Hello all, this is my first post here, I’m looking to see if anyone can help me troubleshoot an emissions issue.
Specifically, I get the P0451 code every other time I drive my car when it is around 40°F outside or above. I live in Minnesota, so during the winter when it gets below 40°F the code doesn’t show up for months.
I installed a new gas cap, had the evap check valve assembly replaced, and installed a new MAP sensor, both of those are listed as the solution per Subaru troubleshooting guide.
None of these fixed the problem so I am wondering if anyone knows something I might be missing related to this issue? I’m wondering why it stops happening below 40°F and if that is telling me about something I’m not thinking about.
I believe your car only does its self-diagnostics on the Evaporative Emissions System when the ambient temperature is above 40 degrees, so the problem is still there but the car doesn’t test for it.
You’re blindly replacing parts. All the parts you mentioned are possible causes (as are a dozen more) but they need to be tested and verified as good or bad before replacing. The fault code tells you the fuel tank pressure sensor is reading something unexpected. The fault could be the sensor itself, or another component in the EVAP system.
Doesn’t the Subaru Service Manual give you a list of possible causes? Could be a sensor problem, component problem (purge or vent solenoids, canister, vapor lines, etc…), wiring issue, physical leak in the vapor system somewhere, and so on. If you have the diag troubleshooting guide start testing components (if you are able) one by one until you find the issue.
It wasn’t very detailed, for that code it basically said to get the pressure reading from the evap check valve assembly and from the MAP sensor and replace the one that is furthest from the actual atmospheric pressure. I’ll try to get some wiring diagrams and troubleshoot it from there
If I were to toss the diagnostic test procedures aside and guess, I would speculate the reason the evaporative emission system pressure value is out of range is because the purge valve is failing to close during the test and applying too much vacuum to the evap. system.
System pressure can be monitored to see if the value is out of range. Purge valves, vent valves and gas caps can be tested before replacing.
That purge valve looks like the same leaky purge valve used on Toyota vehicles.
Do you have the turbo-version of the engine? I’m presuming yours is a non-turbo 2.5L 4 banger. The way to diagnose this problem likely varies b/t the two engines.
Your car’s EVAP system design probably requires the pressure of the airspace above the fuel in the fuel tank to fall in a specific range. It has a pressure sensor to make the tank’s pressure measurement but needs to verify the pressure sensor is accurate. It probably makes this determination when the vehicle is stopped and engine is off, using the map sensor ouput and the fuel tank pressure. If both the map and the fuel tank pressure sensor readings are about the same, that’s a good indication both pressure sensors are working accurately. When you or your shop compares the two readings, map vs tank pressure, what do you get for each? Post here. Normal atmospheric pressure is usually around 15 psi. If it doesn’t show “psi”, but something else (like “inch hg”), use Google to convert from one unit to another.
For example, if you type “convert 30 inch hg to psi” into google, it should be about 15 psi. That’s where I’d start anyway .If the pressure sensor’s are working correctly, you might have a leak in the evap system somewhere. If so, ask your shop if a smoke test might help determine the leak location.
Unless you a certain it is faulty, suggest to keep possession of the map sensor that came original to the car.
Note, that I’m just a knucklehead diy’er, offering an opinion, no specific Subaru experience. Best of luck.