Signs & Symptoms of a Leaking Intake Gasket

By Richard Rowe , last updated March 13, 2012
Intake manifold gasket leaks are insidious, frustrating failures that often follow a frustrating and labor-intensive manifold replacement. The worst thing about manifold gasket leaks is that you won't find out about them until you've put the engine back together and gotten it running again -- at which point, you may have to tear everything apart and do it all over again. It's enough to make you wonder if a bit of knock or water leaking into your oil is really serious enough to warrant going through all that again. Sadly, it is.

Single-Runner Leaks

A leak over or at a single runner causes that cylinder to ingest excess air. This lean condition, at the very least, causes a loss of power, cylinder misfire and a loss of fuel economy. This condition also is fairly dangerous; one cylinder running lean won't affect the engine's overall fuel ratio -- meaning that computer compensation through the oxygen sensor is fairly minimal. The worst-case scenario is fuel detonation -- knock -- and ultimately a hole in that cylinder's piston.

Cross-Cylinder Leaks

There's very little material between the intake runners on most engines -- 1/4 inch or less in many cases. This makes gasket blowouts between adjacent cylinders fairly common. This type of failure is more common than you might think, and it may have very little affect on the engine, depending upon its design. Runners share a common link in the intake plenum, anyway; the runners are only there to isolate pressure pulses. If you do notice an inter-cylinder blowout, it'll manifest as a slight loss in power and possible intermittent misfire.

Multiple-Cylinder Leaks

Multiple-cylinder leaks tend to happen after a botched manifold installation. It could happen because of improper manifold torquing, a bad or damaged gasket, improper gasket installation or warped mating surfaces on the cylinder head or manifold. If you've got multiple vacuum leas, your engine ingests far more air than it should. This causes a loss of power, stumbling and hesitation on acceleration, and poor fuel economy.

Coolant Leaks

Many intake manifolds, particularly those on older V-configured engines, carry water in addition to coolant. The gasket is just as or more likely to fail at the water passage as anywhere else, and the result depends on exactly where it fails. An external blowout causes a visible coolant leak where the manifold meets the cylinder head. A coolant-to-cylinder blowout leaks water into an intake runner, generally causing total misfire and cylinder failure. An internal leak trickles coolant into the oil, which can cause all kinds of problems that may lead to engine failure.

Diagnosing Leaks

There's a time-tested method that mechanics use to find vacuum leaks, and it involves spraying a one-second burst of ether starting fluid on the suspect area. A vacuum leak draws the starting fluid in, resulting in a momentary rise in engine rpm. External water leaks are easy to find just by looking for them; internal leaks generally turn your engine oil into something resembling chocolate milk. If you're experiencing an engine misfire and see white smoke -- actually steam -- coming from the exhaust, pull the spark plug from the cylinder closest to the water passage. If you've got a water leak, this spark plug comes out looking sparkly and new, as though it had been steam cleaned -- which it has.
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