kylebgt

My mustang has always had a bouncing idle, it runs really rough at times. This became more apparent after removing the cats, and replacing the exhaust. I figured the roughness was probably IAC related.

On a whim last week I filled it up with 91octane, something I have never done before. As the engine is not high compression, I don't really see the need. Surprisingly though, it is running a whole lot smoother now. I almost always fill it with 89, which is still better than the recommended 87. I go to diff gas stations, so I know its not because of poor fuel.

Should I continue to run 91 through it? I'm at sea level so hypothetically I can run 87. Why would my engine run better on 91? Thanks!

ct_white_stang

I wonder if you car has a custom tune? Are you the original owner?

kylebgt

I'm the 2nd owner, however it was bone stock when I bought it. I did not gap the spark plugs when I put them in last year. I wonder if that could be causing some problems.

mrtstang

If your car has the stock tune, there's no benefit to running higher than 87 octane.

Nestang

Yeah If I was you I'd pull them back out and gap them.

K Z

If u put a platinum plug in then there is no need to gap, but if they r factory replacments u need 2 gap them.

Danny_Boy

I have never gapped any of my spark plugs and my cars have always ran fine if not better than before.Not saying that gapping is not good,just saying....I normaly get single or double platnum Autolite spark plugs.

I have owned 5 Crown Vic Police Intercertors w/ the 4.6L and never had issues with the motor.I have ran 87-93 octane gas and noticed no difference.

It could be that your Stang had a tune done by the previous owner.It could be your timing was bumped up quite a bit requiering higher octane fuel.Maybe even a lower temp thermostat,or even colder rated spark plugs.

Also try cleaning your MAF wire,if you are running a oiled air filter that is.

tbirdscwd

Without a tune, there is no need for anything above 87.

Danny_Boy

Ummm yeah...we established that already,see below...

tbirdscwd

I'm very happy that you can read dannyboy, welcome to the forums

OP, look.....You were on the right track with the IAC before. If your car isn't running correctly, in this case the idle, it's important that you check the common causes first. These cars are notorious for having idle issues which include stalling, hunting, hanging rpms, you name it. 9 times out of 10 the problem is solved by either replacing or cleaning out the IAC. This is much more likely to be the problem than the possibility that you might have an other than factory tune, or that you have idle problems due to spark plug gapping (which would have to be pretty far off I might add) that only present themselves at idle. Do the math. Most likely the change in octane/seemingly improved running is coincidental.