Need help with 98 Civic evap emissions problem
 

Anyone know how to correct this problem? I get 2 OBD codes:
P0450 - Evap emissions control system pressure sensor malfunction
P0453 - Evap emissions fuel tank pressure sensor high input

I've changed the 3-way valve leading to the charcoal canister, the canister vent shut off valve, the pressure regulator from/to the intake manifold. I tested the purge control solenoid and it seems OK, but I could be mistaken.

I've reset the PCM each time, but after running for a bit the check engine light comes back on. I also have a lope in the idle once the engine warms up, but I've assumed this is related to the Evap problem, and not something separate. Haven't rechecked the ODB codes since before I started replacing things either though.

I'm at a loss for what to do now ...

You need to find out why the FTP is sending a high output. Most likely the actual FTP has failed, but I would check out it's data PID while I purge the system to make sure it's functioning properly.

Agreed if you can access data through your scanner you will notice system pressure/vacuum. On a good scan tool you will be able to command the purge valve. If not you will have to manually do it by backprobing the connector.

You can also do a vacuum test with a vacuum pump. I think you have a fault in either the wiring or the sensor but you can monitor it through data in the scanner.

Thanks for the feedback.

Unfortunately I don't have the code reader anymore though. It was on loan, and was just a basic model anyway.

Hopefully it's just a bad hose that I can replace easily. Otherwise this is turning into a much bigger job than I had anticipated.

Any thoughts on the idle cycling up and down? I haven't found any bad vacuum lines or leaks, and I'm now thinking this is not related the emissions sensor issue if that is due to a bad fuel tank sensor.

The idling fluctuation is caused by the IACV. Try cleaning it before you replace the part.

the ftp sensor is located on the tank so there is nothing inbetween.