Hello

My heat exchangers (from 1.7 engine) are leaking air. The thin outer metal has holes where it is suppose to be bonded to the thick tubes carrying exhaust gasses. There is no exhaust gasses leaking into the heated air.

Is there a way of plugging these holes? Can silicon rubber withstand the heat? Is there a better substance to plug the hole?

Thank you.

Clean them up good, use some JB: Weld high heat version. See if that will clean up the problem.

Thats the problem. Typically leaks come from rust of the jackets. As much as you clean the outside the tin worm is still working from the inside.
JB will be only be a temp fix . Its a bit of work but to kill the rust try to completely immerse the HE in Apple cider vinegar (min 5% acidity)
A couple days in that bath will kill any rust & allow the JB seal to work.

Not sure why you recommend Apple cider vinegar. There are stronger versions available from pickling vinegar (7%) to cleaning vinegar (10%), and also more commercial varieties at 20%, 30% 45% and 75%. However I generally use muriatic / hydrochloric acid with appropriate safety measures. And the Apple only adds flavour.

The muriatic eats steel as well as the rust. An inhibitor will slow the damage down. Usually a manganese based. Phosphoric acid does a good job on the rust removal without eating the base metal.

Simply its cheap, accessible at most any food mart & is safe for the environment when comes time to dispose of.
Not everyone has access to "commercial" level pickling agents nor hazardous material disposal facilities.
Oh .. & it works