Is smoke coming out of this engine part normal?
Smoke escaping from an engine component during a smoke test can result from normal sealing tolerances or indicate a failed part, such as a bad valve. When people cannot tell the difference, they risk replacing functioning components unnecessarily or missing active vacuum leaks that cause rough idling and performance drops.
What the numbers show
Counted from the threads quoted below. How we count
- This comes up 10.9× more often in r/MechanicAdvice than across the site as a whole.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1Consulting online forums or repair manuals to verify if a component leak is normal behavior.Did this help you?
- 2Wiping off the residue to see if it returnsDid this help you?
- 3Asking online mechanics for opinions on whether the runoff is normalDid this help you?
- 4Using the vacuum brake booster hose to inject smokeDid this help you?
- 5replacing old hosesDid this help you?
- 6reading about adjusting the carburetorDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“I just installed a new carb and am getting really low (5 inHg) reading at idle, so want to check for vacuum leaks using a cheap amazon smoke machine.”source ↗
“I know I need to seal off carb throat on top, and assume there's a "correct" port to hook the smoke machine to (which one?), but do I need to plug any of the other hose ports, or do anything in particular?”source ↗
“I performed a vacuum leak (smoke test) and air was coming out of the valve where I circled. Is this normal or is the part bad?”source ↗
“I replaced hoses, and it didn't improve anything.”source ↗
“I read that this means a potential vacuum leak, so I bought a smoke tester on Amazon and want to smoke test for vacuum leaks, but need instructions on how to do it.”source ↗
“I know I need to seal the "top" of the carb (I'm sure this part has a more important name than "top") but is there a correct port to introduce smoke, and are there any ports etc that need to be capped?”source ↗
“Is this just dirt and rainwater/condensation making its way down? Or would there be some sort of leak.”source ↗
Where this came up
People with this problem also raised
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