Like a lot of others, I seem to experience an intermittent rough idle on my 4.6. The other day, I go to take off the fuel tank cap and there appears to of been a "vaccum" in my tank....... You could hear the air rushing in when I removed the cap. After fueling, the truck seems to idle better for quite a while, then eventually get rough again, until, I remove the cap relieve the "vaccum" from the tank. Anyone else had this happen?? Any thoughts?? Anyone else notice a vaccum in their tank as they use fuel??

TIA
Rick

 

that definately should not be able to happen... sounds like something is kinked in either the rollover valve hose or the vent tube (or both!). Plus, given that gasoline has a relatively high vapor pressure (hence the warning about spray on the fuel cap) it doesn't seem likely that there is vacuum forming, but it's definately possible... I'm thinking I might start by replacing the cap... seems to me that there was a post a while back about this exact problem, and changing the cap solved it. While you're at it, you could upgrade to a locking cap for a whopping $20... that would eliminate that from the equation... I'd still look real closely at the vent tube, though... it could be obstructed or kinked someplace preventing the tank from breathing.

Question... when you crack the cap with the engine in its sputtering mode, does it get better immediately?

-Joe-

 

Any idea where either the rollover valve hose or vent tubes run??

Last nite, I tried removing the cap while the truck was running and after driving it for a bit. It seemed to stumble, then, it eventually smoothed out....... Definetely affects engine performance.

R...

 

Are you sure it is vaccume you are hearing? The tanks in our trucks are pressurized to prevent vapors from excaping into the atmosphere. Could the noise you hear be pressure excaping when you open the cap?
If pressure is building up then replace the cap. If you leave it off or loose then you will get a code from the eprom.

 

Well, I think it is vaccum............ I'll check closer and see if it's pressure..... I would think though, if it were pressure, you'd have a fairly strong gas odor when you removed the cap??? I definetly don't...............

R...

 

Where is the vent tube? I have a whistle coming from the tank area when the truck is running, it goes away when I open the gas cap to relieve the vacuum. Tried my original cap with no luck. Using a locking cap presently.

Thanks,
John

 

totally normal! It even talks about it in your owners manual.

if the tank is low the pressure is higher. once you fill it up the pressure is lower.

if your idle is affected by fuel level in the tank you prolly need your fuel filter changed.

 

I agree with hcmq; by the way, that seal on the gas cap must be good if it hisses at you.

From what I've seen and understand, if you leave that cap loose, the check engine light illuminates because the system senses the pressure in the tank. If the pressure is too low, the check engine light comes on because your vehicle is contaminating the atmosphere. Weird how EPA laws work!

I have also seen other vehicles (Chevrolet van) throw codes when a rough idle is encountered and check engine light illuminated, caused by a loose gas cap. I expect the same happens with the F150.

Have you run any gas treatment/gas drier through the tank yet? Perhaps you have a condensation accumulation in the tank. I don't use alcohol, I'm on an acetone kick right now. You might try up to 8 oz. of acetone or one bottle of your favorite brand of gas treatment to 1 tank of gas.

Good luck!

 

Actually it works by checking for vacuum. The only time there may be pressure in the tank would be if both the purge and vent solenoids were stuck closed and the truck sat in a hot parking lot all day.

Principles of Operation


Evaporative Emission (EVAP) Canister Purge Valve

The EVAP canister purge valve is controlled by the powertrain control module (PCM). The EVAP canister purge valve controls the flow of fuel vapors from the EVAP canister to the engine intake manifold during various engine operating modes. The EVAP canister purge valve is normally closed.


Evaporative Emission (EVAP) Canister

Fuel vapors from the fuel tank are stored in the EVAP canister. When the engine is running, the vapors are purged from the EVAP canister for combustion.


Canister Vent Solenoid

During the Evaporative Emission Running Loss System Monitor Test, Evaporative Emissions Repair Verification Drive Cycle, and the Evaporative Emission System Leak Test, the canister vent solenoid is closed to allow either a vacuum to be drawn on the fuel tank or to hold a specified pressure in the system. The canister vent solenoid is normally open.


Fuel Tank Pressure (FTP) Sensor

The fuel tank pressure sensor is used to measure the fuel tank pressure during the Evaporative Emissions monitor test. It is also used to control excessive fuel tank pressures by forcing the EVAP system to purge. The fuel tank pressure sensor is tank mounted.


