I got the CEL after I started my Blazer to go to work last night. It stayed on after a few stops and starts. Well, when I went out for lunch, I decided to check the gas cap. I had filled my tank on Friday. It wasn't all the way loose but it wasn't "clicked" all the way over. Well, after I had secured it and started the engine, the CEL light stayed on but after I drove off to get lunch, it shut off after a while. It hasn't come on again.

I am aware that leaving your gas cap off can cause the OBD2 to throw a CEL. What suprises me is that it took 3 days for it to come on after filling up. I suppose it could be because it wasn't tight and the vibration from all that driving caused the fuel cap to work it's way loose breaking the vacuum, or maybe because the temperatures have gotten cooler the last few days. At any rate, the light is off now, I was just curious as to why, if it was due to the fuel cap being loose (which I am assuming is the cause) why it would have taken 3 days for it to throw a code. Wouldn't it have thrown a code right away?

It's funny, my ex wife has our 1997 Honda Odyssey minivan and she lost the fuel cap once for 2 days and it never set the CEL. However, we did find that the aftermarket fuel cap that we got to replace the lost one ended up causing the fuel to siphon out for some reason when the van sat out in the heat. That was solved by buying an OEM fuel cap from Honda.

 

It takes quite a few cycles for the PCM to register a fault with the evap emissions system. It is quite possible that the cap was the only thing wrong and it cleared itself.

 

Ah, cool. Thanks, learn something new every day. That would explain the 3 day lag and why we never got a CEL in the Honda. I'm not used to CEL's in gas engine vehicles. My Jetta is a diesel, so it doesn't have the same kind of evaporative emissions control. Doesn't have any evaporative emissions at all, since diesel doesn't give off vapors, lol. In fact, my jetta has a little switch just inside the fuel neck that allows you to vent out air from the tank so if you so chose, you could fill it up all the way to the rim of the fill neck. It's good for an extra gallon or so. The diesel Jetta uses the same fuel tank and neck as the gas version, it's just that the connection that would normally lead to the evaporative recovery system loops back to the fill neck where the vent switch I mentioned is. The fill neck is also missing that little mini flap so it can accept large truck diesel fill nozzles.

FWIW, the CEL in my Jetta has been on for 4 years. I fixed the problem that set the code but it requires a hook up to a VAGCOM (VW's OBD2 reader) to clear the code and I don't want to pay VW $75.