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Hi, Can someone point me where to diagnose the problem. thanks Ron
EVAP System Pressure Sensor Malfunction

that means when the EVAP fuel tank vent system is tested this fails.
first make sure gascap not missing or loose.
there are many tests for this complex system.
what car, and year?
then broken hoses from (rubber) from tank vents to evap charcoal canister up front. (and the regulator in line and the tank sensor wires not broken)
that is a start .... more later.
Its a 98 tracker I have checked for broken hoses and loose connections. I have been using the shop manual to check the individual parts. So far I haven't found anything.
in a shop they do full tests using pressure tests.
some even use nitrogen gas to do that ,as adding air to fuel tank is a danger .
there are 2 causes (phyisics) 1: the sensor lies. or 2: there is a leak.
they pressurize the tank at the cap (first testing cap first,did you try new cam first? the vacuum breaker or gasket can fail)
the shop uses a special cap with a nipple on it.
they go to 1/2 PSI (as the FSM shows) and see if the leak down test fails.
in EPA smog station in most states , they do this first. if fails you are sent home with nasty letter, LOL
if it leaks next is finding the leak, that too is not easy.
from bad hoses.
cracked hoses.
loose or fell off
or or top of tank rust though.
or the 4 or more things on top of tank (pump plate, level sender plate or 2 vent plates all leak at the gasket)
using soapy water can find it (bubble while pressurized)
you can see here the sensor maxs out at 0.5 pSI (1/2)

the TPC valve can fail and leak.
my page coveres this
but you have 1998 and that has EVAP2 , more parts to fail.

about all you can do is
replace cheap parts, cap, TPC,
pressure ize it and do leakdown tests.
or try to replace the sensor.
the 1998 has more solenoid valves that evap2 , now does more tests using more valves that can fail.

The 1998 EVAP2 IIRC does 2 tests now, at least.
not just the 1 pressure test
it does 2 tests, one to make sure pressure is wrong, using the solenoid called, bypass valve, then closed the valve and then checks pressure only if the tANK IS 1/2 FULL OR LESS.

I think the TPC keeps the tank at 1 psi pressure all the time, (not 100%full) but as the fuel worms up it evaporates and the air and fuel fumes open the TCP at 1psi.
so if you fill up with fuel the cap goes WHOooosh, this is the 1psi leaking out fast.
we all know this at fill up time
the trick is the ECU must only run the test if it knows for sure, the tank has pressure.
and that is time and temperature issue. IDK.
the tank does not go to vacuum due to vacuum breaker in cap.
so only 0 psi and 1psi possible, and on a hot day it is 1psi for sure.
thanks for the info

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