It has been very cold in Texas the last couple of days. Low 20's upper teens.
It has warmed up in the 50's today, I warm up the car and I hear like she is skipping a beat every few secs, like it is running lean or something. I have full tank of gas. I warmed up fully drove around no change. Reloaded my tune, same thing. Up to 3000 rpm skips a bit every few secs, some backfire occasionally when down shift.

AF ratio between 14.7 to 15.1 (it has always been like that till I get on it then goes to 12.2-3)
Fuel rail pressure 39.9

I added octane booster, no difference that I can tell. Still some skipping.

What is happening? any thoughts? The car has run in cold temps before with the current setup without issues.

 

check your spark plugs and gaps. regap or replace as necessary. you may want to try some heet to remove any possible water you have in your tank. it may just be a bad batch of gas. also octane booster is crap.

here is a link in-case you have not heard of it. really cheap and can be bought at any auto parts store or walmart.

 

what grade gas you running? Do you have any high octane gas around like 110 or something .. maybe as Moneyshot said you might have a bad batch of gas .. this should help bring the octane up if that is the problem

 

+1 on the octane booster being crap...

Since it continues to do it after the car warms up I'd suspect it's not tune related. Other than mechanical, it could be that you have some water in the fuel tank and you're sucking a bit thru the fuel system every so often.

 

93 octane. I am not sure where I can find 110 gas. (the tank is full none the less)

I will check the plugs although they are pretty new. Brisk one stage colder than normal spaced at 35 If I remember correctly for supercharged application coper anti-seize applied.

 

i had what seems to be the same issue. One night i backed my car out of the garage and i noticed it sounded different (almost like it had a "fatter" cam in it). So i started to drive it around the block and noticed that it was down a cylinder. When I pulled it back in the garage I took plugs out one at a time until I found one that was full of fuel (plugs were nearly brand new at the time). So, I cleaned the plug off and put it back in. Started it up and it ran like a champ... therefore, I would suggest checking the plugs... if you do have a fouled (or just not making a spark for some reason) plug you will know it instantly. Good luck, and hope this helps!

 

Is the CEL light on? If so, get it scanned to see where ya need to start looking. Pull the plugs and see how they look. If not, I'd guess its fuel related since you did just fill up. Try some Heat in there to get rid of the excess water.

And another +1 on octane boost. If you read the bottle, it claims it raises the octane 1-3 points but really only tenths of a point. 87=87.2 octane. Regardless, its crap.

 

Pulled the Plugs they all looked fine. Cleaned checked space put back in issue still there.

I think it is the gas. I will get some heat and see if it makes any difference.

 

No lights on.

 

its "HEET" guys all "E"s no "A". hope that fixes it. if not, we will find it. fuel is making it into the cylinder but not burning so checking the ignition coils would be my next step. are you running at 10 psi steady? does it fluctuate? i you have a scan tool i would try to read the codes anyways. i have never checked a CEL in my mustang but in my eclipse, the computer would have to see the problem a few times in a row before it would throw a CEL. Good luck dude!