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Old 07-21-2015
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Join Date: Jul 2015
Location: Oroville ca
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Evap test for ca smog requirements keeps failing

Hi everyone my name is John I'm very new around here. I hope someone can help me I have a 1994 with 4.0 litr 2 wheel drive and its smog test time here in ca and my truck will not pass the evap test. For those of u who don't know that is where they pump about a pound of nitrogen pressure into the fuel filler and measure how much leaks out well my truck says a gross leak so I ask what they recommend the guy tells me usually it's the fuel filler neck needs replaced. So I replace that with a new one and return they test fail again same gross leak they recommend having a smoke test done on fuel system I have this done but nobody can find this so called leak now I've had it tested 5 or more times and still fail If anyone has had a similar problem and maybe able to help me fix mine please I'm ripping my hair out trying to figure this out before my tags expire. Sry this is so long it's my first post please help thanks in advance
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Old 07-21-2015
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Outside of the seal/gasket on the access hole at the top of the tank there are 4 "holes" in the tank that need to be air tight when test is done.

The filler/vent hose is the largest but you have replaced it and I assume sealed it well.

Next is the OUT fuel line, you would smell gas if this was leaking, because it is a high pressure(35psi) line.

Then there is the Fuel Return line, again you should smell gas if this is leaking, but it is a low pressure line.

4th is the EVAP line, it goes to the filter/canister, usually located on the rad support, black box.
After the filler tube this is your best bet for a leak, could be the filter box has a hole, and it is far enough away from tank so smoke test may not show there, or the EVAP solenoid is not closing so smoke is going into the intake and can not be seen.

EVAP system is pretty simple, when engine is running vacuum from the intake manifold is used to create a negative pressure in the filter box, this is controlled by the computer using the EVAP solenoid, it is usually located near filter box, has 2 vacuum lines and an electrical connector.
1 vacuum line from intake, other goes to filter box
Filter box has a line to the fuel tank

Hose/line path
Engine-------solenoid-------filter----------------------fuel tank

When engine is off solenoid should be closed so tank is air tight.

In this type of older EVAP system there is no pressure sensor, the gas cap has a pressure release valve that will allow air to be sucked in if vacuum gets too high in the tank.

I would remove the EVAP line to the gas tank from the filter box, apply a vacuum to that line(suck on it) and make sure tank is holding pressure, if not then problem will be at the tank, maybe gasket at access ring.

If it holds pressure then try the same on the filter box, add a hose to the now open fitting and apply vacuum, it should hold a vacuum if solenoid is closed and filter box is sealed.
If it doesn't, trace the vacuum line from solenoid to intake manifold, remove that line from intake and plug it, test if filter box now holds a vacuum.
If it does then solenoid is leaking and bad.
If it still doesn't hold a vacuum then filter box has a hole.

EVAP solenoid hoses need to be heated to be remove and installed, heat gun or blow drier

Last edited by RonD; 07-21-2015 at 11:40 AM.
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