This post describes one of the diagnosis steps for the P1491 codes on a 1999 Nissan Altima. Specifically the testing and replacement of the vacuum cut valve bypass valve. This applies to all second generation Nissan Altimas which are 1998, 1999, 2000, 2001.
The vacuum cut bypass valve is installed in parallel with the vacuum cut valve on the EVAP purge line that runs between the fuel tank and the charcoal canister.
The purpose of the vacuum cut valve (white part in the pictures below) is to prevent the intake manifold vacuum from being applied to the fuel tank. The vacuum cut valve bypass valve is energized (opened) for diagnostic purposes to bypass the vacuum cut valve to apply the intake manifold vacuum to the fuel tank. This verifies the intake manifold vacuum.
The vacuum cut valve bypass valve (what a mouthful!) is located just inside of the passenger side rear tire, next to the gas tank and spare tire compartment. You can locate it by looking for the charcoal canister which is a large, black square box on the drivers side of the gas tank, and then following one of the lines until you see a white cylinder with four tubes coming out of it. That is the vacuum cut valve. The vacuum cut valve bypass valve is located on the same metal bracket as the vacuum cut valve. Here is a picture of the bracket/valve assembly after it has been removed.
In order to check the vacuum cut valve bypass valve we need to first remove the assembly.
Make sure your car is off, disconnect the battery and block the tires.
1. Slide the two hose clips away from the white vacuum cut valve and carefully remove the hose from the vacuum cut valve tubes. Twisting sometimes helps.
2. Remove the two 10mm bolts that hold the metal bracket assembly to the chasis. Be careful to no strip these bolts. I had one of mine strip and had to use mechanic wire to fasten the bracket in place. I suggest using PB Blaster well in advance of doing the actual removal and letting it soak.
3. I like to save the electrical connector for last so that you can have the assembly dangling low for easy access. Push the green tab "in" and pull "out" on the exterior. You should be able to hear/feel a click when you get the green piece pushed in far enough.
4. Unbolt the vacuum cut bypass valve in order to diagnose it using the below steps.
Perform the exact same battery energizing test that I used on the similar EVAP purge solenoid valve. If you don't hear the solenoid valve click/move (or air passage) when power is applied then yours is bad!
I was only able to find a replacement over at Quality Resale Auto Parts (the website is a bit sketchy but they shipped the correct part, as described, and quickly) for $38 shipped. It's hard to beat that. Here's a comparison shot of the old vs. new solenoids. Why don't they encase/seal these parts better? Darn you northeast salt!
The steps to replace the assembly is just the reverse of the removal.