Idle Surging.... Gas Cap tightness

[almostordinary]

Well I've been searching for an idle surge (rich) situation for a while, just put in a new IAC and still had the issue come up.

On a spirited drive down the PCH I stopped for gas in the Palisades and when I went to put the cap back on, which I alway tighten to it's max, I decided to just give it one click (I think it's usually 2-3 then it's fully sealed).

Drove down the PCH and the surge never showed back up.

Wondering if my cap being fully tightened had any affect on fuel pressure? No clue, but wanted to share my experience.

Drove for about 50miles and didnt have the surge on any of the long lights I was waiting at.

Likely taking the car on a tech scout tomorrow that will have me wading through LA traffic, hope she stays in the same state.

-Ian

[nospiners]

I'm goin to tighten my gas cap on the 87 Targa now, just to make sure..With these cars we are learning something new everyday..Thanks for sharing, i wish i could help..

[almostordinary]

I may have wrote that prior to having coffee... I left it looser than normal and it helped. I usually tighten 3 clicks and have it fully snugged up.

This time I turned just to one click. so there's some opportunity for ventilation. And it's better

[NotStock]

Oil cap yes, but not fuel cap. Oil cap creates vac leak, but fuel tank is vented and shouldn't care if there is a cap or not.

If the fuel cap actually helped, it may point to an issue with fuel tank venting and fuel pressure, but It's hard to imagine that causing issues with surge at idle and nowhere else in the map. I don't know what the stock vent line is supposed to look like (my car is a bit of a mongrel), but there is a tank vent tube on the filler neck that might warrant a look.

[EC900]

I agree, it’s not the gas cap. You could have developed a vac leak, more probable.
Using any fuel cleaner additive? Sometimes this can help if the injectors are a bit dirty.
CIS is in my opinion more finicky, curious being a CA car has any work been done to meet a CA BAR emissions test?

For what it’s worth, the gas cap has an internal mechanism that prevents it from being over-tightened and will click when fully seated. The tank itself is vented thru the expansion tank, you will see the hose on the top of the tank next to the filler neck inlet.