If I use an intake valve cleaner that dislodges chunks of carbon from the intake valves or piston head, and they flow through to the exhaust, could they harm the catalytic converter?

I'm interested in testing the efficacy of intake valve cleaners by inspecting the intake valves before and after using the product. The carbon buildup is mostly on the intake valves in my direct injection car. I'm concerned that the dislodged debris could cause harm down the exhaust line since they could be relatively large for a system designed for gases.

I've thought of a few options:

  1. This isn't actually an issue
  2. Take off the exhaust before the cat or redirect it temporarily
  3. Put a temporary, high flow air filter at the exhaust between the engine block and the cat.

In number 2, would the reduction in exhaust resistance be OK?

Number 3 would be very interesting to me to see how much debris was removed. Some special air filters may be able to handle the high exhaust temperatures.

Any suggestions? Thanks!

  • Why do you think there are "chunks" : Why do you think the cleaner will remove them ? Of the hundreds + of dirty valves I have seen ; the dirty ones had a varnish-like film. Not considering the good old days of leaded gas when there were lumps of oxy lead sulfate type stuff. Commented Jun 18, 2018 at 19:09
  • Search on here - similar question was asked before
    – Solar Mike
    Commented Jun 18, 2018 at 19:45
  • "Take off the exhaust before the cat or redirect it temporarily" you can but it will set OBD codes, but they can be erased after you are done.
    – Moab
    Commented Jun 18, 2018 at 20:32
  • @blacksmith37 I can see a decent amount of carbon buildup, and if a solvent is able to cut through it some may come off in chunks or flakes. Note this is on a direct injection vehicle. A cleaner may or may not work - that's part of what I'd like to see. There is some evidence that they can - just search youtube to find people examining piston heads with a borescope. Commented Jun 18, 2018 at 21:36

3 Answers 3


My understanding is the reason DI engines get carbon build-up on the intake valves is because the fuel (with it's detergent package) get sprayed directly into the cylinder, not on the back of the intake valve like in a "normal" engine.

So, adding a cleaner will get injected exactly the same way (giving the intake valve a miss) and will likely not have much effect on intake valve carbon. Assuming the cleaner is poured into the fuel tank. If you're spraying it directly into the intake, it will depend on the efficacy of the cleaner. Does it actually perform as described?

My understanding of why DI engines generate so much carbon (look at the back bumper above the exhaust pipe) is because combustion chamber temperatures are lower and fuel is not burned as completely.

That's my understanding from when I looked into this a couple of years ago.

Personally, I'd start by verifying that there is actually a meaningful problem to solve; i.e. how much buildup do you actually have on your intake valves.


if pieces travel to the cat it will definitely block it partially. Yes, this happened to me using this junk!


Any "chunks" that end up on the converter honeycomb will not last long once the catalyst gets to 1200-1600 degrees F.

I wouldn't worry. But you won't have meaningful results, either - as Tim Nevins posted the "cleaner" won't be getting anywhere near the valves on a DI engine.

The flip side is that DI engines have much worse carbon buildup on intake valves, specifically because the detergents and solvents in gasoline (or cleaners) never get close. The preferred method of intake cleaning is walnut shell blasting.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .