This is similar to, but not the same as my question Effects of Dirty Piston Heads and Intake Valves.

Basically, I often see people talking about taking the intake manifolds off older cars to clean them, or to seafoam them, separate from just trying to get the valves clean.

However, I would think that unless there is a major problem with the engine which has caused a truely massive buildup of gunk then a typically dirty intake manifold shouldn't really be much of an issue.

The only things I could think of would be that maybe a dirty intake could throw off ECU fuel calculations relating to the assumed Tau pool on the intake walls, but that would seem to be a pretty small effect I would think.

The other thing I could think of might be clogged EGR passages, but I don't think seafoaming would help with that.

Anyways, what are the effects of a typically dirty intake manifold?

1 Answer 1


Upstream the throttle body

A dirty intake by itself isn't the problem, but it is a sign of contamination of related components:

  • MAF sensors don't take kindly to dirt

    This will usually lead to the fouling of the hot-wire(s), resulting in the underreading of air mass flow, which will lead to positive fuel trim correction.

    If the contamination is bad enough, the engine will feel sluggish and slow to respond.

  • Throttle plate actuation can be hindered

    Crud tends to build up on the back of throttle bodies. If bad enough, they will hinder the ability for the actuation of the throttle plate, which can lead to driveability problems.

  • A clogged EGR hose/valve can throw off fuel management

    Again, this will show up in fuel trims, albeit not dramatically. The obvious thing to suffer from lack of EGR will be the environment due to increased NOx emissions.

Downstream the throttle body

The only losses between the throttle body and intake valves will be caused by ductwork.

In almost all situations, the deposits of oil, fuel and dirt will not have a significant impact on pressure drop. I'd expect the performance gains from cleaning this part of the manifold to be minimal.

Of course, the lesser the dirt that goes past the intake valve, the better.

However, the right solution is to ensure that dirt doesn't enter in the first place: Keep the intake tract air-tight and use a good quality air filter.

  • When I was talking about dirty, I meant more in the vain of carbon, oil and sludge whose source is in the closed system of the engine, not actual dirt coming from the external environment. Commented Jan 25, 2016 at 16:54
  • @RobertS.Barnes I updated the answer somewhat... let me know if it does or doesn't address your question
    – Zaid
    Commented Jan 25, 2016 at 19:53
  • @Zaid Why do people often suggest cleaning the throttle body when facing poor idle, stalling, etc.? The only thing it can fix is a sticking throttle, as I see it. Does it have anything to do with sucking the deposits inside the cylinders and causing momentary rich conditions? Commented Jan 26, 2016 at 8:11
  • 1
    @IhavenoideawhatI'mdoing beyond a sticking throttle I cannot think of any other reason to do so.
    – Zaid
    Commented Jan 26, 2016 at 8:23
  • @IhavenoideawhatI'mdoing As part of the intake tract is the PCV. The PCV sucks oil vapor out of the crankcase and into the engine. It is commonly before the throttle body and can leave carbon deposits in the port for the Idle Air Control Valve.
    – rpmerf
    Commented Jan 27, 2016 at 14:47

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