Fuel Vapor Vent (FVV) Valve Assembly

The fuel vapor vent valve (FVV) assembly is mounted on the top of the fuel tank. It is used to control the flow of fuel vapors entering the EVAP system. The head portion of the assembly prevents the fuel tank from overfilling during refueling. The assembly also has a spring float, which prevents liquid fuel from entering the vapor delivery system under severe handling or vehicle rollover conditions. In the upright position, the open bottom of the float will lift and shut off the orifice. Under severe handling conditions, the spring will push the float closed when angles allow liquid fuel to reach the orifice. In a rollover condition, the weight of the open bottom float and spring pressure will close the orifice.


Fuel Filler Pipe Check Valve

The fuel filler pipe check valve is an integral part of the fuel tank or the fuel filler pipe. It is intended to prevent liquid fuel from re-entering the fuel filler pipe from the fuel tank on refueling or rollover conditions.


Fuel Filler Cap

The fuel filler cap is used to prevent fuel spill and to close the EVAP system to the atmosphere.


Evaporative Emission (EVAP) System Monitor

When a fault occurs, the EVAP system monitor is reset to NO and a diagnostic trouble code (DTC) is set in the PCM memory. After the DTC is repaired, the vehicle drive cycle must be completed to reset the monitor in preparation for inspection and maintenance testing.


EVAP Running Loss System Leak Test

To start the testing, conditions of stable purging and vehicle speed must be satisfied. During the first stage, the canister vent solenoid is closed, while the EVAP canister purge valve remains open, applying and building vacuum in the system as indicated by the FTP sensor. This phase checks for major leaks in the EVAP system.

In the second stage, the EVAP canister purge valve closes and the system looks for minimal decay rate in the EVAP vacuum, indicating the absence of any small EVAP system leaks.

The last stage is entered only if stage two of the leak test has failed and checks whether the failed test was due to excess vapor generation. It monitors fuel vapor generation rate. Initially, the canister vent solenoid is opened to equalize EVAP system pressure to the atmosphere. Then the canister vent solenoid is closed, allowing pressure to build if vapor generation is present in sufficient quantity. If the rate of generation is found to be too high, the EVAP running loss system leak test is aborted. If not, then a small leak is diagnosed.


Inspection and Verification

1. Verify the customer concern is with the evaporative emission (EVAP) system.

2. Visually inspect for the following obvious signs of mechanical damage.


Visual Inspection Chart
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³ Mechanical ³
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³ ù Fuel filler cap ³
³ ù EVAP test port ³
³ ù EVAP canister or vent solenoid ³
³ ù EVAP canister hoses ³
³ ù Vacuum lines or hoses ³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

3. If the concern remains after the inspection, connect the scan tool to the data link connector (DLC) located beneath the instrument panel and select the vehicle to be tested from the scan tool menu. If the scan tool does not communicate with the vehicle:

l check that the program card is correctly installed.

l check the connections to the vehicle.

l check the ignition switch position.

4. If the scan tool still does not communicate with the vehicle, refer to the scan tool manual.

5. Carry out the DATA LINK DIAGNOSTICS test. If the scan tool responds with:

l CKT914, CKT915 or CKT70 = ALL ECUS NO RESP/NOT EQUIP, refer to «Section 418-00».

l NO RESP/NOT EQUIP for PCM, refer to the Powertrain Control/Emissions Diagnosis Manual («OBDII», «OBDII-Bi-Fuel LPG/CNG»).

l SYSTEM PASSED, retrieve and record the continuous diagnostic trouble codes (DTCs), erase the continuous DTCs and carry out the PCM KOEO self-test.

6. If the DTCs retrieved are related to the concern, go to the PCM Diagnostic Trouble Code (DTC) Index to continue diagnostics.

