Okay, this one seems a little strange ...

Sunday, the yellow "Service Engine Soon" light came on. My fiancee was driving the truck at the time and swore to me that she did nothing other than drive it. Gas was below 1/4 tank.

Yesterday, I heard a commercial on the way home from work (in another car, did not drive the Disco yesterday) - the OnStar commercial where a loose gas cap causes a check engine warning. So, when I got home I checked the cap - appeared tight - and started the truck. Engine light still burning, so I figuredcommercial was BS, or only applied to Chevys and other, more primitive, automobiles.

Today, I filled up on the way to work and the light went out when I restarted the truck.

So, what is going on? Coincidence?

Bob

 

Could be a bad gas cap. Or a leak somewhere else in the lines involving evaporative emissions.

That's a code Auto Zone can read for free. Have them plug in their scanner - if it says P0440 that's it.

 

If you catch a loose gas cap within the next, so many start cyles, the ECU will check and clear that fault code without the use of a scanner. Understand though you have to catch it over the first hour or so of driving or the code will lock in.
I wouldn't worry about your truck unless the check engine light comes on then you might want to go have it scanned.
Mike

 

Thanks for the replys, guys.

What makes this odd is that IF it was the gas cap it took at least a week between the last fill-up and the time the light came on. After that it was maybe four starts and 1.5 hours drive time before I filled up and the light went off.

I'm not worried about it as long as the light stays off, but will have to watch and see if it repeats next time I am below 1/4 tank.

 

I believe the truck checks pressure in the tank, so if the cap had a really small leak and you weren't driving it much, it wouldn't show until you had a long day of driving. I had a bad seal on mycap, and it didn't register until I drove to Chicago from Tulsa.

Like Mike said, don't worry about it too much until your check engine light comes on, then have the code read at Autozone to see what it is.

Cheers!

 

thats funny thats when my light came on. it was codep1448. did tune up and filled up it stayed on went and bought the obd11 code reader and cleared it and it hasn't came back on. oh it still ran with a slight miss until I cleared it.

 

P1448 Evap System Purge Valve Flow 2 Fault , well, it is not the gas cap, got a problem with the purge valve under the hood.
Mike

 

A dirty gas cap will tighten enough to cause it to ratchet (click) but still be loose enough to leak. The same thing will occur if you cross thread the cap. Remove the cap, clean the threads and reinstall properly and all should be well. The typical fault code for an improperly installed gas cap is for a vacuum leak.