Fuel Tank Pressure

[We Gone]

Just a quick ? On removal of the fuel cap on the 93 I get a lot of pressure release. Normal? I ask as I get very little on the 91. Do I have an issue with the evap system? Would to much pressure cause a rich idle?

[mike100]

It only takes about 3-4 seconds to vent the fuel tank pressure through the purge solenoid so it won't enrichen the idle for all practical purposes. If you still have a stock exhaust, you can sometimes hear the hiss if things are just right.

I would have to review info in the FSM, but I don't think the purge lasts very long, just during the engine start-up to suck the vapors out of the charcoal canister and the top of the tank. I think it may turn on during lean cruise- or maybe a small duty cycle on/off as a controlled vacuum leak, but I can't say for sure on the obd1 GM cars, but some of the emissions stuff is pretty interesting.

[We Gone]

When I remove to fuel cap regardless of fuel level after a drive I get a noticeable whoosh of air. Just want to be sure the system is working properly.

[fletchusmc]

My 95 has a lot of pressure as well when I remove the fuel cap, interested to see if this means anything?

[scottfab]

only thing I could find on gas cap pressure testing wast this from the EPA. It seems to allow for a certain amount of leakage.

Are people seeing a true pressure build up OR a vacuum build up?



Gas Cap Test
(1) Cap Installation. The fuel cap, or caps, shall be removed from the fuel inlet(s) and installed on
a portable or bench test rig using the adapter appropriate for the gas cap as specified in
§85.2227(d)(1)(ii).
(2) Leak Measurement. The gas cap leak rate shall be measured and compared against a 60 cc/min
at 30 in. WC flow standard. Pressure decay measurement using instruments with a 1 liter head
space shall be made from an initial pressure of 28 in. WC and be compared against a loss of 6
in. WC in 10 seconds.
(3) Cap Replacement. The fuel cap(s) shall be replaced on the fuel inlet and tightened
appropriately.

[We Gone]

Originally Posted by scottfab

only thing I could find on gas cap pressure testing wast this from the EPA. It seems to allow for a certain amount of leakage.

Are people seeing a true pressure build up OR a vacuum build up?

That is a good question Scott,

Found the same info. There are a few posts over on the other site but no definitive answer.

[Schrade]

Originally Posted by scottfab

only thing I could find on gas cap pressure testing wast this from the EPA. It seems to allow for a certain amount of leakage.

Are people seeing a true pressure build up OR a vacuum build up?



Gas Cap Test
(1) Cap Installation. The fuel cap, or caps, shall be removed from the fuel inlet(s) and installed on
a portable or bench test rig using the adapter appropriate for the gas cap as specified in
§85.2227(d)(1)(ii).
(2) Leak Measurement. The gas cap leak rate shall be measured and compared against a 60 cc/min
at 30 in. WC flow standard. Pressure decay measurement using instruments with a 1 liter head
space shall be made from an initial pressure of 28 in. WC and be compared against a loss of 6
in. WC in 10 seconds.
(3) Cap Replacement. The fuel cap(s) shall be replaced on the fuel inlet and tightened
appropriately.

Anytime I have my front end up on ramps, and air temp increases 15' - 20' F, I start smelling strong gas near cannister while wrenching. I go to the back, loosen the cap, and A LOT of vapors release.

Makes me think that tank angle, and gaseous pressure increase, forces fuel through the EVAP line, instead of just vapors... (I keep the tank full too).

On LT1 top end rebuild, I was greeted on second day by steady drip of fuel from disconnected lines, whereas it was NOT dripping shortly after dis-connecting the first day.

[WARP TEN]

Originally Posted by fletchusmc

My 95 has a lot of pressure as well when I remove the fuel cap, interested to see if this means anything?

Mine too. Seems like pressure, not vac. --Bob

[scottfab]

Originally Posted by WARP TEN

Mine too. Seems like pressure, not vac. --Bob

Well, I don't get a pressure build up on mine. Maybe it's time to visit the FSM at page 6E3-C3-2.
At the bottom it speaks to "FUEL TANK PRESSURE CONTROL VALVE" and on subsequent pages shows that valve, operation theory and the Evaporative Emissions Control System Schematic.
Maybe the fuel tank vapor pressure is not getting relieved?

[Schrade]

Originally Posted by mike100

It only takes about 3-4 seconds to vent the fuel tank pressure through the purge solenoid so it won't enrichen the idle for all practical purposes. If you still have a stock exhaust, you can sometimes hear the hiss if things are just right.

I would have to review info in the FSM, but I don't think the purge lasts very long, just during the engine start-up to suck the vapors out of the charcoal canister and the top of the tank. I think it may turn on during lean cruise- or maybe a small duty cycle on/off as a controlled vacuum leak, but I can't say for sure on the obd1 GM cars, but some of the emissions stuff is pretty interesting.

3 - 4 seconds...

What if the purge of vapors was CONTINOUS?

That's something I always thought on, when I spent $300.00+ / day, keeping a small delivery fleet rolling (@ $1.79 / gal).

One night, I was talking about that with the Ops Manager, when the line hauler was late. He told a story about a friend who devised something like that with a carb'ed car late '70's, who then submitted it to one of the big 3.

He said shortly afterward, his friend disappeared. He said he got a letter from his friend several years later, from Canada, who was doing 'very well', after signing something, and then getting a big check...

Only some safety mechanism is necessary, to prevent a mis-fire from lighting up the tank...