bad gas cap?

Messages
884
Location
San Diego, CA
My 97 subaru occasionally idles too fast. I found that this was correct once after buying gas so suspected might have been a loose cap. Tightening the cap does not always help, however. I have cleaned the gasket on the cap and the filler neck with alcohol. This helps, but sometimes for only a day. Is the cap itself bad? Coincidence that it stops usually when I clean the cap? The is a very small amount of non-loose rust in the filler neck also where the gasket meets. It is nearly as smooth as the surrounding metal. Bad gas cap maybe? It is the original.
 

joaks

Thread starter
Messages
884
Location
San Diego, CA
There may not be a connection. My guess was a pressure disruption. I can remove the cap and not get a check engine light and I have no light now so that doesn't help.
 
Messages
5,091
Location
USA
Maybe there is something wrong with the vapor vent system.The cap isnt vented,but when its loose,acts as a vented cap.Tight,its sealed,and if the vent system isnt working maybe the fuel pump is trying to collapse the fuel tank (suck it inward) and it alters the air/fuel ratio? (when fuel leaves the tank,air needs to replace it).
 

joaks

Thread starter
Messages
884
Location
San Diego, CA
Okay, so it looks like it is NOT the gas cap. I opened it and got a nice "hiss" suggesting a good seal. Also I took the car for an oil change and asked the shop to check it outg but it of course was running fine. it does not idle too fast all the time but if it does and I turn it off and restart the problem is solved! So I'm thinking something is sticking perhaps? Any thoughts?
 
Messages
19,479
Location
Chicago Area
Gas cap designs [and venting systems] can be varied. Some need to breath in. The hiss may show a problem, not a good cap! I don't know every cap of every car, with their venting systems. So check a bit further to see if this is right for your particular car.
 
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