Vacuum pressure when removing fuel cap - solenoid?

[iamdub]

Yeah, both of those refer to the circuit and not the EVAP flow (valve stuck open or closed). A faulty solenoid could be shorted (code P0445) and it being inoperable or the circuit being open could cause the P0443. Of course, the valve and solenoids could be fine and there could be a wiring fault. Testing this is simple if you have a DVOM and are proficient at using it. Since the PCM commands the valve to open with a ground signal, then the ground wire would be what's shorted and keeping the valve open at all times.

But, I really think the circuit is fine and the valve is just stuck open and allowing the engine to constantly suck air from the tank, giving it a vacuum.

 

[DonnieBrasco]

Yeah, both of those refer to the circuit and not the EVAP flow (valve stuck open or closed). A faulty solenoid could be shorted (code P0445) and it being inoperable or the circuit being open could cause the P0443. Of course, the valve and solenoids could be fine and there could be a wiring fault. Testing this is simple if you have a DVOM and are proficient at using it. Since the PCM commands the valve to open with a ground signal, then the ground wire would be what's shorted and keeping the valve open at all times.

But, I really think the circuit is fine and the valve is just stuck open and allowing the engine to constantly suck air from the tank, giving it a vacuum.

Just for my own clarity here...the "solenoid" is the mechanism (for lack of a better term) that opens and/or closes the valve, correct? The valve is, well, the valve. Also, I've got a multimeter, think I've got two actually. I know one is not digital, can't remember the other one. Does it have to be digital?

Also, is the "PCM" the main computer? I think I always called it a body control module, the BCM. Or are those two different things?

 

[iamdub]

Technically, yes, the solenoid is the electromagnetic actuator that control the valve. In this sense, we're referring to the purge valve solenoid located on the manifold as the "solenoid", and we're pretty confident that this is the failed part. The other part near the rear axle is something else- a valve that may be solenoid-activated and part of the EVAP system, but not what we're going for here.

A digital volt/ohm meter is best for this sort of diagnostics as you're really just checking the circuit for continuity. Even a $3 Harbor Freight meter with audible continuity tester would be fine.

PCM: "Powertrain Control Module" This is what most refer to as the "computer" or "brain". It controls the engine and emissions functions and communicates with the TCM (Transmission Control Module) that has been a separate computer since the GenIV motors (I believe this is when that change occured). The BCM is an entirely separate computer that controls the body functions- power locks, gauges, infotainment, lights, etc.

 

[DonnieBrasco]

Technically, yes, the solenoid is the electromagnetic actuator that control the valve. In this sense, we're referring to the purge valve solenoid located on the manifold as the "solenoid", and we're pretty confident that this is the failed part. The other part near the rear axle is something else- a valve that may be solenoid-activated and part of the EVAP system, but not what we're going for here.

A digital volt/ohm meter is best for this sort of diagnostics as you're really just checking the circuit for continuity. Even a $3 Harbor Freight meter with audible continuity tester would be fine.

PCM: "Powertrain Control Module" This is what most refer to as the "computer" or "brain". It controls the engine and emissions functions and communicates with the TCM (Transmission Control Module) that has been a separate computer since the GenIV motors (I believe this is when that change occured). The BCM is an entirely separate computer that controls the body functions- power locks, gauges, infotainment, lights, etc.

Excellent! Thank you. Ok...

So, you're recommending to first check the circuit for continuity with a DVOM, yes? Then, depending on whether that passes/fails, replace the valve (at the manifold, not underneath at the rear), yes?

I want to get it taken care of sooner rather than later since it's effing up my fuel gauge accuracy and I'm sure it's killing my fuel economy.

 

[iamdub]

One of the wires going to that solenoid is a constant 12V+ and the other is a negative ("ground") that is controlled by the PCM. When the PCM wants to open that valve to let the engine suck in the fuel vapors, it connects that wire to a ground which completes the circuit so power can flow which activates the solenoid. It's just like having your tongue always on the positive terminal of a 9-volt battery: you will never feel anything because the power isn't flowing. As soon as you rotate the battery so that the negative touches your tongue, the power flows and your tongue gets zapped. The PCM grounded the EVAP circuit to open the solenoid, but saw that the circuit was still open, so it threw a circuit code. Or maybe it saw current flow (closed circuit as commanded), but it saw TOO much flow, indicating a short in the circuit (shorted solenoid).

With key on, and negative lead of your DVOM on a known good ground, you should have 12V+ at one terminal. Switch your DVOM to test continuity and probe the other terminal and you should not have continuity unless the PCM is commanding an EVAP purge (and it shouldn't be). The solenoid is easy to remove. Pop it off and try to gently blow through it. If you can, then it's stuck open and needs replacement.

