# Code P0441: EVAP System – Incorrect Purge Flow - tank not holding vacuum, is it normal?



## maestr0 (6 mo ago)

TLDR;
Should a gas tank hold a vacuum?


I'm getting P0441 EVAP code and Check Engine Light is on.

I replaced the purge valve, the vent valve and checked if they hold a vacuum and if they have the right voltage and if the EVAP canister pressure sensor is ok. All good.
The whole system holds a vacuum if the EVAP hose from the tank is disconnected from the EVAP canister and the inlet blocked. I tested it with a vacuum pump and it holds a vacuum up to 15kPa.
When I tested the tank, I attached the vacuum pump to the hose that goes from the gas tank to the EVAP canister. A fuel cap was closed (I got a new fuel cap too). The tank was not holding vacuum, I wasn't able to get any vacuum pressure at all. Is it normal? Does it mean that the problem is somewhere between the EVAP hose and the tank or maybe even in the tank? Should a gas tank hold a vacuum?

Murano SL 2013


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## VStar650CL (Nov 12, 2020)

maestr0 said:


> Should a gas tank hold a vacuum?


It should hold vacuum if the cap is on and both the Purge and Vent Valves are closed.


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## maestr0 (6 mo ago)

VStar650CL said:


> It should hold vacuum if the cap is on and both the Purge and Vent Valves are closed.



My ODB2 scanner shows 0 vacuum even when I use a vacuum pump on one of the EVAP canister inlets. The canister holds the vacuum, the pump shows a negative 10kPa. The engine is on, 5V, ground, and a signal wire seem to be fine. The EVAP pressure sensor signal voltage changes according to the vacuum level but the ODB2 scanner is always showing 0. Is it expected? Maybe the computer ignores the sensor readings unless it runs its EVAP test? There's no second gas tank pressure sensor in the gas tank, right?


Technically, if I have a leak in the gas tank, I can unplug the EVAP hose that goes from the tank to the EVAP charcoal canister and block the inlet in the canister. It should remove the tank out of the equation. Is my thinking correct?


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## VStar650CL (Nov 12, 2020)

Only one sensor, and it's not like FTT that only reads during EONV. The ECM should be looking at it whenever the engine is running, and it shouldn't be flatlined. Make sure you have power to the sensor, Blue should be 5V and Gray is ground. The Sky Blue wire is signal. The FSM doesn't spec an absolute signal voltage, but it shouldn't be zero or 5V and it should drop at least 100mV between idle and 3000 RPM. Check for water intrusion into the connector too, that's very common.


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## VStar650CL (Nov 12, 2020)

Well, wait a sec, those colors are at the ECM. They change in the Body Harness. At the sensor, Yellow is power, Purple is ground, Light Green is signal.


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## maestr0 (6 mo ago)

VStar650CL said:


> Well, wait a sec, those colors are at the ECM. They change in the Body Harness. At the sensor, Yellow is power, Purple is ground, Light Green is signal.


The voltage on the sensor is correct and it changes as expected according to a vacuum level but my scanner is always showing a flat line. 

Sensor ground wire checked, continuity ok
Sensor 5V is ok.
Sensor signal wire, ~4V for atmospheric pressure. ~0.1-4.9V depends on a vacuum/pressure level (vacuum pump directly attached to the sensor)

When I unplug the sensor I get a DTC code about a bad sensor, I believe it's P0452. As you said the ECM checks if the signal wire voltage isn't 0V neither 5V to verify if the sensor is good. Since I don't get the sensor error code I assume that ECM is getting a valid signal from the sensor. Why then the scanner isn't showing any pressure and p0441 comes up after driving for around 20 miles? 

The scanner used to be able to show the evap pressure but it was when I was getting a "large leak" code (check a gas cap warning) that somehow went away.

Can you confirm that I should be able to read some EVAP pressure any time the engine is on? If so I will focus on figuring out what's going on with the sensor rather than trying to find a vacuum leak.


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## VStar650CL (Nov 12, 2020)

Purge generally opens up at around 2500~3000 RPM. Until then the sensor should read tank pressure. Once the Purge Valve starts to open you should see partial manifold vacuum so the voltage should drop. Since that seems to be behaving normally in your screenshots and your voltmeter says the sensor works and the reading drops like it should, I'd say the pressure signal 
1) isn't making it back to the ECM _or_ 
2) you have a bad ECM _or_ 
3) your scanner is messing up.
There's only one intermediate connector in the circuit, B4:E104 in the left side kick panel where the wires go from the Body Harness to the Engine Room Harness. If you have a sunroof make sure those connectors aren't waterlogged and/or corroded.


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