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Discussion Starter #1
Has anyone experienced issues with misfire and poor running after leaving fuel cap off and driving ? After months of effort I finally got my 67 fastback 390FE running great, I then had to fuel up and used a 98 unleaded fuel and some octane booster but accidentally left fuel cap unsealed and drove off . 5 minutes later car began to miss and now will not run or idle properly and misfires , I’m not sure if it’s the fuel I used or the fact the cap off may have caused ingress of air possibly ?:eek:
 

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Discussion Starter #3
Problem is the car just came back from mechanic whom cleaned out tank with new filters etc and added fuel and this is a Fran restoration with all new bits including new tank. Car had a ballast resister issue (causing start issue) which I fixed and everything was running fantastic until I added 98 fuel and left cap off . Frustrating one Problem to the next ?
 

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Discussion Starter #5
Yeah I’ll drain tank again and try standard unleaded with octane booster. Pain in the ass
 

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Pour a can of Seafoam in the tank. Is it worth $8 to save you from the pain of having to drain your tank? You can also run a hose into a gas can from the input side of your fuel pump to see if the problem persists before draining your tank.
 

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Discussion Starter #7
Yes good idea , I was thinking the same . Do you have an opinion on which fuel to use for a 390fe 4v ? Is unleaded 91 with octane booster better than 98 ? I’ve read mixed opinions
 

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Yes good idea , I was thinking the same . Do you have an opinion on which fuel to use for a 390fe 4v ? Is unleaded 91 with octane booster better than 98 ? I’ve read mixed opinions
Sunoco or Shell Supreme if you can get it in your area. In the US 93 octane is equivalent of 98 octane in Germany. Is it the same as Australia? At least that is what it says on the pump here in Germany.
 

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Discussion Starter #9
Yes I think so . I’ve heard that the 98 works worse on a high compression classic v8 give n it’s additives and properties . Some people advocate using 91 ( 89 USA I think ) with an octane booster .

I’ll try a couple of types
 

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Discussion Starter #11
Yeah I checked that . I had driven the car for 2 hours running perfect and issue only presented when I added fuel and the issue was gradual ( I guess as fuel mixed with existing fuel) but I did notice when I limped back home I left fuel cap unscrewed and hanging like an idiot
 

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I have a stock 1970 390 in my 67 Cougar and currently run regular 87 octane with no issues. I tried the 91 octane but didn't see any great difference in power, etc.
 

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87 octane runs the same as 93 octane in my 390. Non ethanol 89 runs fine, and I prefer it due to its longevity.
 

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With my 67 390, aluminum heads (lowers compression ratio) I've always super unlead - 93 octane, car runs just fine
 

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With my 67 390, aluminum heads (lowers compression ratio) I've always super unlead - 93 octane, car runs just fine
In my 67 390 FE with 4v and Eddy aluminum heads, I’ve observed that my car runs best with 93 octane, and believe it or not, runs worse with non ethanol fuel ( I would have never believed it). My advice is to experiment with all choices, until you have established your own conclusions.
 
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