P0446 & gas tank pressure sensor info

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P0446 & gas tank pressure sensor info

Postby spaceghost » Sat Apr 17, 2021 1:00 am

Picked up a 2000 Isuzu Trooper 3.5l late October, no issues during winter use in town but threw P0446 on occasion. With Spring's warmer weather we has us a puzzle. Getting P0446 more frequently, often during startup and now also getting P0101 & P0300 during highway trips (sometimes). Noticeable intermittent issue that only happens during 40F-ish or warmer temps - With engine hot or cold, sometimes dies when shifting into reverse or when stopping (not always). Accelerating sometimes makes check engine light flash, sometimes stays flashing at highway speeds. During accelerations sometimes misfires/loses power for a moment and continues again. Idle sometimes surges 500ish to 1200ish, but pulling a vacuum line off the top intake brings idle back to norm or slightly high. Is my first drive by wire Isuzu, so learning as I go.

History - Prior owner said the engine was replaced under warranty around 60k and tranny rebuilt around 80k, fuel pump around 100k. After buying, precautionary replaced: spark plugs, timing belt & water pump (did appear to be a newer engine, but did it anyway). Also changed OS2 sensors drivers side (codes), but probably was a loose connector after going through the change.

Current finding - Gas tank seems prone to overfill, and we see a small gas drip from very top of tank until it uses some fuel. Feeling around the top of the gas tank, found what I think is the gas tank pressure sensor attached the the wire but not attached to anything (has looks like the part that would go on top of the round fuel pump cage and has a vacuum port sticking out of it). Can't' see much to confirm, using up gas to drop the tank and see better. Guessing wasn't attached correctly during fuel pump change. Not finding pictures online to see what it should look like.

After seeing issue before finding fuel tank pressure sensor (if that is what it is), I tried a few things but no effect thus far:
-changed upper/lower intake gasket, cleaned up ERG tube/valve
-Fuel pressure measures good, vacuum measures good, didn't find leaks/issues (could be still).
-swapped throttle position sensor, MAF, air intake sensor and crank sensor from working Trooper
-new fuel filter

My guess is somethign to do with fuel system, but not sure how pressure should work. I'll try to drop the tank to see what I see, anyone has some pictures for what I should hope to find?
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Re: P0446 & gas tank pressure sensor info

Postby Buster28 » Sat Apr 17, 2021 6:58 am

Your truck has a fuel evaporative emission system ("EVAP") installed. The EVAP system captures gasoline vapor and stores the vapor to be burned in the engine. This prevents gasoline vapor from escaping into the atmosphere. The major components of the EVAP system are the Vent valve, Purge valve, Vapor Canister, Gas Tank, Gas Tank Cap, various pipe/hoses and the fuel Vapor Pressure Sensor.

The Gas Tank Cap is non vented, the Gas Tank is vented through the Vapor Canister and the Vent Valve. The Vapor Canister is connected between the Vent Valve and the Gas Tank. The Purge Valve is connected between the Vapor Canister and the intake manifold. The Vapor Pressure Sensor measures the absolute air pressure inside the gas tank.

The fuel tank Vapor Pressure sensor measures the pressure in the fuel tank and associated components. The sensor mounts at the top of the fuel pump assembly. The PCM supplies a 5 volt reference voltage and ground to the sensor. The sensor provides a signal voltage between 0.1 Vdc and 4.9 Vdc to the PCM. When the air pressure inside the fuel tank is equal to the atmospheric air pressure, such as when the gas cap is removed, the output voltage of the sensor will measure approximately 1.3-1.7 volts

In normal driving conditions the Vent valve is open so the pressure inside the gas tank is close to atmospheric pressure which is about 14.7 psia at sea level. To run the EVAP emission test the PCM commands the Vent valve shut and opens the Purge valve. This allows the engine to draw a vacuum (negative pressure) in the fuel system. The Vapor Pressure Sensor is used to measure the pressure in system, If there are no atmospheric air leaks in the system a vacuum can be draw, if the vacuum as measured by the Vapor Pressure Sensor is correct the various EVAP tests will pass.

