what are the side effects of disabling fuel tank ventilation? Especially for petrol cars. Is it true that the ventilation system, which constantly allows the air to move into and out of the tank worsens the quality of the fuel?

  • Welcome to Motor Vehicle Maintenance & Repair! Commented Nov 8, 2023 at 15:41

2 Answers 2


Your view of the EVAP system is a little ascew. The EVAP system isn't open to the atmosphere. It doesn't "constantly" allow air to move into and out of the tank. The whole purpose of the EVAP system is to NOT allow this to happen. As this reference states:

The Evaporative Emission Control System (EVAP) is used to prevent gasoline vapors from escaping into the atmosphere from the fuel tank and fuel system at all times.

If you are talking about disabling the EVAP system, you're actually defeating the purpose, which will cause more contamination to the fuel, allowing what ever amount of ethanol which is in the system to absorb more water from whatever air is present.

There has to be a certain amount of air going into the system to accommodate the lessening amount of fuel being in the tank. If you are talking about this part of the system, then if you disable it, you'll cause the system to be in an vacuum state, which at some point would not allow the fuel to flow to the engine, which would not allow it to run. I'm pretty sure you'd also get to the point where you couldn't get the gas cap off, either.

The best way to keep your fuel from becoming contaminated for any length of time is to keep the tank full. This only allows a small amount of air to be in the system. Less air equals less moisture to be absorbed. You can also use a fuel stabilizer, which helps prevent the fuel from absorbing water (forms a barrier between the air and the fuel). This only works for a period of time depending on the pro

  • Thanks for the reply. Maybe it's not constantly but from time to time(I've watched live data). So, the ventilation does not worsens the quality of the petrol?
    – locke
    Commented Nov 8, 2023 at 16:32
  • There is no "in and out" was my point. And yes, the air going in is a detriment as it brings moisture in with it and ethanol absorbs it. However, that isn't what your question is about, so I didn't include this into my answer. Commented Nov 8, 2023 at 20:28

In addition to the points raised in Paulster2's answer, disabling the EVAP system will give you a check engine light as soon as the EVAP monitor runs. From that moment forward, you will have a permanently-on check engine light. If some other issue arises that would turn on the light, you will never know unless you attach a code reader and check for other codes.

Some issues that turn on a check engine light can cause damage if the issues are not repaired within a reasonable time. For example, misfires can dump unburned fuel into the exhaust, causing catalytic converter damage. A vacuum leak can create an air-fuel mixture that is too lean, burning your valves.

In addition, a large number of faults that would turn on the check engine light without causing actual damage will result in poor fuel economy, reduced power and excess emissions.

So it's against your best interest to make any kind of modification that will result in a permanently-on check engine light.

  • Yea, check engine light will came. I've found a guy who disabling the light but wondering what side effects will appear later in using the vehicle (already read about increasing the pressure, possible hoses or fuel pump damage)
    – locke
    Commented Nov 8, 2023 at 16:34
  • @locke If you disable the check engine light, that is no different from a check engine light that is always on. You will never know if there is another problem that would turn on the light. So the result, same as above, could be engine damage, poor fuel economy, etc., if some other problem occurs and you do not learn of it from the check engine light.
    – MTA
    Commented Nov 8, 2023 at 16:41
  • Actually, I mean that this guy can disable flashing the check engine light because evap valve is disconnected. Not to disable it permanently
    – locke
    Commented Nov 8, 2023 at 17:48
  • Wherever you heard air fouls fuel in the tank is bogus. Try searching how the evap system works. Engine off; evap vent valve is off and allows gas station fillups. Engine on; vent valve closes. During engine runs, the evap system uses intake vacuum to pull fuel vapors from the carbon canister instead of venting it into the atmosphere. Fuel will not go stale since some air gets into the tank, enough for vacuum pulling vapors into the canister before going to the intake manifold. You'll do more harm interfering with emissions systems unless your state doesn't require emissions tests.
    – F Dryer
    Commented Nov 9, 2023 at 0:19

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