What's the deal with my gas cap?

[71CHEVYSHORTBED402]

Got some gas in the beast and figured it was time for a new gas cap. 1/4 mile up the street to Napa, I end up with seems the same cap I had on my 71. Cool.

Drive 1/2 mile to the store to pick up the wife some flowers, you know, anniversary. Start the truck and it's running like crap. Sounds funny too. I don't see anything, but my old gas cap is in the garbage at Napa, so a 180 to save my newly appreciated MotoRad gas cap.

My old cap doesn't seem to help, so drive home and discover the cap from the vacuum port on the manifold was gone. Huh?

Forgot to mention: It blew a cork when I pulled the gas cap off. Gush of air and gas. Seems to me these caps would be vented both directions. My old one worked on the 71 no problem. I don't claim to know much about caps.


Wish I knew more about caps and EEC. This truck no longer has the canister. As far as I can tell it's set up like my 71 was. Pump has a return line, which I believe would otherwise be to the canister.

[dmjlambert]

Well it seems you found the problem with the vacuum cap on the manifold missing. That is probably not related to your new or old gas cap. I have made a habit of hanging onto the old stuff I remove when I install a new part, at least until the new part is very proven to work well. So my advice is enjoy the new cap on the truck and hang on to your old cap for a couple months.

[71CHEVYSHORTBED402]

Originally Posted by dmjlambert

Well it seems you found the problem with the vacuum cap on the manifold missing. That is probably not related to your new or old gas cap. I have made a habit of hanging onto the old stuff I remove when I install a new part, at least until the new part is very proven to work well. So my advice is enjoy the new cap on the truck and hang on to your old cap for a couple months.

Got rid of the new cap. I'll try the cap off the 71 tomorrow. Great timing too, the tank and front/rear windows are all that's in the cab and I pull the tank next.

Sounds like you're saying it was a coincidence. I thought perhaps some vapor exchange blew the cap off the vacuum port. I wonder why there was a gush of air (and gas) when I pulled the gas cap off? First I've seen that happen.