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Take a look at JB Weld.

Their advertising says that a farmer repaired the engine block of a tractor with it.

I just discovered last night that the coolant expansion tank in my wife's truck has a crack where it's leaking severely when hot and pressurized - it nearly emptied the tank in a 45 minute drive.

Someone at one of the stores I called mentioned that some people have used high temperature epoxy to fix holes and cracks in these. I of course didn't think to ask what kind or if they had it in stock, since I had in mind replacing the whole thing, so now I'm waiting for a call-back about it. In the meantime I thought I'd ask here if anyone knows what to use, or if I should just replace it. I can get a new tank locally or online, but not for a day or two - I was hoping to find something right away, like today or maybe last night, so patching it is appealing. Or it could be a temporary fix until I get a new one.

I pulled it out and can see there's a ding type crescent crack about 1 cm across where it may have been hit with a tool or maybe a pebble kicked up from the road. There's plenty of room for a big glob of epoxy on the outside. Unfortunately, the inside is not accessible, so I can't put anything there, and all the patch force will have to be taken up by adhesion on the outer surface. The plastic material is that usual black glass-filled alloy. The only marking that may indicate the type is ">PA6.6-GF30<" molded in with part number and date info. The tank is otherwise in good shape, so it seems it should have plenty of life left. The truck is a 2007 BMW X5 4.8i.

An alternative is to drill out a small hole in the crescent, then put a screw with rubber washers etc if I can at least work a piece of hardware into the inside. Or, a hole would allow me to get some of the goop on the inside, but it's not enough to do much cleaning of the surface around it.

Does anyone know of this kind of repair or suitable adhesives? It will have to work in the under-hood environment with oil etc, and at coolant temperature, and be able to bond properly with the plastic.

Ed

Yes, it runs fully pressurized as an integral part of the cooling
system loop. The coolant filler/relief cap is on it, rather than on
the radiator.

The deed is done. I used the kit NAPA #765-1544, which is basically 1
oz of epoxy, a few square inches of woven fiberglass material, a piece
of sandpaper, and an applicator brush. I decided to goop up the whole
concave wall section - there was also a stress crack about 2 cm long
farther up the side of the tank, in line with the main crack. I
figured maybe that whole section was deforming too much under working
conditions. It came out beautifully, with nice layers of epoxy with
embedded glass, about 1-2 mm thick all the way up.

This was a pretty expensive kit - about $21 - but if it works, then
OK. From the appearance, viscosity, and smell, it seemed like regular
"5-minute" curing mix, but a little heavier due to more filler
materials. I'm sure the glass layer will help a lot too. So, if it
bonded to the polyamide OK, then it should be "permanent" enough, but
if not, then it will blister off eventually from the pressure.

Ed

I did mention it in the original post, but it's hard to see older
messages with this wonderful yahoo system. It's a 2007 BMW X5 4.8i.
It's amazing how much plastic is used in modern cars, and how well it
works - up to a point. In the good old days of copper and brass you
could just solder these kinds of things and know it would be OK.

I put it back in tonight and ran it at idle until the water started to
boil out, then capped it up. So far it has held at pressure and
temperature for a half-hour or so of idling, so it's probably OK as a
temporary fix, but there will always be some doubt. Right now the
coolant is almost all water, since so much was lost and refilled - it
tends to boil off easily, putting more stress on the system. I'll be
correcting the mixture to the usual 50:50, after I put in a new tank.
For now, we'll probably do a car switch and I'll drive it for a while,
and carry some extra water, tools, and epoxy for possible field
repairs, until it's resolved.

Ed

Well, I put in a new tank and correct AF mix today and it seemed to work and hold pressure. Then I opened it up to check the coolant and it came gushing out again as before, so I thought the worst, that a head gasket leak was shoving it out from pressure buildup.

After stewing about it for a while and looking into repair options, it suddenly dawned on me that (I think) every time this happened was with the engine running, while it didn't do that when cool. The difference was that when cool, the thermostat was closed, limiting the loop flow to a civilized trickle, but when hot the flow was full, and the open cap was the path of least resistance to the water pump, so it ejected the coolant. Duhhh.

So fooling around with it less and checking with it shut down seems to show it's OK now - I hope. It also shows that simple things can be forgotten from not doing any serious car work for many years.

Ed

Also note that modern engines operate with a pressurized coolant
system so they can reach temperatures ABOVE 100 C (above the boiling
point). The pressure raises the boiling temperature (10% pressure
gives 2% temperature +/-). Removing the cap immediately drops the
pressure and the superheated water will immediately boil. This is the
reason your tank was pressurized in the first place.

I have never understood why the small increase in temperature was
important but then again, I am not a mechanic, engineer or otherwise.


________________________________
From: "edbreya@... [TekScopes]" <TekScopes@...>
To: TekScopes@...
Sent: Monday, September 15, 2014 11:55 PM
Subject: Re: [TekScopes] OT: high temperature automotive epoxy


I use the orange Dexcool AF for all vehicles - I quit using the old
green stuff long ago. I'll even be putting it in my 1967 Kubota
tractor when I get around to it. Ed