I have a 91 LS400 with 164,000 miles. It has been a great automobile until recently, but currently it has me baffled. The vehicle, without warning, will suddenly lose power and drop rpm. It doesen't die, it will stumble and miss and it just can't get the rpm back up. This may last a few or several minutes, then just as suddenly the malfunction corrects itself and everything is back to normal. The car is fine again for maybe 50, 100 or 200 miles and then it will re-occur. Strangely, the CEL never comes on and I can never find any stored error codes.
At first I concluded that the fuel filter was blocked and replaced it. No change. Then I suspected water in the tank, so I drained and refilled. No change. I can't believe that the fuel pump can be the culprit, since the malfunction recovers. In my experience fuel pumps fail terminally. I still suspect a fuel problem. Everytime I open the fuel filler cap I hear a lot of in-gassing. Does anyone know how the fuel tank vents? Could conditions in fuel tank create a vacumn that the fuel pump is struggling to overcome? Does the tank vent through the filler cap?
Could I be completely wrong about the fuel issue and have some other obvious problem? If you get no CEL or error codes where would you start trouble shooting?
Any insights or suggestions you might have are appreciated.
Thanks