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Euromike
10-21-2010, 07:38 PM
well i have been chasing an "EVAP System Large Leak Detected" code for awhile and found a faulty N80 valve, Replaced it and i still have a leak!

While troubleshooting, I thought of somthing. The way the system determines that there is a large leak is by pressurizing the system and seeing if it holds using the leak detection pump, correct?

So if i were to trick the leak detection pump into thinking it were holding pressure in the system by attaching a plugged line to the end of the leak detection pump, Would the code fade? Would more codes arise? Or is there a pressure switch somewhere else in the evap system that detects if it is holding pressure?

Also, can i somehow via vag com tell the car to perform the leakdown test with the engine off so i can listen for leaks?

thanks guys

Euromike
10-21-2010, 07:54 PM
Read this on another thread



The LDP detects leaks in the evap system. however it does not detect vacuum leaks. fuel vapor builds in the tank and is sent to the charcoal canister for storage. under certain operating conditions, the canister is emptied via the n80 valve into the intake system so it can be burned rather than releasing raw fuel into the atmosphere. the leak detection pump is a checks the integrity of the system to ensure that the gas cap is tight, there are no cracked or broken hoses/lines (large/small evap leak faults), and that n80 isn't stuck open. i forget if it pressurizes the system or creates vacuum, but it does 1 of them, until a flap closes. then it counts how long until the flap re-opens due to re-equalized pressure. if it takes no time at all, large leak. takes longer = small or very small leak. does not re-open - no leaks. In terms of necessity, its not required for the engine to run properly, but it will set a mil, you most likely wont pass any emissions testing if applicable to your state (i love FL), and if you have an evap leak it will be detrimental to the environment.


Makes me think this should work

Dallas09
10-21-2010, 10:04 PM
Since connecting an empty capped line to it would have much less volume then a gas tank, I would wonder whether it would think that the pump is blocked or malfunctioning and throw a different code since it would gain pressure so quickly. (Although a full gas tank doesnt have much empty space).

walky_talky20
10-21-2010, 11:17 PM
I have to say I'm more familiar with how the Toyota EVAP system works, but I would think that the pressure sensor involved here for the LDP would keep track of the canister pressure at all times. If it never sees a change in pressure (because the line is always capped), it would throw a fault for the sensor circuit not responding. It may also use this sensor to measure EVAP flow. Specifically, it wants to see the pressure slowly go down whenever the N80 purge valve is actuated. At least this is basically how the Toyota evap monitoring works, and theirs is one of the simpler ones.

Again, just thinking out loud here. I am not very familiar with the specific operation of the Audi system. I should really learn exactly how it works, though. I'm curious now.

Euromike
10-22-2010, 06:46 AM
Thanks walky. So can I do a leakdown test with vag com? It's impossible to hear where it's leaking with the engine on

Spanky387
10-22-2010, 08:28 AM
you can wire in a 330 ohm resistor where the N80 valve is and you will no longer have a check light

Euromike
10-22-2010, 08:50 AM
you can wire in a 330 ohm resistor where the N80 valve is and you will no longer have a check light

REALLY. Care to elaborate? How would that tell the system there is no leak?

Euromike
10-22-2010, 09:02 AM
Ok, since spanky replied i have been doing some searching. Check this DIY

http://forums.vwvortex.com/showthread.php?3111538/page1

Im a little confused though, How does putting a 330 ohm resistor at the N80 make the LDP think there is no leaks?

BASARAB
10-22-2010, 09:13 AM
you can wire in a 330 ohm resistor where the N80 valve is and you will no longer have a check light

Detailed explanation if possible?

Euromike
10-22-2010, 09:14 AM
I would think wiring a resister in the n80 socket would stop you from throwing an n80 code. Not sure about a leak code though.

dowsett6
10-22-2010, 01:40 PM
Spanky is confusing two different things. And that link you posted would show that if you read through it.

The 330 Ohm resister is put into the connector that goes to the secondary air injection pump. The 330 Ohms acts as the load instead of the pump motor. Also a 10 watt resistor isn't needed as it will just heat up. A 5 watt resistor will heat up half as much, 1 watt even less heat created. Remember 10watts at 12volts is ~.8amps so 1watt at 12volts is ~.08amps = less heat.


Anyways that all means that the car thinks the secondary air pump is still there as it has the exact same load, which means no code being thrown. Basically a hardwired option of coding it out of the ecu all together.



