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Recurring P0140 Code


DSM N/T Wiseman
DSM Wiseman
Nov 10, 2002
Raleigh, North_Carolina
Octane rating has nothing to do with being good or bad to your car or even the quality of the gasoline. The octane rating is an indication of the burn rate of the fuel. The higher the octane the slower the burn so on a stock, good running engine that the manufacturer has tuned for 87 octane, you're actually creating a less efficient burn buy upping the octane you put in.

The reason high performance engines such as turbo car or SVT/HO/ETC ETC engines require a slower burning fuel is because of the pressures inside the combustion chamber from either this forced induction or the high compression. As you increase pressure you increase your chance of setting off the fuel charge prematurely. This is why 87 octane will "Ping" on a turbo car, the fuel charge is igniting before the piston is at the top of the stroke working it's way back down. The ping is the piston ever so slightly being forced BACK the direction it just came causing the noise and damage to the engine.

The only time you should up the octane rating on an engine/PCM set-up originally designated for 87 is if you already hear that pinging. This is usually caused by carbon on the top of the piston being so hot it prematurely sets off the fuel charge. Now you've got an ignition source at the bottom of the combustion chamber and the spark plug at the top igniting the charge as well. This causes the pinging as well and again is bad.

Don't bump up your octane unless your computer is designed to handle it, the compression ratio has been increased or you're running forced induction. If you do so without those changes, you're creating more problems.


Proven Member
Jan 5, 2011
Southern, New_Hampshire
Thank you Doug for that information, I already knew about that and I am sticking with the 87 :).

Just to give you an update, Sunday 3-13-2011 I deleted the code for the P0140 code. I drove the car for the rest of the week (until now, Thursday 3-17-2011) and I went to see if I'd pass, just for the fun of it. Well...all 8 sensors were ready to be tested and NO Check Engine Light codes!!! I passed and finally got my sticker after 6+ months of trying!

Thanks guys/gals for all your info!
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