90 LS400 - Excessive Gas Tank pressure

Warmer weather?

Thanks for your reply. As stated, I have ALWAYS experienced 'some' pressure, but on the return trip it was greater than EVER before. I guess I would like to understand exactly what causes the condition of pressure build up, and if it is a telltale sign of component failure (i.e., fuel pump) or some vent being clogged. As stated, other than this 'pressure' condition, I notice no other performance related issues. Thanks

Modern fuel systems are NOT vented (except maybe through a charcoal canister). EPA does not want volatile organics (gas fumes) getting out. Our '92 LS400 has always exhausted some "air" when the gas tank is opened after the car is driven a long distance, especially in hot weather. Most noticeable with a less than full tank for obvious reasons -- more room for a headspace. It's just heat causing more evaporation of the gas in the tank and pressure build-up. Seen (heard) the same thing with our other non-vented tank cars. Never been a problem.

If you are getting pressure build up in your fuel tank there is something terribly, terribly wrong.

Modern automotive fuel systems are designed to operate with a small vacuum in the tank. The 'whoosh' you hear when you open the fuel cap should be air entering, not leaving the tank.

Well maybe it is a vacuum, but all I know, is that recently the 'whoosh' has been louder and longer than I remember in the past.

Could be a blocked line from the evap control system. Check your charcoal canister and the associated vent line.

If this was the case, should it be viewed as something that needs immediate attention, or something that could wait until the next time the car was serviced? The car is behaving itself as usual (just crossed 200K last week) Thanks

it may be a coincicence, but I noticed it for the first time when I ran the tank to near empty (something I have never done before).

I wouldn't worry about it then... it is true that the vacuum will be more pronounced when the tank is near empty (because of the increased ratio of air to fuel in the empty tank). Besides, if the vent line was completely plugged it is most likely that the car would starve for fuel before there was any damage to the fuel tank (as a result of a collapse under vacuum).

Just in case you misunderstood me, I FIRST noticed the situation after I ran the tank to near empty. It has CONTINUED ever since. You think that it could have anything to do with the fuel filter?

No. Check the evaporative control system (charcoal canister and associated vent lines). If they are plugged it could cause excessive vacuum. I don't know if the OBD-I system has any diagnostic capability for this problem, so you may not get an OBD idiot light.