In the recent colder weather my 745t ran fine on lower octane so I filled up the 855t with the same. I don't drive it much but drove it to work yesterday with the temps in the 70's and a got a CEL on the way home. About 1/2 a tank has run through since the fillup.

I'll pull the code(s) tomorrow morning and see what it points to but the gas is the only thing I've changed. I haven't figured out how to change the weather yet.

I'll also top up the tank withe the 93 octane I usually run.
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Erwin in Memphis, '88 745t 193,000 miles, '95 855t 96,000 miles

When I got home last night I noticed idle rpm was 1000 which is high. Normally around 750. I suspect a vacumn leak causing both. Didn't get to pull the codes today, my work shift is crazy this week.

Tomorrow I'll have the morning off and the wife & kids will be visiting friends. I'll look into it then and pull plugs for a look-see.

Thanks,
btw, it's the first time I put 87 in the 855t. No particular reason other than the 745t didn't mind. I'll be filling it up with 93 tomorrow as well.
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Erwin in Memphis, '88 745t 193,000 miles, '95 855t 96,000 miles

Most likely a missfire code caused by the gas you used. The ECU is supposed to compensate for the lower octane, but maybe you got a little water and/or bad gas. Your 740 would probably not be as affected by bad gas as your 850 (is a much cruder system in the 740).

I would never use less than 89 in my Volvos, and almost always use 93. I have sometimes (for a long trip) mixed 89 and 93 to get 91 octane, the minimum recommended by Volvo. You might try that to save a little $$. In 35 years of driving, I've never used less than 91 in any car I've owned. Performance and fuel economy is better, system is kept cleaner, and I've only ever had to do one valve job, and never a ring job (the typical problem caused by using too low octane fuel). If you ever get pinging (usually from low octane), remember that is the sound of independent explosions occurring at the wrong place and time within the cylinders (i.e., not a nice smooth explosion wave front). It is the sound of expensive damage to your engine. Pay me now or pay me later.

I've been running 85 octane in my 97 850-R since I bought it with 64 miles on it and it runs fine. All plugs I pull look great. I get 32 mpg on the hwy. I had bad gas in my Volvo one time that made it ping, but that was in another state, but never had it ping here in Denver.

85 octane? Why have an 850-R?

Remember that high altitude really does change the ocgande requirements. 85 is not even generally available in most states, and would certainly test the compensating abilities of the anti-nock system down here at sea level. (Denver = the mile high city for a good reason)
--
Mike Sullivan ('91 745 (171k), '93 965 (147k), '95 855T (60k), '98 V70XC (78k)

Good point. 85 in Denver is functionally the same as 87 in Dallas.