I still don't see how this would have any effect on fuel gauge accuracy or fuel economy.

UNLESS... The EVAP system has a leak somewhere and the fuel is dripping out or evaporating. You're not getting worse MPG, you're just losing gas. And maybe this leak is allowing air to get in the tank which introduces moisture for the ethanol to absorb which causes the rheostat of the level sensor to corrode and read incorrectly...

That last paragraph was really farfetched. Besides, that would throw something like a code P0455.

 

[DonnieBrasco]

One of the wires going to that solenoid is a constant 12V+ and the other is a negative ("ground") that is controlled by the PCM. When the PCM wants to open that valve to let the engine suck in the fuel vapors, it connects that wire to a ground which completes the circuit so power can flow which activates the solenoid. It's just like having your tongue always on the positive terminal of a 9-volt battery: you will never feel anything because the power isn't flowing. As soon as you rotate the battery so that the negative touches your tongue, the power flows and your tongue gets zapped. The PCM grounded the EVAP circuit to open the solenoid, but saw that the circuit was still open, so it threw a circuit code. Or maybe it saw current flow (closed circuit as commanded), but it saw TOO much flow, indicating a short in the circuit (shorted solenoid).

With key on, and negative lead of your DVOM on a known good ground, you should have 12V+ at one terminal. Switch your DVOM to test continuity and probe the other terminal and you should not have continuity unless the PCM is commanding an EVAP purge (and it shouldn't be). The solenoid is easy to remove. Pop it off and try to gently blow through it. If you can, then it's stuck open and needs replacement.

I still don't see how this would have any effect on fuel gauge accuracy or fuel economy.

UNLESS... The EVAP system has a leak somewhere and the fuel is dripping out or evaporating. You're not getting worse MPG, you're just losing gas. And maybe this leak is allowing air to get in the tank which introduces moisture for the ethanol to absorb which causes the rheostat of the level sensor to corrode and read incorrectly...

That last paragraph was really farfetched. Besides, that would throw something like a code P0455.

I did have a 455. See the last post on page one. I'd link it but my mobile is acting screwy.

 

[iamdub]

Sure did! I looked up P0445 and not P0455. My mistake.

Hmmm... That code refers to a large leak, but clearly you don't have a leak in your tank as it's under vacuum. Unless the constantly stuck open solenoid (although yet to be confirmed) lets the tank "breathe" in through the intake manifold when the engine isn't under load (vacuum).

Let's start with the free and easy tests for now- remove the purge valve from the manifold and see if you can blow through it.

 

[DonnieBrasco]

Sure did! I looked up P0445 and not P0455. My mistake.

Hmmm... That code refers to a large leak, but clearly you don't have a leak in your tank as it's under vacuum. Unless the constantly stuck open solenoid (although yet to be confirmed) lets the tank "breathe" in through the intake manifold when the engine isn't under load (vacuum).

Let's start with the free and easy tests for now- remove the purge valve from the manifold and see if you can blow through it.

No worries! Ok perfect. That's what I'm gonna try this weekend. Might as well check the circuit for continuity as well.

To your point about it not being related to the fuel gauge being weird...
The huge pressure vacuum build up in the tank couldn't affect the fuel gauge? I ask because I don't know how those gauges work.

Also, finally, some or all of this could be stemming from my computer. I've suspected it being messed up for a while. Just weird stuff for years. But nothing detrimental enough to deal with replacing and reprogramming a new one. But all those issues are for another post another day.

Thanks a ton for all the excellent advice and assistance.

 

[iamdub]

The fuel level sensor in the tank is just an arm with a float on the end of it, just like what's inside of a standard toilet tank. Well, at least the older ones. The newer ones have some sort of tower with a float that rides up and down on it. Anyway, it wouldn't matter if it was in a vacuum or not, it would still be able to move around just as astronauts, spaceships, comets, planets, aliens, etc. are able to move around in the vacuum of outer space.

If your computer was throwing bogus codes, then you wouldn't be experiencing physical symptoms such as the the tank being under vacuum and breathing when you removed the gas cap. I'm not saying that your PCM is NOT messed up, though.

 

[DonnieBrasco]

A digital volt/ohm meter is best for this sort of diagnostics as you're really just checking the circuit for continuity. Even a $3 Harbor Freight meter with audible continuity tester would be fine.

I've got this meter: http://www.harborfreight.com/7-function-digital-multimeter-90899.html

Don't believe it's got audible continuity but I can still check continuity with it. Should be fine, yes?