Can P0446 Be Caused By Air Leak On Top Of Fuel Tank?

[dgorrie]

I've done quite a bit of reading about MIL P0446 on a 1999 RX300 (2WD), in this forum and elsewhere, so I think my next question is new.

In looking at the large hose off the vent control valve (at the charcoal canister in back), I read that one driver had found his hose clogged by dirt dobbers. So to test, I just cleaned it off and blew into it (lung power). I heard air leaking out "somewhere", and with the help of a neighbor, it appears to be coming out somewhere on top of the fuel tank (not anywhere near the gas filler cap).

We did note a rubber hose coming from the canister to a point in front of the LR tire, then up and over the tank, toward the area where the leak sounds like its coming from (can't see that area on top of the tank as yet).

My questions:
1) Since the EVAP system is supposed to be enclosed (holds vacuum, basically). I suspect that air leaking out toward the top of the tank indicates a bad hose or other issue. Is there any reason such an air leak would be "normal" whe it's under some lung pressure?

2) If this air leak is a legitimate problem, is there any way to see the area of the leak without dropping the tank? I understand there is an access port to get to the fuel pump -- if I get that open, will it make the top of the tank more visible, or accessible?

3) I finally found a tank diagram at AllData, and it shows two fuel cutoff valves on top. Is this what allows fuel vapors to reach the charcoal canister, while keeping liquid fuel inside? Even if a cutoff valve has failed (theoretically letting raw gas out), I'd think it would still be connected by the hoses to the charcoal filter to prevent an air leak to the atmosphere.

4) If I need to drop the tank to get to the area of the air leak (assuming it's an issue), it appears I've got to remove the fuel tank protector and exhaust pipe, but I've not seen anything further about the procedure to drop the (empty) tank. What else needs to be disconnected first, and how do you get to those points that are hidden? I suspect there is an access from the cargo area to the gas filler hose, and maybe the breather tube - correct?

Lots of questions here, so I'm obviously expecting the worst. Thanks for any guidance.

[beachlover]

Our rx300s are known to throw this code every few months. I just reset it, and go along happily for the next few months.....

[dgorrie]

I appreciate the response beachlover, but I'm pretty sure this one has a legitimate issue somewhere. I've had this RX since 2002, replaced 3-4 gas caps along the way, but this is the first time the issue/code has stayed. I've disconnected the battery at least once and cleared the code, only to have it come back within an hour of next use. I replaced the gas cap again when all this started, of course.

I've actually had this issue for several months, and pretty much just lived with it without any problem. But I now need to get it inspected, and it won't pass in Texas until I get it fixed. While I still need to do some basic testing of the Vacuum Switching Valve and probably the charcoal canister (at least), the fact that I can hear air leaking out somewhere above the fuel tank makes me think that air can also leak in when the ECM is doing its testing. Hence the interest in determining whether what I’m hearing is normal.

I could be on a wild goose chase, but I just don’t want to ignore the symptom without cause.

[bbratz]

dgorrie - Did you ever receive an answer on this post? Just received a call from Lexus dealership for the very same thing after a smoke test revealed there is a leak in the top of the tank near the main seal is. Repair is $1,200. I have also tried new gas caps and other recommended repairs from local shop in order to clear this error throughout the last 13 years I've owned it, but it seems to resurface more often now (every 6 mos). Does seem to happen when I try to squeeze every ounce I can into the tank. Maybe I'll just stop doing that! :) Bonnie in AZ