peg.

Dave
 
Thanks Dave! ...Yikes, this doesn't sound good. I actually have not tried it without the cap. Now that you put it that way, maybe it is a pressure release.

So if the engine is cool, and I do have a geyser without the cap, does that definitely indicate that combustion pressure is entering the coolant system? What do I do at that point? Remove the head and inspect?

The cap seat looks good.
 
If you remove the cap (cold engine) and start it, and you get a geyser almost instantly, you need to investigate combustion gasses in the coolant.

Like, pull the head and inspect.

If it doesn't geyser on you, but rather runs for a bit then squirts, look for belt, water pump, stuck t-stat.

Cold engine should not have a lot of pressure in the radiator.
Heck, if it did, we'd never be able to add coolant with it running.
 
I have a vague recollection that some of our cars require a deeper rad cap than stock - does anyone else remember this?
 
Do you top up the radiator when it's cold?
Water expands when hot and will spill out of the overflow until the proper lever is found. Now if you top it up again, cold, you will get more coming out as soon as it's warm.
Let it spill out, do not top it up.
Like others have said, you may have a blown head gasket putting combustion into the cooling system.
 
Slow down Dont rule out the cap!
IF you have a BE with the down flow rad it most difinit ly takes a lond ingaugemnet cap.
it more than likely not a long enough ingaugement and has been sealing on the soft second seal. I had the same thing happen. The first rad cap I got from vic had a to short ingaugement.
I thought it turned down to easy but heck it was a lighter pressure cap than I ever used before 8 PSI. Well it turned out it was not even sealing.

Take your cap off dry the face of the rubber seal thoughly.
on the sealing surface of radiator neck smear a little oil .
Put your cap on then take it off and have a look chances are you will find
no evedence of it contacting the cap.If this is the case order one from Moss.

This was my only rebulid gremlin the symtom was warms up quick not really hot but under sporty driving or really warm weather it would over flow and get hot.
the new correct cap cured all of this. Now it runs a constant 185 190 during the dallas area summers only the driver now over heats and melts.
 
Good news!! No geyser with the cap off.

Strangely though, I let it warm up for 5-10 minutes and neither were there any leaks!! I even let the temp gauge get up to 160-170 or so. Maybe it was too full? I did fill up the radiator cold without it running.

I definitely have the vertical flow radiator. I should probably get a cap from Moss, to rule that out.
 
Do not fill when cold. You should see the liquid but it should be about an inch below the seal. It is not critical.
 
See?

Now you have eliminated the big stuff.

Don't overfill, and now check your cap depth.

You can use your finger.

Reach in and touch the seal area in the radiator for the cap.

Hold your thumbnail against the same finger to indicate top sealing ring.
Remove hand from radiator.

Hold marked hand up to cap.
Touch upper sealing ring with tip of finger.

See where thumb lies.

Remembering you have spring crush to deal with, if your thumb rides short of lower sealing surface, you have the wrong cap.
 
I went high-tech and used the depth gauge on my dial calipers. My goodness!! It's almost 1/4" short! Duh, why didn't I think to measure it in the first place?
 
Yeah, well, I gave you the LBC Shortcut you could use on a road trip when the dial calipers with depth gauge were back home on the workbench....
 
Here was my solution to fixing the car while I still had a couple hours of NICE Sunday afternoon daylight. After spending all afternoon Sat getting this thing running like a top, ordering a cap from Moss and waiting 2-3 days was simply not a correct solution.

Advance Auto Parts was right, their caps do not fit...... until now:
cap1.jpg

cap2.jpg

cap3.jpg

cap4.jpg

cap5.jpg

cap6.jpg


I've been driving the wheels off this car for the last 3 days now with not the first drop of leakage!!

Order your custom Radiator Cap extension kit made from Billet Aluminum TODAY! Never be stuck with your Down-flow Radiator leaving you short! Uses generic caps found at nearly all parts stores. All for the low introductory price of $49.95 + s&h. (or buy a correct replacement cap for $6.95)
 
I want to be you! I can't imagine anything more gratifying than a $50 solution to a $7 problem. Seriously, I would love to be able to mill stuff like that.

well done!

:cheers:
 
Heh, wonder why you can't get the right cap.
 
Back
Top