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E39 M5 (1998-2003)

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Old 05-11-2012, 07:15 AM
Jsborn Jsborn is offline
always learning...
Location: Oakland, CA
Join Date: Nov 2008
Posts: 394
Mein Auto: '89 Porsche 944 S2
Avoid CA e39 M5?

Hi all,

I've been searching nationally for an e39 M5 for several months, and I've found one in northern California. I'm wondering whether I should be wary of purchasing a CA e39 M5, though, because of the lower octane you guys get.

Would I be better off buying an M5 that hasn't been used in CA?

E93 335is DCT w/ Invisihitch + 1Up, split armrest, stainless lines, track pads...super fun
Recent ex: Porsche 944 S2
Former Bimmers: Dinan E39 M5, E90 335i 6mt sport JB+, E36 M3
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Old 05-14-2012, 06:59 AM
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timfitz63 timfitz63 is offline
MT = Doppler Shifting
Location: Lorena & San Antonio, TX
Join Date: Aug 2010
Posts: 928
Mein Auto: 2007 X3 (MT)
I presume you're referring to the relatively lower octane rating of premium fuel in California versus that available in the Pittsburgh area? But I follow the gist of your question.

I know the Owner's Manual of my X3 recommends premium unleaded (91+ octane), and would presume the Manual for the M5 recommends the same; that would be typical of high-output engines with high compression ratios. I don't think (but admit I'm drawing on sketch memories) that the octane rating of premium unleaded is below 91 anywhere in the nation, so as long as the previous owner(s) always used premium unleaded, there shouldn't be any problem whatsoever purchasing a California-based vehicle.

That being said, even if the previous owner(s) used regular unleaded (usually 87 octane) in the vehicle, this is generally permissible, per the Owner's Manual, with some caveats about reduced performance. The computer (via knock sensors) will try to compensate for the lower octane rating by altering the timing to the extent that it can to avoid detonation. So long as the previous owner(s) didn't push the engine hard enough on regular unleaded to encounter detonation on a frequent basis, again there shouldn't be any mechanical problems with the vehicle. The way octane plays into the long-term effects on the engine is somewhat complicated, and dependent on the habits of the driver and conditions encountered (low vs. high altitude; very cold or very hot temperatures; moderate or wide-open throttle; etc.). Unless the paperwork history of the vehicle includes gasoline receipts, there's probably no way to verify what type of fuel was routinely used in the vehicle, and you'll have to determine for yourself whether to accept someone at their word if they tell you they always used premium unleaded.

I think in general you shouldn't have any fuel-related issues with purchasing a California-based vehicle; at least I wouldn't, given a thorough test-drive, and pre-purchase inspection. But ultimately the decision will be yours, and if you're not suitably comfortable with the unknowns, you might look at alternatives.

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