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I loosened the nut on the front of the filter and the fuel flowed out freely. Would this indicate that the filter is ok and the pump is pressuring the system sufficiently?

Replaced the O2 sensor in the exhaust pipe and the check engine light went off. Now to see if the mileage improves.

If you loosen the nut on the front of the filter (the side away from the fuel tank) and fuel flows out freely even with some pressure, would this indicate the filter is ok and the pump pressuring the system sufficiently?

This is for a 94 SC400.

Similar problems seems to be fuel related. I'm going to change the fuel filter in hopes that works else it may be the fuel pump which is in the gas tank.

The car is a 94 SC400. Just purchased. Had a basic tune up done wires and plugs.

I had the engine rebuilt about 6000 miles ago and mileage was steadily improving to an average of 23-24 mpg. Total miles to date 142000. Then it just dropped off to 18-19 mpg and the check engine light came on. Checked the codes and came up with 25 & 26. I have a original service manual and I followed the flowcharts as best able and seem to keep coming up with the Main Oxygen Sensor as the culprit. Just thought I would see if this is a common cause for these codes. My intention is to replace the sensor with a Bosch sensor which has the proper harness connector.

Don't know if you have got a response yet but mine was doing the same thing and it was a solenoid in the valve body. Cost a couple of hundred to get fixed.