I've always ran 87 as per the owners manual...but at times I've put 89 in and on occasion the car would crank then just stop, then would usually start on the second shot. Would never do this while using 87 though... Now, I just got the Ford Racing CAI and tune done over the weekend and the tune calls for 91+ octane. Had about a quarter tank of 87 then filled it with 93, this morning the car had that start up issue again... is this common with higher octane?? Don't want to try and explain this to the dealer, probably tell me I need a new fuel pump or starter..

 

No, it should start up just fine. But having a mixture of octanes in the tank is not ideal for troubleshooting. Run that tank down and use only the octane that your tune calls for. Then see if the issue still exists.

 

With the mix ratio in your gas tank,your engine should run fine.
Sounds like a mass air issue to me.
As far as tunes go... I'd recommend a 93 tune cause 91 fuel is not that great in my opinion.whats your mileage?
Check your worm clamps and make sure every thing is good and tight, even pull the mass air and check that all is good there to.
Just curious.... Where did you get your tune from?

 

I have the Ford Racing pro-cal tune in it and had it done at the dealer, just can't run anything below 91 octane.. and like I said, it only happens when put anything higher than 87 octane in the tank. I read that if you put anything other than 87 in a stock tuned car it'll cause these type of issues.. Not sure why it happened again with this tune, could it be a cam sensor issue?

 

Just FYI, I'm completely 'stock' here (the shaker hood intake shouldn't really matter) and I've always run 89 with no issues. When on the highway for long trips I'll switch to 87 since I'm not shifting much and just cruising. I switch back to 89 afterwards and never have starting problems.

 

Now I'm starting to think it's not a fuel related problem.. The other day when I got out of work and put the key in, turned it, the gauge lights came on but nothing.. then it fired up on the second shot. Thinking this is ignition related now, maybe it's on it's way out? I was looking at my keys a few nights ago and noticed dirt in the groove of both of them, so I cleaned them out thinking maybe this could be an issue.. been starting up fine since. This has me baffled...