EVAP System diagnosis

Our rallyx car (B15 sentra) has an evap leak.  In the past year, we have replaced the canister, purge valve, vent valve, and check valve all with junkyard parts.  

Even E36 M3ty rockauto replacements are not cheap, so I want to be sure I've isolated the problem before ordering one.  To find the leak, can I do the following:

• Disconnect the evap line from the intake manifold.
• Shove air compressor nozzle into it.
• Turn pressure down to very low - 10psi or so?
• pressurize
• have some at the back of the car check around for the leak

Am I crazy?  Will I blow my gas tank apart?

Thoughts?

10psi should be low enough to not hurt anything.  And get a bunch of soapy water to spray on all the hoses and joints.  

Find someone with a smoke machine for EVAP testing.  A lot of the bits are buried in places where you may not get a good visual on what may be leaking.  The smoke should make it more obvious.

I almost bought one to diagnose the EVAP system on the ex's M3 (the E36 is apparently famous for these), but she sold the car (14 years ago). Since I have a R53 MINI now, I'm sure I'll end up getting one eventually.  Fortunately they come down in price since then.

I would only use regular see-through air out of desperation. A smoke machine is made for this. They're expensive for DIY'ers, but you may be able to beg some time with one if you have a connection with someone who works in a shop that has one. 

Well, sounds like I probably wont blow anything up or hurt anyone, so that's the important part.

I might be getting desperate :P  

Is regular see-through air just too hard to detect?  I can basically lay the whole system out on the ground if that helps, with only the lines running to the tank.  Everything else is pretty much attached to/around the evap canister in one big mess and can be dropped out fairly easily.  I feel like that with the soapy water like alpha suggested may work?

I'll think about who we may know with a smoke machine, but I don't really want to pay for a $$$ diagnosis on this car at the moment.

I would definitely do the soapy water and compressed air Route. For one thing it is cheaper

I don't know man. 10 psi doesn't sound like a lot, but remember psi means pounds per square inch, and a fuel tank has whole bunch of square inches. That equals a lot of force. I wouldn't do it. Call around to local shops and at least see what they would charge to use a smoke machine on it. Plus it's not as simple as seeing where it comes out. You need to use a scan tool to command the vent and purge valve closed otherwise your test means nothing.

Did you replace the gas cap?

Actually... no, I haven't replaced it.  That should be an easy check.

Any idea what the check valve in a typical system is rated for?  I could regulate down to that range.  If it sees engine vacuum in the 20in" range, isn't that roughly equivalent to -10psi?

Hrmm.... Aren't both the vent and the purge closed by default (so no vapors can escape when the car is sitting), and the check valve is parallel to the purge valve for safety reasons?  This diagram says otherwise, but it is confusing me:

If the vent is normally open, what is defined as a 'leak'?  Wouldn't it always be leaking?

It operates in a similar way to a PCV valve on the engine. It's designed to keep the system under just a bit of vacuum so the fumes get sucked into the engine - but not too much vacuum to cause problems with the vacuum fighting the fuel pump. Early systems were often T'd into the PCV, for instance. 

gearheadmb said:

I don't know man. 10 psi doesn't sound like a lot, but remember psi means pounds per square inch, and a fuel tank has whole bunch of square inches. That equals a lot of force. I wouldn't do it. Call around to local shops and at least see what they would charge to use a smoke machine on it. Plus it's not as simple as seeing where it comes out. You need to use a scan tool to command the vent and purge valve closed otherwise your test means nothing.

The tank is pretty regularly subjected to the opposite pressure, as the system is designed to leak test the system by drawing a strong vacuum.  Which is why I think 10psi positive should be ok.

In reply to ProDarwin :

In normal operation, yes, everything is "open" and flowing. But quite often, the system is totally shut off, the PCM has the engine draw a strong vacuum, and then the pressure sensor tracks the pressure change.  If the change happens too fast, there's a leak, slow enough, there is not a leak or the leak is small enough to not be a problem.