.
DTC P0446 is set when the pressure in the tank is not at the correct value when the Purge valve is shut and the Vent valve is open. If the Vent valve fails to open or the Vapor Canister is plugged a vacuum will develop in the gas tank as the fuel pump removes gas from the tank.

Use a hand vacuum pump to test the Vent Valve and Vapor Canister.
The Vent valve is normally open, +12 Vdc is applied to one side of the Vent valve solenoid coil at startup. When the PCM wants to close the Vent valve it grounds the other side of the solenoid coil.
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Re: P0446 & gas tank pressure sensor info

Postby spaceghost » Sat Apr 17, 2021 2:44 pm

Thanks for the walkthrough, that helps me dig in. Right off the bat, looks like the vent valve 8-97233-997-0 is shot, but I am having trouble finding a replacement. Anyone have a good source for this one, or have come up with a different replacement approach (fitting something similar)?

EDIT - I think i found a Toyota option locally that may work, going to pick it up and try it tonight (Dorman EGR Vacuum Solenoid 911-606). Also getting new tank pressure switch, will report back if they do the trick.
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Re: P0446 & gas tank pressure sensor info

Postby spaceghost » Mon Apr 19, 2021 9:16 pm

The replacement fuel sensor and vent sensor (using available compatible Dorman Solenoid 911-606) seem to have resolved the check engine light and flashing when accelerating, but still seeing the stall when stopping or putting into reverse at times. RPMs drop to low when slowing down and getting under 25 to 20 mph it seems. Is there a way to adjust the idle on the drive by wire models? Or where should I start looking next? Appreciate the help and advise!
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Re: P0446 & gas tank pressure sensor info

Postby Buster28 » Tue Apr 20, 2021 3:30 am

DTC P0101 indicates there is a problem involving the Mass Air Flow (MAF) sensor. Monitor the measured air flow using a scan tool plugged into the DLC. The PCM is using measured air flow to control the air/fuel ratio.

Additionally, the engine will not idle properly if the EGR valve pintle does not shut tightly, make sure the pintle seat is not blocked with carbon and pintle shaft is not sticking. A leaky Positive Crankcase Vent (PCV) valve introduces a vacuum leak which will affect the idle. Replace the PCV valve. Also consider replacing the Fuel Pressure Regulator (FPR) valve to ensure correct fuel pressure.

The engine should idle around 850 rpm without having to adjust the mechanical throttle valve stop.
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Re: P0446 & gas tank pressure sensor info

Postby spaceghost » Sun Apr 25, 2021 9:20 pm

Swapped ERG and PCV from working Trooper, then took a long drive today to try to reproduce the symptoms and troubleshoot. After a jaunt on the highway, slowing down the idle issue showed up where the engine almost cut out and then revs up and down on repeat. Issue continued after parking at a nearby Autozone lot, muffler sounded like misfires, pulling vacuum hose off intake seems to improve things, pulling off other areas didn't do much. Got P0101 mass air flow code afterwhile, shutdown and restart kept symptoms. Shutdown, clear code and restart things were back to normal. Person in the parking lot said it might be a plugged cat, not sure will look into hot to test for that.

Didn't swap fuel pressure regulator yet, and my scanner only shows codes so I didn't get to see air flow air/fuel ratio stuff. I'll have to have a local mechanic help with that part. Does the drive by wire model have a mechanical throttle valve stop? I didn't see when digging around.
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Re: P0446 & gas tank pressure sensor info

Postby Buster28 » Mon Apr 26, 2021 5:57 am

The adjusting bolt on the left can adjust the throttle valve so that it does not shut completely. When the key is On and the engine is not running the throttle valve is not on the stop. I reasonably sure adjusting the throttle stop is not the cure for your engine problem.
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Re: P0446 & gas tank pressure sensor info

Postby spaceghost » Wed Apr 28, 2021 11:05 pm

Update - I forgot about the MAP sensor on top of the intake. I swapped that from another trooper and took a few spins over the last few days. So far so good, no idle issues. Gonna take a 200 mile highway trip this weekend, will see if the MAP was the cure.
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