P0440 Error code - what usually causes it - & fixes it?...

I recently scanned for and found my 1998 Pontiac Transsport (aka Montana) to have a P0440 dtc, and I'm wondering if the ECM makes any changes at all to compensate for the failure causing the P0440 dtc to be set. In other words, does the ECM alter any fuel or timing characteristics in an attempt to compensate for the P0440 dtc being set. I ask because I also own an '85 Fiero 2M6 SE (OBD-1), and it had an ignition timing issue for which the ECM compensated by defaulting to a pre-programmed table of values in regards to injector timing / pulse width to compensate for the ignition timing issue. Fixing the ignition timing issue made the ECM restore the injector timing/pulse width back to normal operation values. I realize my '85 Fiero uses OBD1 and my '98 Transsport uses OBD2, but could the ECM in the '98 Transsport be trying to compensate for whatever has caused the P0440 DTC?

I'm aware that the various valves in the EVAP system can go faulty as well as cracked hoses in the system, but this problem is only intermittent. I also understand that somthing as simple as a poorly fitted gas cap can set the P0440, but I would sooner expect a bad gas cap to set a low leak DTC instead of just the P0440. Anyone agree?

I regards to the bad gas cap idea, I have the OE gas cap which prevents one from over-tightening the cap by clicking like a child-proof cap on a medicine container. Might replacing the gas cap solve the P0440 dtc?

- Al

It might...and it's the easiest fix, but get a dealer cap and not an aftermarket one.

Have you checked for leaks, cracked hoses, etc.?

For EVAP issues, the ECM won't change anything to compensate for it.

Once I get a chance to get back under it I plan to check & replace any suspicious hoses showing signs of wear, abrasion, cracks, etc.

However, I still wonder why, if it is the gas cap at fault, why it hasn't set the P0442 low leak DTC.

On a side note, is the fuel tank pressurized, or under a vaccum?

- AL

Because the leak (as you said) is intermittent and not continuous.

The fuel tank can be both, depending on the EVAP cycle.

Planning on buying a locking gas cap actually, what with the price of fuel these days, that combined with the neighbourhood we live in the potential of having ones fuel being syphoned isn't unheard of these days...

What of locking gas caps, anything to watch out for? - do dealers offer locking gas caps? - or only aftermarket?

-AL

Haven't seen many locking gas caps that are worth the money and don't cause the evap leak. If the dealer doesn't have a listing for a locking one in ther books then I wouldn't try it unless you know the cap isn't the cause of the leak at the moment.