I have a 1989 740GL with an aftermarket radiator. The radiator has the plug style temp sensor/fan switch. Up here in Minnesota, we never had an issue with the thing leaking. However, on a recent long distance drive through some higher terrain and doing several mountain climbs, the plug either worked itself loose or gave in to higher pressure and we lost some fluid (thought we blew a head gasket for a second...) I pushed the plug back in and rigged it with some high-temp tape. Back in the flat lands, we never had an issue with it. Could the mountain climbs caused higher pressure and the plug to unseat? If so, is there a fix for this. I was wondering about some gasket sealer or RTV, but am worried about how this would hold up and how it would adhere to the plastic radiator.

One thing people do to prevent the plug/t-stat from blowing out is to loop a couple of zip ties through the rad and around the side tank. The rubber bushings aren't supposed to be re-used when a rad is replaced, but some people/shops do. The bushings are most readily available from Volvo. Aftermarket sources list them, but are often out of stock. Everything is supposed to be installed dry, but you'll likely want to use water or perhaps slightly soapy water, but nothing oily. The bushing easily installs dry, but installing the t-stat is often difficult. Carefully pressing it in dry using a clamp and wood blocking for protection is possible. Although you might get away with it, I wouldn't recommend using RTV on the existing bushing.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now

Thanks for the replies. We're taking the 740 on a long trip, so I zip tied the plug/t-stat and will hope for the best. I'll probably wind up putting another aftermarket rad in, but will look for one of higher quality.

Thanks Dave. I installed the rad myself and only used soapy water on the bushing (and it was not easy to get the thing in). I'll use your zip tie trick, as I already have something similar rigged. I'm going on a long drive, so am trying to prevent a re-occurence (though it's been fine since). I think I'm going to find a junkyard with an original rad and have it re-cored. The originals had the screw in plugs and I never had an issue with them.

On another note, why wouldn't RTV work?

I haven't really heard of anybody re-coring a rad. The originals were better than average, but not great. It's the plastic side tanks that are the issue with used rads. The plastic becomes brittle with age, cracking around the base of the hose necks with little provocation. I don't know what kind of rad you put in, but if you really don't trust it then for not much more than you were planning to spend I'd go for a half-decent aftermarket one (Nissens is popular) where you can expect higher manufacturing tolerances on the inside diameter of the t-stat plug hole. A new rubber bushing will be included.

As for my not wanting to use RTV, there's likely to be precious little compound staying around the rubber bushing as you push it together so it probably wouldn't do much good. Plus you don't want any to get loose in the cooling system and plug things up somewhere. Chemically it will be okay. It also doesn't look very professional, a sign that somebody either doesn't trust their handiwork or the parts they used. Not that zip ties are much better in that regard, but at least it looks like an afterthought rather than an integral part of the installation.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now

Dave Stevens says "I haven't really heard of anybody re-coring a rad."

Back in the day when cars had carburetors and rads were made of brass, re-coring was very common. It's still done with vintage rads by real rad shops. While Volvo's plastic/aluminum rad is remarkably rugged and reliable (I'm nudging 490,000 klicks with an original rad) these throw-away rads got a bad reputation on throw-away domestic cars. I sure hope Volvo used better plastic for the end caps than they used for door panels.
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1992 745 approaching 500k km

That was a rather incomplete statement on my part. LOL Of course you're right, all-metal rads can be re-cored, although with labor costs the way they are it's not the cost saving it used to be. Even had that done once myself. It's those new-fangled ones with plastic side tanks and aluminum core I was referring to. Unfortunately the original Volvo ones do have plastic side tanks, as do all the aftermarket ones (that I'm aware of). Nissens did make a three row, all-metal rad with aluminum core and brass side tanks for 700/900s, but those haven't been available for a few years. Now if you found a used one of those it might be worthwhile.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now