On a 1996 GMC Jimmy. For the past 3 months, I regularly (once every 2
weeks?) get a engine warning light resulting in code 440.
Initially, I was advised maybe it's just a bad batch of fuel or
tightness of fuel cap. Obviously after so many repeats, this is not the
So I don't get hosed going to a mechanic right off the bat, what would
be the causes of this issue and the methods of diagnoses and possible
repairs or replacements required?
I don't notice any coincidingissues with the vehicle and fuel economy
has not dropped since the start of the problem (although in the past
month, since weather is colder, fuel economy has dropped noticeably!)
This is a leak in the evap system, after the fuel cap, then it becomes more
difficult to diagnose, a smoke machine is the most common method of tracking
this, otherwise lots of guesswork and parts may be on the horizon.
Yep it can be really fun to find the leak without a smoker. There are a
few places you could look before you take it to a shop though IF you
want to DIY.
The tank vent solenoid could be stuck open. Drop the spare and look for
a black plastic line going to the valve. Disconnect the line and the
wires and bend the retainer tab to get it off the mount.
The other end of the system is connected to the purge valve solenoid
mounted on the intake manifold toward the back. One bolt on either side
and it will pull out of the manifold. With it disconnected it should be
closed, you can test it with a hand vacuum pump (or you can even blow
through it). It shouldn't pass air to the intake side from the external
NO HIGH PRESSURE AIR on either valve or any part of the EVAP system.
The Vent Solenoid is about 30 bucks.
The tank purge valve solenoid is 23 bucks.
The catch is that failures run about 50/50 for those parts.
Then it could also be a cracked or rusted line off the tank or along the
frame. That can be checked with a hand vacuum pump by blocking one end
and applying vacuum to the other end.
I've had success by adapting a bicycle pump to the vapor system. (Nothing
fancy - I ripped a valve out of an old bicycle tube and stuffed it in the
end of one of the vapor hoses!) Pumping with a leak produced a hissing sound
I could hunt down. When the leak was fixed the tank held pressure as felt on
the pump and confirmed when I removed the fuel cap.
I forgot to mention - easy does it!!! Fingertip pressure on the pump only.
The pump is made to put out 100 psi or more, which can do spectacular damage
to a fuel system. The hissing starts at under 1 psi, but you have to have a
quiet location to make it work.
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