16 December 2014

In my October column, I addressed theproblems associated with attempting totest for an evap system leak using pres-sure and closing the system vent valvewith a bidirectional control. When oneuses this method, the pressure tends to

bleed out through the vacuum pump assembly’sone-way valves, which are intended to hold vacu-um, not pressure.

To deal with this problem, I built an assemblyto apply vacuum and test pressure decay, watch-ing the gauge and recording the vacuum decaywhile monitoring the evap system pressure sen-sor on a scan tool. This assembly gives me com-plete control to test an evap system under allconditions. All that’s necessary to begin testing isto close the vent with the scan tool’s bidirectionalcommand and start applying vacuum. I opted touse my five-gas analyzer to supply the vacuum.

There’s one potential problem associated withthis method. The fuel cap’s relief function is sup-posed to protect the fuel tank by venting whenexcessive pressure is present or allowing atmos-pheric air to enter the tank when excessive vacu-um is present. These changes in tank pressure

naturally occur due to expansion or contractionof the fuel in the tank that takes place duringchanges in ambient or fuel temperature.

Is it possible for a large vacuum hose to flowmore vacuum during purge than the cap’s relieffunction can handle, causing damage to thetank? The definitive answer is…it depends.

I have seen two vehicles whose tanks were col-lapsed by excessive purge with the vent closed.But don’t all gas caps have a pressure and vacu-um release built in? See Fig. 1 below from Stant(http://www.stant.com/index.php/english/prod-ucts/consumer-products/caps/unique-stant-safe-ty-features/). Visit the site for an interesting les-son on gas cap design and safety features.

I believe all gas caps do have a pressure andvacuum release, but will the release handle fullflow from an evap purge? Or did the vehiclesI’ve seen with collapsed tanks have a defectivecap? I can’t say for sure. Both vehicles were inshops I was visiting.

Okay, so let’s put my 2010 Toyota Tacoma4.0L V6 to the test and see how it responds. Iconnected a manually controlled valve betweenthe intake manifold and the purge hose under

When it’s working properly, a fuel cap should protect the fuel tank

from damage due to vacuum or pressure extremes. However,

caution is required when applying vacuum to test an evap system.

Driveability Corner

MarkWarren

smwarren@motor.com

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the hood, bypassing the purge controlvalve. Fig. 2 on page 18 is the recordingof the test. I also wanted to test the totalpurge flow in grams per second, usingthe difference in the mass airflow(MAF) reading. Baseline readings atidle with the valve closed are shown atthe green cursor. They are:

Idle: 700 rpmMAF: 3.76 g/secondFuel tank pressure (FTP): 91.454

Kpa, or 27.01 in.-Hg at 3000 ft.Referring to Fig. 2, the first series of

changes are with the vent open andopening and closing the manual purgevalve:

Vent open and purge manually wideopen: 900 rpm

MAF: 3.26 g/secondFTP: 90.88 Kpa, or 26.84 in.-Hg.Note a drop of .17 in.-Hg in the fuel

tank pressure with full purge and thevent wide open. The drop relates to therestriction in airflow through the ventand canister. Looks like the systemflows purge air pretty well.

Now, how much did the evap purgesystem flow? Note that the MAFdropped from 3.76 to 3.26 g/second as Ibypassed air around the throttle/MAFsensor directly from the vent to the in-take manifold. The difference would bethe purge flow if rpm had remained at700. The rpm rose to 900; therefore,the MAF had to rise as well. Testing lat-er showed MAF to be 5.28 g/second inPark at 900 rpm. So, the 3.26 MAF(purge open at 900 rpm) subtractedfrom the 5.28 reading (900 rpm MAFin Park) shows the difference in purgeflow is 2.02 g/second, or a 53% increasein airflow.

Now let’s close the vent and see if wecan damage my gas tank. Look at thedata on the right side of Fig. 2. Therpm is almost 900, MAF is slightly low-er than the 3.26 g/second (vent open)and FTP dropped to as low as 77.82Kpa and then stayed at about 83 Kpa,or 24.5 in.-Hg.

Okay, the tank did not implode andthe gas cap vented at 24.5 in.-Hg, adrop of 2.5 from the baseline of 27.01

in.-Hg. The cap handled all of the air-flow of 2 g/second from the evap purge.

What are typical cap pressure andvacuum release points? Hard to say. I

did find a couple of specs once for a1998 Ford Explorer. They were: pres-sure release: 14 Kpa, or 2 psi; vacuumrelease: 3.8 Kpa, or .53 psi. The Tacoma

17December 2014

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dropped from 91.45 Kpa to about 83Kpa, a drop of about 8.5 Kpa vacuumrelease.

What’s the take-away from all ofthis? You can use MAF to determineevap purge flow, but be sure to com-pare the minimum MAF at wide-open purge to MAF with closed

purge at the same rpm. Be cautiouswhen applying vacuum to an evapsystem to watch vacuum decay or testflow; the cap should relieve excessvacuum before tank damage can oc-cur, but don’t depend on it. Watchthe tank pressure and don’t apply toomuch vacuum.

18 December 2014

Driveability Corner

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