People on the forum frequently say that a gas cap gone bad can trigger EVAP and other smog error codes.

My current code is P1493... but it could be something else tomorrow.

Is there a reliable test, using nothing fancier than routine shop tools, to determine if a gas cap is OK or bad?

When I twist off the gas cap, there is a hiss or woosh but that might be pressure releasing instead of vacuum recovering. I read somewhere that the gas cap engages a valve function for air pressure both ways: opening a port to the world when positive tank pressure is too high, and closing to keep the world out when internal vacuum is too low.

So two questions:

-- If I open the tank and hear a woosh or hiss, does that mean the cap is good to go, or could it still be causing an error code?

-- Is there a method for determining whether it's operating within specs in both directions?

Otherwise stated: should the cap be replaced EVEN IF it hisses or whooshes, because there really is no practical way to test it?

 

For $18 delivered, I'd remove that possibility from the equation, despite that it's probably not the cause.

http://www.ebay.com/itm/NISSAN-OEM-F...-/262553843530

Do more research to prioritize the logical list order to find the cause based on your findings. Many times with the EVAP system, there's no easy answer. But one thing's for sure, always test your EVAP system vacuum lines to make sure they aren't leaking even a little FIRST.

https://www.autocodes.com/p1493_nissan.html

CS_AR is going through this right now.

 

Here's a link to a post and thread about how a faulty gas cap as preventative maintenance can save you a lot of headaches, time and money chasing an evap code P1447.

https://maxima.org/forums/4th-genera...ml#post9148476

Repost from the link above.

This morning I removed and tested the gas caps on the 98, the 99 I30, the Q45 and the new Stant cap and found the ones without problems all have one thing in common.

"When vacuum is applied to the inner part of the cap, they make a slight whining noise that sounds almost like a saxophone or a low pitch reed instrument." Then when pressure is applied, the valve closes clean and tight to stop the air flow. The faulty cap would not make the whine noise and the valve did not appear to be making a clean tight closure.

After replacing the cap, I drove the car for over an hour on the trip where the code P1447 likes to surface. The code stayed off with the new cap.

To prevent this from happening on my other 4th gen, I bought another Stant cap and will replace it later today.

I see where CourtesyParts and others also sell Stant locking gas caps, so they really can't be all that bad. The Stant caps appear to have a good seal.

Here's a link to CourtesyParts locking Stant caps.

http://www.courtesyparts.com/maxima-...sc?parent=1069


From the EC section of the FSM about Evap system diagnostics, here's the page that describes how to test a fuel cap.



Here are some pictures of the different Nissan caps that I tested this during the process of elimination. I purchased the Stant cap in the picture (for the 98 and 99 models) at AdvanceAuto for $10.34 + tax.



Here's the front view. Notice how the Stant and the OEM cap look like they come from the same mold.




Bottom Line: For less than $13, you can avoid evap code headaches by just replacing an old cap as preventative maintenance.