Humming or buzzing gas tank

I have a 2014 access cab FWD taco. After turning off the engine and removing the key occasionally I can hear a humming sound coming from gas tank. The sound stops when I restart the engine for a couple seconds then turn it off. It also begins sometime after I turn the truck off and return after a couple hours later. I think it must be the gas pump, but it so far hasn't caused any problems. Should I just ignore the noise?

 

Its the evaporative emissions system (charcoal canister) doing a leak test. Nothing to worry about

 

People at work would tell me my truck is running/making noise in the parking lot ...hahaha

 

Thanks for the responses! This is a great forum, Taco people rock!

 

Is this something that only occurs on newer models? I've never heard it from my 2010.

 

Wait. You have a front wheel drive Tacoma?

 

I had a 2014 that did it

 

I just read an online article that during a routine colonoscopy Doctors found a live fly in some guys colon. Maybe you have the same in your gas tank?

 

05+ Tacomas have the Leak Detection Pump.

1. Key-off monitor This monitor checks for EVAP (Evaporative Emission) system leaks and canister pump module malfunctions. The monitor starts 5 hours* after the ignition switch is turned OFF. More than 5 hours are required to allow enough time for the fuel to cool down to stabilize the fuel tank pressure, thus making the EVAP system monitor more accurate. The electric leak detection pump creates negative pressure (vacuum) in the EVAP system and the pressure is measured. Finally, the ECM monitors for leaks from the EVAP system, and malfunctions in both the canister pump module and purge VSV, based on the EVAP pressure. HINT: * : If the engine coolant temperature is not below 35°C (95°F) 5 hours after the ignition switch is turned off, the monitor check starts 2 hours later. If it is still not below 35°C (95°F) 7 hours after the ignition switch is turned off, the monitor check starts 2.5 hours later.

 

Question, on most cars the evap system test is inhibited unless the fuel tank is also between 25 and 75% because the system requires a certain amount of headroom airspace in the tank to then perform a leak down test being that fuel (fluid) doesnt compress but air does. i would think that if the tank is more than 75 percent full then the evap leak down test is inhibited. but i dont know the tacoma specifics. laws of physics still apply here and i would expect that ample airspace would still be required before entering into the ready for test window. this is also why those that have put the larger fuel tanks in are having issues with the leakdown test passing..... all of the leak rate level limits engineered into the leak down rate table limits programed into the ecu were accounted for by the expected volumetric readings of the stock tank

 

Below are the factors for enabling the EVAP test on 2nd gens:

Atmospheric pressure: Between 70 kPa(abs) [525 mmHg(abs)] & 110 kPa(abs) [825 mmHg(abs)]
Battery voltage: 10.5 V or more
Vehicle speed: Less than 4 km/h (2.5 mph)
Ignition switch: OFF
Time after key off: 5 or 7 or 9.5 hours
Engine coolant temperature: Between 4.4°C (40°F) % 35°C (95°F)
Intake air temperature: Between 4.4°C (40°F) & 35°C (95°F)
Canister pressure sensor malfunction (P0452, P0453): Not detected
Purge VSV: Not operated by scan tool
Vent valve: Not operated by scan tool
Leak detection pump: Not operated by scan tool


Both of the following conditions 1 and 2 are met before key off:

1. Duration that vehicle has been driven 5 minutes or more
2. EVAP purge operation: Performed


Toyota's strategy for EVAP leak detection differs from other manufacturers in that it doesn't seal the system to check for leaks, instead it draws a vacuum through a reference orifice of known size as a baseline and then draws a vacuum on the tank to see if the pressures match within a certain range.

 

interesting, makes a whole lot of sense to overcome the need for headspace before initiating test to allow for pressurizing (compression of air), then determining leak rate by measuring bleed off rate of that pressure by instead using vacuum where you dont have to worry about compressibility of air/fluids. vacuum is not affected by compressibility of the medium and therefore one can perform test regardless of headspace volume in tank.....if you cant pull a minimum vacuum level using the pump than you are drawing in too much from a leaking point to the containment area .....and they compare it to a set orifice calibrated for a set amount of leak vs vacuum daw against it........ the tank has to have less leak and more vacuum buildup than the orifice path....thats brilliant

Hey Dm93, thanks for providing that

 

All these technical diagrams and talk are way above me, but I do have a question. If I have a leak in the system, how would I know it?

 

You'd have a check engine light and a trouble code stating as such..

 

Your CEL and Trac Off light would turn on and you would have EVAP code(s).

The typical codes that set are P0456 (Small Leak) if it's a small leak and a large or gross leak (system is unable to obtain any vacuum) will typically set a P0455 (Large/Gross Leak) & P0441 (Incorrect Purge Flow).

 

My truck sounds like a trumpet when the test is active.
I've had a few people look at me funny.

 

I've never heard mine run, I have ran the automated test on mine with a scan tool just to see what know good looked like but the pump is pretty quiet at least on mine.

 

ha ha your exhaust port line coming from the vacuum pump is probably kinked or restricted somehow and causing it to be tuned to a musical note

 

Evap tests use vacuum, not compression to find leaks. The vacuum pump is not compressing the air, but evacuating it. This how it drops pressure. Do not know the official reason why the test is limited to certain fuel levels.