There is nothing to do with evap in my car at all anymore. Here is some info people will need.

The purge valve when in fine working condition will have a resistance between 22 - 30 Ohms anything outside of the range and your purge valve is junk. So at that range of resistance you can use a 27 Ohm resistor at 5% tolerance.

I took out my evap canister and leak detection pump completely as well. That pump has a 3 pin connector. When testing it to find out of your LDP is in good shape or not you would find that
pins 1 and 3 have anywhere between 640 - 720 Ohms and pins 2 and 3 have 17.5 - 27.5 Ohms (this is on a 1999+ car)

So again using the 5% tolerance resistors you would need a 680 Ohm for pins 1 and 3, and 20 Ohm for pins 2 and 3.

The car then thinks the system is plugged in as its the exact resistance it should be yet your reaping the benefits of not having the system in the car.



edit: P.S. taking the LDP out is a b****. Well more so getting to where it is.

BASARAB
10-22-2010, 01:43 PM
So without removing all the components, how to trick system that it does not leak? Plug something on the LDP itself?

dowsett6
10-22-2010, 01:51 PM
before i removed everything i was running with the purge just plugged in not connected to anything with the nipples plugged. And plugged the evap line behind the fender liner on the drive side. The one that runs with the brake lines.

The evap line on the passenger side i just had venting behind the fender liner. the line that runs with the gas lines. So instead of the purge valve purging the gas vapours into the intake system it was just going to atmosphere like cars used to do.

walky_talky20
10-22-2010, 02:20 PM
The 330 Ohm resister is put into the connector that goes to the secondary air injection pump. The 330 Ohms acts as the load instead of the pump motor. Also a 10 watt resistor isn't needed as it will just heat up. A 5 watt resistor will heat up half as much, 1 watt even less heat created. Remember 10watts at 12volts is ~.8amps so 1watt at 12volts is ~.08amps = less heat.

Anyways that all means that the car thinks the secondary air pump is still there as it has the exact same load, which means no code being thrown. Basically a hardwired option of coding it out of the ecu all together.

Wow. None of this is correct.

1 - The ECU does not measure the resistance or current flow of the SAI pump at all. The ECU signals a relay to turn switch it on, so it (the ECU) is completely abstracted from the motor's draw. It measures the flow via the O2 sensor. There is no reason to put a resistor in place of the pump...ever.

2 - IF a resistor was put in place, the wattage *does* matter. The wattage signifies how much power the resistor can safely dissipate without melting down. A fixed resistance across a voltage supply will ALWAYS draw the same current (in amps). As such it will dissipate a certain amount of power (in watts) that it must be rated for. Putting in a lesser wattage rated resistor than required is dangerous. It will get hotter than it can handle and melt down. It won't just draw less current. This is Ohms Law.

3 - As stated before, the SAI system flow is measured via the O2 sensors. No amount of resistor bypasses will change this. The ECU wants to see a certain amount of change in the O2 sensor reading. Nothing can do this other than actual AFR change accomplished by pushing fresh air down the exhaust. The only option is coding it out of the ECU program or dealing with the CEL.

4 - No worries, just make sure your info is correct before posting.

As for the N80 bypass. If the EVAP readiness, etc has been set in a BT tune for example, it doesn't bypass the device circuit checks. The ecu still wants to see that all the solenoids are there. In that case a simple resistor in place of the N80, N249, N75, N112, etc would work for keeping off the CEL. This is not the case for the OP. His EVAP must pass the leak test.

Euromike
10-22-2010, 10:14 PM
Thank you for clearing that up walky.

It looks like Im not going to trick the ecu on this one. I think what I will do is take the line off the ldp and blow in it, or use a very small amount of compressed air, and look for leaks. The system isn't that complex, I should figure out what's leaking. Thanks everyone!

dowsett6
10-23-2010, 07:13 AM
2 - IF a resistor was put in place, the wattage *does* matter. The wattage signifies how much power the resistor can safely dissipate without melting down. A fixed resistance across a voltage supply will ALWAYS draw the same current (in amps). As such it will dissipate a certain amount of power (in watts) that it must be rated for. Putting in a lesser wattage rated resistor than required is dangerous. It will get hotter than it can handle and melt down. It won't just draw less current. This is Ohms Law.


my mistake, i was converting backwards when remembering the ohms law and the triangle V over I * R