Back in the days when most engines had cast iron blocks, heads and manifolds, the intake manifold could be easily sealed with simple and relatively inexpensive die cut fiber-faced metal gaskets (solid or perforated core). Sealer was usually required to ensure a leak-free installation.
But in the late 1980s and early 1990s, fuel systems and manifolds began to change. Multiport fuel injection meant the intake manifold no longer had to flow an air/fuel mixture, only air.
This gave engine designers more freedom to optimize airflow. Manifolds became more complex, with long curving runners and intricate plenums engineered to improve throttle response and low speed torque. Plastic became a popular material for casting manifolds due to its low weight and cost.
Some were also fitted with tuning valves that could reroute the air inside the manifold to change air velocity and flow according to engine speed and load.
On engines with aluminum cylinder heads, corrosion typically erodes the area around the coolant ports. If the plastic under the sealing bead on the intake manifold gasket is eaten away, the gasket can’t hold a seal and will leak. In this case, you can’t blame the leak on the gasket.
Many aftermarket gasket manufacturers have redesigned intake manifolds and gaskets for some problem applications that are outside the OE warranty. They’ve changed the sealing bead designs and the materials.
When servicing or disassembling high-mileage engines, pay close attention to the condition of the sealing surfaces around the coolant ports on the heads. If this area is eroded or pitted, the intake manifold gaskets may not seal properly when the engine is assembled.
In some cases, it may be possible to clean up the mating surface on a pitted head by lightly resurfacing the intake port side of the head. Pits can also be filled and sealed with high temperature epoxy fillers, then sanded or machined flat to restore a smooth, flat surface. If the erosion is severe, it might even be necessary to build up the damaged area by TIG welding the head or replacing it altogether with a new or salvage casting.
According to one aftermarket gasket manufacturer, the recommended surface finish for the intake manifold and cylinder head mating surfaces should ideally be 30 to 60 microinches Ra (Roughness Average). Another gasket manufacturer said anything from 20 to 80 Ra should be good enough.
Unlike the super-smooth surface finish that’s usually required for late-model MLS head gaskets (20 Ra or less), the surface finish for most intake manifold gaskets isn’t as critical. Even so, it must be smooth, flat and clean, with minimal pitting and waviness.
If the intake manifold or intake ports on the head are milled at too high a feed rate, it can leave an undulating wavy finish with ridges and valleys that can be difficult to seal. This may result in coolant or vacuum leaks, as well as premature failure of the intake manifold gasket sealing beads.
Overall flatness on both mating surfaces should also be checked with a straightedge and feeler gauge. Flatness should be 0.003” or less on the mating face of the intake manifold and cylinder head intake ports on V6 engines, and it should be 0.004” or less on a V8 or straight six.
Equally important, the intake manifold and head surfaces must be clean (no grease, oil or coolant film) and dry. RTV, other sealants or adhesives should not be used around the coolant or intake ports on carrier-style gaskets. The only place RTV silicone may be required is to seal the area where the ends of the intake manifold gaskets mate with the end strip seals on the block under the intake manifold or valley cover.
It’s also essential that the threads for the intake manifold bolts in the head be clean and undamaged, as this can affect clamping torque. New intake manifold bolts are recommended. But if you must reuse the old bolts, make sure the threads are clean and undamaged. Also, follow the recommended torque procedure when tightening down the intake manifold bolts and make sure it is the latest procedure as the original procedure and torque specs may have been revised.
If the intake manifold gasket does not make a tight seal around the coolant ports, it may leak coolant into the crankcase. If it fails to seal tightly around the intake ports, it may allow vacuum leaks that upset the air/fuel mixture and cause idle and driveability issues.
Testing For Air Leaks
Even the smallest leak in an intake manifold can cause a fuel trim problem. Finding a leak can be time consuming using your eyes and ears.
A smoke machine allows you to diagnose multiple leaks in less time compared to other methods. A smoke machine can pressurize the intake manifold and put smoke or vapor into the system. If there is a leak, you will see smoke come out.
Connect the smoke machine to a vacuum port like the supply line to the brake booster. Make sure to block the throttle body with the right sized plug. Also, block off the PCV system.
Leaking injector seals can cause lean and misfire codes. Conventional testing methods often involve flammable gases or putting thick oils on the seals and looking for a change in rpms. But this test can’t be performed on engines where the intake manifold restricts access. Smoke machines can help spot these leaks without any disassembly.
Smoke coming from the oil filler or PCV system on an engine that is not misfiring could mean the bottom of the manifold is leaking or has cracked. It can also indicate worn valve guides or seals.
Coolant does not magically disappear; it has to go somewhere. External leaks can be seen with the naked eye or by using dyes. Internal leaks can find their way into the combustion chamber or the oil. Spotting these leaks can be difficult.
Always inspect the overall system. Check the oil for any foaming or signs of contamination. It may be counterintuitive, but take the time to pull the codes. Efficiency or oxygen sensor related codes can indicate that coolant is leaking into the combustion chamber. Coolants contain phosphates and other chemicals that can damage the oxygen sensor and the catalytic converter.
If the engine is a V6 or V8, the codes can even tell you which bank is leaking. If the leak is large enough and located in a runner, it might cause a misfire code. If the problem has been occurring for a long period of time, pull the spark plugs. Coolant will leave chalky white deposits on the electrodes. These techniques can help to narrow down what is leaking and determine if further tests should be performed, like compression or leak-down checks.