Paris MkVI

And not even a huge change in octane.

I run 87 octane, and it isn't an issue. Once in a while I will hear a little rattling in hard acceleration, but nothing major.

Lately my fine steed (with 168k miles) has been stumbling a little bit at low RPM, and more rattling under hard throttle. I figured it's probably just time for new plugs, throttle body cleaning, the usual. And it really is probably time for those items.

But this last fill-up, I poured 89 octane into her, more out of curiosity than anything else.

Stumbling? Gone. Hard acceleration rattle? Gone. Truly dramatic improved throttle response and driveability.

Mind you, I did not have these expectations. I am truly surprised by the difference.

I am not suggesting octane replaces maintenance. But 2 points in octane made an immediate difference. Pretty weird. And my understanding is the 2006-era 4.6L ECS doesn't adjust for octane differences.

Bert

Octane is for one thing and only one thing: preventing detonation and/or "pre-ignition" also known as pinging or knocking.

So I'm betting that all of your problems stemmed from detonation / pinging / knocking. The stumble was probably because the ECU was pulling timing when it senses the detonation.

Which makes me wonder what was causing the detonation, because a factory tuned Mustang "should" run fine on 87 octane. If there is a tune installed that was written for 91 or 93, that would explain it. Carbon build up in the cylinders can cause it. Or if it is going lean for some reason I think that can cause it also.

Some "premium" fuels also supposedly contain better detergent additives than the "regular" but octane is the main difference. The better detergent package "might" help with the carbon build-up over a few tanks, but I doubt it.

SpectreH

It wouldn't hurt to run a bottle of Chevron Techron fuel system cleaner through. It is supposed to be one of the few additives that actually works.

Paris MkVI

I may give that a try.

I agree with you, Bert. No tune on her; she is completely stock. She's due for the routine maintenance, anyway. And she's not a spring chicken anymore. But I may run on 89 for a while now. It is really surprising what a difference it made.

Glenn

Do you think it might be time to change out timing chains and gears?

Paris MkVI

That's also true Glenn. I don't know if the 4.6 3V is an interference engine or not. But even if it isn't, I don't want the timing parts to fail on me while she's running.

And I do want to keep her running.

Other thing I've noticed is she is using more oil now. Not huge, but noticeable.