vivshank

Recently bought the well-loved wagon with 140k miles on it. Made a rookie mistake of putting in regular 87-octane gas in it. Filled the tank. The manual, I think, says minimum 87 but the check engine light is on. Realized later the fuel cap says use only premium gas.

I should flush the tank, right? Or is the check engine light on cos initially the gas cap wasn't on very tight?

Wife drove about 50 miles today and she said it didn't feel as smooth as when we were test driving it. Don't think there was any knocking. Co-worker and long-time A4 owner saying he wouldn't worry about it.

What do you recommend? Thanks in advance!

acarney

I probably wouldn't worry about it either. Seriously, think of all the people that bought these cars, I bet you had plenty of people with too much money and not enough smarts dumping whatever the cheapest gas was in. You probably won't get the designed performance from it and it might not run as perfect as it should, but I highly doubt it'll damage anything. Just run it almost empty and fill it up with premium then. I suspect the ecu will sort out timing to avoid knocking and such... Though it might take more then one tank of the top shelf stuff for the ecu to readjust to max performance.

vivshank

Thanks, acarney. Do you think the gas-type set off the CEL?

jcman

acrney's right no damage will occur with low grade gas the pre O2 sensors and knock sensors will adjust accordingly, octane is just a pre-detonation additive that does not make gas more volatile but does give a fractional higher power stroke on high compression engines.

vivshank

Tks jcman - but would the wrong kind of fuel trip the CEL? tks

jcman

I'm sure that acarney and myself would agree, NO.
I hate to thread jack a question directed to someone else.

SloopJohnB@mac.com

The answer is generally it depends. There is a finite range the ECU can alter/retard timing to make up for low quality fuel. In an engine optimized for 91 (R+M?)/2 it may or may not be able to compensate for 87 thus resulting in knock/preignition and tripping a misfire(s) and thus a CEL.

Moreover, carbon buildup in the combustion chamber (including the piston top/dish) effectively increases the compression ratio which makes the chamber more likely to experience detonation.

That being said, I suspect the 87 is more sensitive to turbo and high relatively high compression engines. During one protracted electrical outage after a large snowstorm (5 days) the local gas stations ran out of premium very quickly and only had regular delivered. I left the S6 and BMW M3 in the garage and ran the 2K4.2A6 on 87 regular with no CEL or problems..I also feathered the gas pedal a lot more than usual, no full throttle low rpm blasts!

If you're really worried about the low octane gas and running it until the tank is almost empty, just toss in a can of 104+ or some such SAFE octane increase additive. Yeah, it's $5 or so but what's your comfort worth?

jcman

Glad you chimed in Sloop, it's almost like I was expecting you on this subject and I bet your ears were burning too, Ha Ha.

My daughter who drives the 01 2.8 has used low grade gas for the past 5yrs with not one cel related to gas, I can't seem to convince her about all the stuff we know therefore I let it go, it's her car now.

Yes sir you are correct to a point with certain engines for sure and an example would be my 04 3.0 because of the higher compression ratio over the 2.8 I'll stay with 93 only but that's just me.

N_Jay

Ouch!

First; to the OP: What engine do you have? If it is turbo, then I would keep the load down till you have a chance to top up about 1/2 the tank with higher octane fuel.

Now to the mental "Ouch"

Probably no damage as the KNOCK SENSORS will protect the engine as long as the ignition system can pack off the timing far enough to avoid pre-ignition and detonation (knocking).
Higher boost, higher heat and higher load all make this more difficult.

Second, Octane is a measure of the fuels ability to resist pre-ignition and/or detonation. It is not an additive, juts a reference point.

Third, higher octane has no more energy (and may actually have very slightly less energy).

Airbag

just to add my 2 sents . I have a 2000 a6 2.8 and have NEVER put mid grade or high grade gas in it. i just put regular in it and dont worry about it . i have 400,000km and have not had a check engine light on for over a year .
I dont " horse" the car either as for me its just an extra car for my son to drive and i drive it a few times a week.