4doorH22

Okay I hope I don't get flamed this time. Why do some gas tanks "suck" (no jokes please) when you open the tank for filling up and some don't? Seems to me some cars must have a one-way valve on the gas tank to allow the tank to remain at atmospheric pressure instead of creating a vacuum as fuel is depleted. Do some cars depend of a vacuum in the tank? Seems to me the fuel pump just needs to work harder drawing fuel from a sealed tank. Would opening the fuel filler cap at, say, 1/4 tank to equalize the pressure be beneficial in any way? (Nah I'm not looking for power gains, just curious about fuel systems).
Thanks for info.

BreakStuff

By the year of 1999 the EPA mandated that every car sold in the US be equipped with a Onboard Refueling Vapor Recovery System (ORVR) The purpose is to capture fuel vapors and prevent the vapor to reach the atmosphere..

You hear this very vacuum when you remove the fuel cap and the tank is below atmosheric pressure, it draws the fuel vapor to the charcoal cannister where it can be stored and later drawn into the intake manifold where it can be burned..

Very good observation 4doorH22.. the fuel level in the tank is always changing so the ORVR system has to detect the changes in the atmospheric pressure , thats where the Fuel Tank Pressure sensor comes into play, it will detect those changes and adjust the vacuum accordingly.. atmospheric pressure also varies in certain geographical locations, Bingo! Barometric Pressure Sensor! The are many things involved and the list goes on and on..

The ORVR vacuum is rather small compared to the draw from the fuel pump, the fuel pump will pump more fuel than air being sucked out , The fuel pump will accomadate a great deal more pressure than ORVR vacuum thus the need for a fuel pressure regulator.. On a normal working system, If you were to block the regulator return line the pressure from the fuel pump will blow a seal somewhere in the system, ... The ORVR system consists of quite a few components and I know its quite a bitch to diagnose but you already seem to have a handle on whats involved... I just filled in the blanks

fixhondas

Depending on how much fuel is in the tank and what the temperature is and the amount of fuel slosh and what model the car is will determine if you will get a vacuum or pressure release when you remove the gas cap. If you have an almost empty tank the fuel will slosh around alot and evaporate (hence the term evaporative emissions) and create pressure. Pressure is also created by heat from the fuel pump and fuel passing through the hot engine compartment via the return line. The ORVR system that breakstuff is referring to is actually for preventing gas vapors from spewing out of the filler neck during refueling. It works by sensing pressure change through a pipe located near the top of the filler neck when you remove the gas cap. IF you want more detail on the ORVR system let me know- It will take a few paragraphs to fully explain it.
Now, usually gas tanks will have pressure instead of vacuum, depending on the system. One way to get a vacuum is to fill up the tank on a warm day and park it for a while. As the temperature cools the gas in the tank condenses and creates a vacuum. There really isn't a vacuum formed as fuel is used beacause of the heat that is added and the physical movement of the gas through the return line causes evaporation and pressure. IF you monitor the fuel pressure on a car that has sat all night it will usually start as a vacuum (when it is cool outside). When you start the engine the tank pressure slowly rises- the rate it rises depends on temperature and how much fuel is in the tank.
Newer cars will have a gas cap that states," tighten at least 3 clicks or check engine light may come on." How does this happen? The ecu monitors fuel tank pressure with a fuel tank pressure sensor (similar to a MAP sensor). When the car is started the ecu looks at tank pressure and depending on the temperature of the engine coolant and intake air temperature- it determines whether or not to monitor the fuel tank pressure. When it does monitor tank pressure, certain things must happen to pass the monitor. The ecu watches how fast the pressure rises to determine approximately how much fuel is in the tank. (remember- less fuel=more slosh=more pressure faster). If it determines there is enough fuel in the tank to continue the the test- it will open the purge solenoid and close the canister vent shut valve and pull a vacuum on the system up to the bypass solenoid. Then it closes the purge solenoid and monitors the fuel tank pressure sensor as it opens the bypass solenoid. It should see a pressure drop. This is how the ecu tests the front half of this sytem- if it fails this part 2 times consecutively, the MIL comes on and a code P1457 (evap leak canister) is set. So, let's say it passes this test the first time. The ecu will then test the fuel tank half of the system. It opens the bypass solenoid and the purge solenoid and closes the CVS valve to put a vacuum on the fuel tank. Once the FTP sensor has reached a certain voltage (vacuum) the ecu then closes the bypass solenoid and monitors the FTP voltage. If it holds steady for a determined amount of time- the system passes. If it detects a leak here 2 consecutive times, the MIL comes on and code P1456 is set. This is where the loose gas cap comes into play. Leave the gas cap off and after a while the MIL will come on and you will have a code 1456. If you put the gas cap back on tightly and drive it for a few days or weeks or even months- the light will eventually go out and the code cleared. It has to pass a certain number of "Trips" for this to happen.
It is very important to note that you should NEVER "Top off" the gas tank in these cars. If you do- fuel gets down inside the vapor signal tube of the ORVR valve and is sent into the 2 way valve and bypass solenoid and FTP sensor. These are then damaged and can give a 1456 or 1457. So- when the fuel pump shuts off don't go clicking it a bunch more times to the next dollar- you might end up spending a few hundred to fix this problem- that is if-you can find someone to diagnose it properly.
Hopefully I answered something or confused you excessively. Whatever.