7. If the concern remains after the inspection, determine the symptom and proceed to the Symptom Chart.


PCM Diagnostics Trouble Code (DTC) Index


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³ DTC ³ Description ³ Source ³ Action ³
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³ P0442 ³ Small Leak Detected in ³ PCM ³ GO to Pinpoint Test A. ³
³ ³ EVAP System (as small as ³ ³ ³
³ ³ 1.02 mm [0.040 inch]). ³ ³ ³
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³ P0455 ³ Major Leak or No Flow ³ PCM ³ GO to Pinpoint Test B. ³
³ ³ Detected ³ ³ ³
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄ´
³ P1443 ³ Very Small or No Purge ³ PCM ³ GO to Pinpoint Test B. ³
³ ³ Flow Detected ³ ³ ³
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄ´
³ P1450 ³ Excessive Vacuum Detected ³ PCM ³ GO to Pinpoint Test C. ³
³ ³ in the Fuel Tank ³ ³ ³
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄ´
³ Ä ³ Any Other PCM DTC ³ PCM ³ REFER to the Powertrain ³
³ ³ ³ ³ Control/Emissions ³
³ ³ ³ ³ Diagnosis Manual. ³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄÙ


Symptom Chart


Condition(s):

o Hissing sound when removing fuel cap

Possible Source(s):
§ Canister vent solenoid.
§ Evaporative emissions canister.
§ Fuel vapor control valve tube assembly.
§ EVAP canister tube.
§ EVAP canister purge outlet tube.

Action(s) to take:
§ GO to «Pinpoint Test D».

o Excessive fuel odor

Possible Source(s):
§ Canister vent solenoid.
§ Evaporative emissions canister.
§ Fuel vapor control valve tube assembly.
§ Evaporative emissions test port.
§ EVAP canister purge outlet tube.

Action(s) to take:
§ GO to «Pinpoint Test E».


Pinpoint Tests

Note:
Reinstall or install new evaporative emission hose clamps removed or damaged during testing procedures.

 

Here's another section that talks about the components....

Evaporative Emissions

The evaporative emission (EVAP) system:

l prevents hydrocarbon emissions from reaching the atmosphere.

l stores fuel vapors in the EVAP canister that are generated during vehicle operation or hot soak until they can be consumed by the engine during normal engine operation.

l routes the stored fuel vapors to the engine during engine operation.

l is controlled by the powertrain control module (PCM) which, using various sensor inputs, calculates the desired amount of purge flow. The PCM regulates the purge flow, induced by the application of intake manifold vacuum, by varying the duty cycle applied to the EVAP canister purge valve.

l has an evaporative emission test port for test purposes.

The fuel vapors are routed:

l from the fuel tank (9002) through the fuel vapor vent valve.

l to the EVAP canister (9D653) through a vapor line.

l to the engine when the EVAP canister purge valve (9C915) is opened by the PCM (12A650).

The fuel tank pressure (FTP) sensor (9C052):

l monitors the pressure levels in the fuel tank.

l communicates the pressure reading to the PCM during the OBD II leak test.

The evaporative emissions (EVAP) canister (9E857):

l is located under the rear of the vehicle above the rear axle.

l contains activated carbon.

l stores fuel vapors.

The fuel tank filler cap (9030):

l relieves system pressure above 14 kPa (56.21 inches H2O).

l relieves system vacuum below 3.8 kPa (15.26 inches H2O).

The canister vent solenoid (9F945):

l is normally open.

l seals the EVAP system for the inspection and maintenance (I/M 240) test and OBD II leak and pressure tests.

l is mounted to the EVAP canister.

The evaporative emission (EVAP) canister purge valve (9C915):

l is normally closed.

l regulates the purging of the EVAP canister.

l is controlled by the PCM.

The evaporative emission (EVAP) system test port:

l is attached to the EVAP canister purge valve.

l is used to connect the Evaporative Emissions System Leak Tester to the EVAP system.

The evaporative emission (EVAP) system monitor:

l is a self-test strategy within the PCM which tests the integrity of the EVAP system.

l monitors the EVAP system for leaks.

l monitors electronic EVAP components for irrationally high or low voltages.

l monitors for correct EVAP system operation.

l uses negative and positive leak test methods to test and activate the EVAP system.

The EVAP Running Loss System leak test:

l utilizes intake manifold vacuum to test the EVAP system and involves several stages.



By my read, it should almost always be purging the cannister when the engine is running or venting to the atmosphere except during the self-test. If you sealed the system up on a hot day, you'd have a bomb waiting to go off!!

Just my 2 cents to muddy-up the waters.

-Joe

 

Just looked back up at the original post.... could definately be the vent solenoid stuck in the closed position and/or the canister purge solenoid (which leads to the intake manifold) stuck in the open position. I would think that the self test would catch this stuff, but I guess not. The dealership should be able to connect to the diagnostic port and troubleshoot it in short-order.

-Joe