Should You Replace That Damaged Plastic Intake Manifold?

It seems like plastic is everywhere under the hood of a modern car. You even have to remove a plastic cover to find the engine! But plastic and composite parts like distributor caps and ignition coil housings have been in use for over 100 years. In the 1970s, automakers tinkered with plastic intake manifolds, but it wasn’t until the 1990s that they gained wider adoption. Some may remember GM’s iconic 3.8L V6 from the mid-90s, which sported an ill-fated plastic intake manifold, but the overall idea proved workable. While using plastic for intakes saves weight and allows for more creative engineering tricks, it obviously means that you can’t repair them the same way as their metal cousins. Here’s why replacing, not repairing, a plastic manifold is usually the best solution.

When plastic intakes go bad

The high-tech plastic used to make an intake manifold is designed to withstand heat, vibration, vacuum, and even boost pressure. But there is a limit, especially as they age. You may inspect a plastic intake and find:

• Cracks: Repeated heating and cooling cycles combined with vibrations can lead to cracking over time, particularly where the plastic casting is thin or where there are seams. Cracks can lead to chasing down vacuum-related OBD-II codes or coolant leaks.
• Warping: Both air and water passages depend on absolutely flat mating surfaces to prevent leaks. Over time, these mating surfaces can bend and warp, leading to leaks. Unlike metal, a plastic intake cannot reliably be machined to bring back a flat surface.
• Breakage: Just like a metal manifold, in places where clamping forces are uneven, there is a risk of breakage. Vacuum ports, mounting bosses, and bracket mounts can snap and break off.

Photo: Mike Apice.

Why you shouldn’t repair a plastic intake

There have been plenty of attempts to repair plastic intake manifolds, but for a shop, the risk far outweighs the benefits. First off, the repair may not hold, and now you have an upset customer who might have also lost faith in your capabilities. Glue, epoxy, and even plastic welding repairs won’t have the same expansion-contraction rates as the original plastic, possibly causing another failure. There may also be complicated internal passages inside the intake manifold that you can’t see. If your repair causes an internal air leak or blockage, everything may look fine on the outside, but a hidden problem will be lurking. Some intake manifolds use multi-piece construction, making it nearly impossible to reach some areas without causing further damage. Features like variable-length intake runners rely on flaps and complex passages, where repair-generated blockages or debris could hide.

Photo: Mike Apice.

Benefits of replacing a plastic intake

When you are faced with a damaged intake manifold, the easiest solution is to just replace it with a new one. In some cases, a new intake manifold is actually better than the original factory design. For example, certain GM 1.4L Ecotec engines have an internal PCV valve that tends to fall out, causing the engine to burn oil.

Replacing a damaged OE plastic intake manifold is a good time to install a revised or upgraded manifold if available from the aftermarket. Dorman, to name one, has an improved version available for Buick and Chevrolet vehicles that solves the problem by using a PCV valve retainer. In the case of certain Kia 3.5L V6 engines, the intake manifold uses variable intake runners that depend on shaft-activated flapper valves that often break. An improved version (also available from Dorman) for Hyundai and Kia vehicles uses stronger materials and additional shaft supports for a more durable intake manifold.

There’s also the issue of time. If you have to remove the intake manifold anyway, why spend labor time putting a questionable part back on? Plus, there is the time trying to figure out not only if a repair can be made, but how it can be done. Wasting time playing “arts and crafts” on a repair that fails as soon as you start the engine means a shop bay that isn’t making money.

Using a new intake manifold means not worrying about dirty or clogged internal passages, stripped bolt holes, unseen cracks, warped mounting flanges, or other lurking problems. Fixing one bad spot on an old plastic intake doesn’t keep new issues from popping up later. If you see one crack now, there is likely another forming somewhere else. For most techs, cobbling together an old intake isn’t worth the time or risk when a new intake is just a phone call or mouse click away.