4doorH22

Hey you are the two cool guys who answered my last question about bleeding ABS.
Good stuff guys. Thanks a lot for the very informative and fascinating information.
Yeah I'm sure negative pressure is preferable in a gas tank, don't want those nasty hydrocarbons or anything coming out, just going in.
They say a modern car is cleaner at 55 mph than a car from the sixties is sitting in the driveway (due to emissions from paint and gas and stuff, but not proven).

allenp

Talk about bringing a thread back from the deep! The archive search seems to be working well.

My 00 GSR threw code P1457. I have since refilled the fuel, cleaned the cap, made sure it was tight, and reset the ECU with a battery disconnection. Alas, the MIL came back on this morning. I have to assume it's the same code.

Do I understand correctly that 1457 is NOT indicative of tank pressure, but rather canister pressure? And that this code would not be thrown from a mis-tightened cap?

Stew Pidasso

I always wondered this myself. I never knew it was so "high-tech" though. Thanks for the info...

JimBlake

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by allenp &raquo;</TD></TR><TR><TD CLASS="quote">... Do I understand correctly that 1457 is NOT indicative of tank pressure, but rather canister pressure? And that this code would not be thrown from a mis-tightened cap?</TD></TR></TABLE>
Yup... I recently had a P1457 on the '98 Accord, and like you said, its the canister vacuum, not the tank vacuum. The guy at AutoZoned who read the code told me I hadn't tightened the gas cap. But he did tell me the code number, so I could look it up myself in the Helm manual.

The Helm book has about 6 pages of troubleshooting for this code, starting from the purge valve on the intake manifold... You need a DMM & a MityVac. There's several valves, vacuum lines, the canister itself, and a pressure sensor that could be causing it.

allenp

Hey, another northeastern Ohio H-T member. Don't see many of us!

Were you able to diagnose/repair your system? Forgive my ignorance, but what is a DMM?

JimBlake

DMM = digital multi-meter. For checking wiring, to see if any of these parts are actually getting 12v power when they should... You'll need to measure voltages & resistances.

I posted some detail about this for someone else, but I can't find it now. I can't think of a good keyword to search for, and the switch that selects title/body search doesn't work? Looks like it only searches the title so it was probably a response to something with a title like "help me"

I went thru something like this...

Purge valve on manifold - apply 12v power by jumpering some particular pin on the ECM. Then the valve is supposed to hold vacuum. If not, look for short or open in the wiring.

Next check some other valve underneath the car. You disconnect a hose, plug another hose, or something like that. Then check whether it holds vacuum.

Eventually I found it was the canister vent valve. The remainder of the procedure includes dropping the gas tank to check a valve on top of the tank. After you've eliminated the easier stuff...

You need the Helm book for your car, since its probably different than my '98 Accord...

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by allenp &raquo;</TD></TR><TR><TD CLASS="quote">Hey, another northeastern Ohio H-T member. Don't see many of us!</TD></TR></TABLE>
Stew Pidasso's around here somewhere too, I don't know where...

allenp

Thanks much for the advice. I actually JUST got the Helm manual, good timing apparently.

Looks like I have my weekend plans.