Intake manifold leaks are both insidious and irritating; insidious because they can mimic a number of other problems, and irritating because you may have to tear half the engine apart to deal with them. Rarely will the solution prove as simple as just pouring something in your oil or gas tank, but there is one case where it might.
There are two, perhaps three, places where an intake manifold can leak. All intake manifolds carry air from the carburetor or throttle body to the engine, and the gaskets that join the three components can fail and allow unmetered air to go into the cylinder head. Many manifolds used on V-configured engines also contain a coolant crossover that carries water from the cylinder heads, and this portion of the gasket is at least as likely to fail as any other. Lastly, the intake manifold often seals the top of the engine, and the end seals directly beneath the manifold can fail and allow oil to trickle out of the engine.
"Stop-leak" additives and high-mileage oils typically contain one of several combinations of hydro-treated napthenic oil -- heavy mineral oil -- and some type of ether blend. The latter causes the microscopic polymer strands in the rubber seals to relax and uncoil a bit, not unlike the way that hair relaxer does to curly hair. This causes the seals to soften and swell up a bit, sealing around whatever rubber or polymer gaskets they come in contact with. While some propose that this softening could actually reduce seal life throughout the engine, the reality is that such products really only return the seals to something like their original state. Stop Leak won't fix a gaping hole -- it only causes the rubber to swell by around 10 percent or less -- but it is worth a shot if you've got leaking end seals.
There are three basic types of coolant stop leak product: products that use metal particles -- essentially copper or aluminum dust -- to seal leaks, those that use expanding pellets -- like Bar's trademarked Rhizex -- and those that use sodium silicate, or liquid glass. Liquid glass is very good at sealing cracked heads and blocks but requires a lot of heat to work well. For this reason, it's not the best choice for a manifold gasket leak. The metal-suspension type works best when used on a leak in a corresponding metal; aluminum sealant for leaks in an aluminum radiator, copper sealant for leaks in a copper radiator. But metal-suspension sealants don't generally do as well at sealing gasket leaks as pelletized products do, because the pellets themselves are more like the gasket material than metal. But this should be considered as a line of last resort, since both metal-suspension and pelletized products -- in sufficient quantities -- can clog small water channels in the heads, heater core, radiator and water pump.
Finding Air Leaks
The quickest way to track down a manifold air leak is with a can of ether starting fluid. With the engine running and at idle, hold the can about 3 inches from the suspect area and give it a one-second blast of starting fluid. If there's a leak present, engine vacuum will draw the fluid in; once in the engine, the fluid will act as a supplemental fuel, causing a momentary rise in engine rpm. Once you've confirmed a vacuum leak, you've only got two options: you can replace the gasket or you can try to seal it with silicone. There's nothing you can add to the fuel tank or anywhere else that will seal a vacuum leak, since on air goes through.
Sealing the Leak
This solution should work as long as you can physically gain access to the leak. On a V-configured engine, start by running a thick bead of black silicone RTV sealant in an unbroken line all along the manifold-to-head mating surface. Here's the tricky part: after you lay the bead of silicone down, you'll need to wait for a minute or two for the silicone to "skin over," or dry to the touch. Then, you'll want to disconnect the ignition coil or coils, and give the starter three or four five-second bumps over the course of two minutes. You have to do it this way because, if you were to just fire up the engine before the silicone sets up a bit, engine vacuum might just suck the silicone through the leak and into your engine. And that's not good. This can be a bit more difficult on inline engines, since you'll have some trouble getting to the bottom of the manifold, but it can usually be done. Don't worry too much about leaks underneath the manifold on a V-configured engine; they might suck in a bit of oil, but net airflow through the engine remains about the same since you're only drawing gases that make it past the piston rings. Still, gasket replacement is a good idea since those dirty gases and oil are bound to cost you some horsepower and foul spark plugs.