97 Taurus Continues to Run after Shut-Off

97 Taurus 3.0 L Vulcan, 103000 miles.
Hot day today in Denver - temp in high 90s, A/C running on high. All gauges appeared normal.
Engine started to run rough about 4 miles before I shut it off, but became
smooth again. I started it an hour later, and everything seems normal.
Shut off engine, it continued to run for about 30 seconds, stumbling. It felt like 1-2 cylinders were firing.
No codes appeared as a result.
Does this sound like bad fuel? I use mid-grade gas, but the last fillup was premium on 1/2 tank.
My 76 F250 V8 started surging for about 2 minutes on the freeway yesterday, but then became smooth. Different gas station, regular lead-free. Gas again?
At $2.00+ for gas, are we also getting diesel fuel?
GT
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GT wrote:

its turned off and the injectors are electric. Unless there leaking?
The roughness could be a coil pack failing under heat load
Bob
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May be time to clean the injectors again. This is the first time I've had a port injected engine display run-on. And no codes were shown. But it's strange that the rough running was transient - it lasted for about 4 minutes. Much like the behaviour of the truck.
I've been buying gas at the local supermarket stations, and Sam's Club station. It may also be time to pay a little more and switch back to Shell.
Thanks for the input about the coil pack. Probably a good idea to replace it, keeping the old one as an emergency spare.
GT
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ALL Taurus engine only require 87 octane fuel, as do most Fords, even high performance models like the Mustang GT. The owners manual states using a higher octane fuel 'may cause derivability problems.' Perhaps that is your problem. Why not take your vehicle to a competent technician and have him perform a diagnostic test to be sure you don't have a more serious problem that may cost you a lot more money if not corrected?
mike hunt
GT wrote:

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At lower elevations, most all regular fuel is 87 octane.
In Denver, however, all regular fuel is 85 octane, mid-grade is 87, premium is 91. They get away with the lower octane rating because of the elevation. This car has had problems with 85 octane fuel. It runs much better, gets better mileage, no vapor locks, etc. with 87 octane. The manual says to use 87 octane.
I put in a container of Chevron fuel injector cleaner today, and the problems seem to have disappeared. But today wasn't quite as hot as yesterday either. I think the injectors probably need to be cleaned as a result of garbage in the fuel. Fuel filter was replaced 4000 miles ago.
GT

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