Vacuum in gas tank - already replaced almost everything

[HOG4FUN62]

Mine was throwing the 455 code and ended up being the Purge Valve. I started with the Gas Cap on mine because it was the cheapest/easiest to fix. When that didn't work, smoke test showed leak at the purge valve.

 

[DonnieBrasco]

Pressure release when you open the cap is probably actually air rushing in which would mean the tank is under a vacuum. Unless there's something pressurizing the tank and the air is blowing out when you crack open the cap. At least you know the cap's seal is good! But, if there was a leak somewhere, then the tank wouldn't be able to hold air and pressurize- positively or negatively.

Even though you replaced the vent and purge solenoids, the circuit controlling them may be faulty (broken wire, loose terminal in side of a plug, etc.). The purge solenoid (on the motor) should steadily be pulsed with the engine running and the pulses should increase with RPM. With the engine running, unplug the vacuum hose going to it and put your finger over the nipple on the valve- you should be able to hear and feel the air being sucked into it. Unplug the electrical connection to it and see if the pulsing stops. If so, it's responding and likely operating as normal. The vent solenoid near the tank is controlled by the PCM to open when certain parameters are met. This is where a bi-directional scan tool and a simple cheap test light come in handy. It can command the vent solenoid to open for diagnostics. If the plug at the vent solenoid doesn't receive the signal when commanded (test light illuminating), then the circuit is open (broken) somewhere.

I can't tell you how thankful I am for this comment. This explains SO much about this issue. As much as I've tried to track this down this is without a doubt the best explanation I have gotten. Thank you a millions times. I will try the trick with the purge solenoid and see what that does. As far as the scan tool and test light for the vent solenoid, is that something I can do myself? I think you're spot on that this is an electrical and/or PCM issue.

 

[DonnieBrasco]

Mine was throwing the 455 code and ended up being the Purge Valve. I started with the Gas Cap on mine because it was the cheapest/easiest to fix. When that didn't work, smoke test showed leak at the purge valve.

Can you do the smoke test at home? Or is this out of my wheelhouse?

 

[HOG4FUN62]

You need the "Smoke machine" to insert into the evap system. If you know someone with one you can do yourself. There are videos on YouTube to show how to make one or you can buy one for from $40 - $2K and how to hook into your evap system. Smoke test will show you where you leak is instead of just replacing parts.

 

[bigfootchiro]

Purge valve (bottom picture that Rocket Man posted), if faulty, will cause the fuel tank prsssures to drop due to a vacuum leak within the unit. It will not allow the fuel tank pressures to reach high enough to supply enough fuel to the engine, causing the evap code and reduced engine power. I went through this exact same thing with another vehicle.

 

[iamdub]

I can't tell you how thankful I am for this comment. This explains SO much about this issue. As much as I've tried to track this down this is without a doubt the best explanation I have gotten. Thank you a millions times. I will try the trick with the purge solenoid and see what that does. As far as the scan tool and test light for the vent solenoid, is that something I can do myself? I think you're spot on that this is an electrical and/or PCM issue.

You can do the scan tool test if you have one or have access to one and a knowledgeable user. All you're doing with it is making the PCM command a signal on the vent solenoid's circuit. With a test light on this circuit, it should cycle on and off as you command it to. If not, then you may have a broken wire. Trace it back and you may have to probe the wire and test again. You may trace it back to the connector on the PCM and find a loose or broken pin. But if you probe it along the way and get power (or ground), then you passed up the break in the circuit. I'd check the ends of the circuit first. The chances of a break in the harness in between are slim, unless the wire has been moved or rubbed by something.

It's somewhat of a complicated system, but the fundamental physics are still there. Ever tried pouring gas from a can with the breather (vent) cap closed and it "glugs" as the sides of teh can flex in and out? The same thing happens with your fuel tank only at a much slower rate. The pump is pulling fuel out of it just like you were slowly pouring that gas from a can with a small hole. The empty space must be replaced by air. If no air can get in, it starts to create a drop in pressure (approaching vacuum), which makes the sides flex in. At lower fuel levels, the negative pressure is so great that the pump is under more strain trying to pull the fuel against it. The tank having a vent lets it breathe so the fuel flows. That's the basics of it. The actual components and processes are where it gets complicated.

Having a vent to let air in means air (fumes) can get out which is an emissions issue. It can also be nauseating if you park in an enclosed garage. The EVAP system is designed to collect and store the fumes in a charcoal canister to be burned off later. So, as much of a nuisance this system can be at times, it's a good thing to have. Instead of your expensive gasoline slowly evaporating into the atmosphere, it's staying contained and eventually still making it's way into the motor.

The PCM monitors the tank pressure with what is essentially a MAP sensor. When it seems the pressure get too high, it opens the vent valve to let the pressurized fumes go into the charcoal canister, then closes the valve when the pressure is at it's programmed threshold. If the valve gets stuck open or otherwise leaks, then this will create a constant vacuum on the tank, sucking out all the air.

If the electrical all checks out and you wanna test the lines, you can use an air compressor with low pressure. I was helping my neighbor with a similar issue and we disconnected the line from the purge solenoid and from the canister. He plugged one end and I used a blow gun with a rubber tip that sealed tightly into the line to blow low pressure into it. Non-moving air makes no sound. He heard a faint hiss and we discovered a split in the plastic part of the line in one of the curves. Turns out he hit it a bunch of times when changing his oil pressure sensor a few months prior and must've cracked it.