Accidentally filled Evo X with 87 octane... (learn me fuel trims, octane booster?)

So I may be overthinking this a bit... I just pulled a boneheaded move, and filled up while my mind was preoccupied with something else. Muscle memory at the pump led me to choose 87 octane for my Evo X (used to filling up the wife's van at this certain location). This was on a near empty tank.

WWGRM do? I am assuming that driving like this is ok so long as I stay out of boost (sub 3k RPM), until enough time has passed that I can fill up with 93 octane. Or am I assuming too much?

Modern ECU's are able to learn fuel trims based off of O2 readings; does this affect power delivery even after this tank? What is the consensus on locally available "octane boosters" (ie Lucas) - does this add any degree of safety?

This is probably an over-reaction, but the factory tune on this car (2015 final edition) has been called into question as being detonation prone in certain areas where 91 octane predominates. Thanks.

Fuel trims / O2 sensors wouldn't be a concern in my mind. I'd worry about whether it'll safely pull enough timing based on knock sensor feedback to keep it from self-destructing. As long as it does, it might feel a bit funky when it starts pulling timing but it should be safe.

Drive gently.

The fuel tank probably has a drain plug, or just syphon it out.

Find something worthwhile to worry about. This ain't it. If you typically do, don't drive like a loon on this tank

eh, drive it lightly for 50 miles or so and fill back up with race fuel. should dilute down to near 92 ish. Personally I wouldn't worry much at all.

You could just dump in a bottle of octane booster and drive as usual.

I highly doubt the ECU is incapable of pulling the timing back enough for it to live through a tank of crappy gas, engineers keep that kind of thing in mind when designing something that will be sold with a warranty to the general public. Driven gently it will probably just get crappy mileage on the next tank of good fuel as it brings the trims back to normal.

Out of boost it's just a regular low-ish compression motor. It's only when you cram it full of fuel and hot air that the extra octane is necessary. Don't cram it full of fuel and hot air.

Dont even worry about it unless you hear detonation.

Slippery wrote: Dont even worry about it unless you hear detonation.

You can damage it without hearing the detonation.

Personally, I would siphon it out down to a gallon or two, put the 87 into the wife's minivan, and drive gently to the gas station to refill with premium.

You could try driving gently. I'm told it's possible to drive a car for an entire tank of gas without ever getting it into the boost, but I'm not sure I believe it. I've certainly never managed it. :)

BrokenYugo wrote: I highly doubt the ECU is incapable of pulling the timing back enough for it to live through a tank of crappy gas, engineers keep that kind of thing in mind when designing something that will be sold with a warranty to the general public. Driven gently it will probably just get crappy mileage on the next tank of good fuel as it brings the trims back to normal.

NO there is a limit to what modern ECUs can pull as far as timing. The more you retard timing the higher you send EGTs.

And keep in mind. The EVO X doesn't have a "modern" ECU by today's standards, it's already more than a decade old design.

codrus wrote:

Slippery wrote: Dont even worry about it unless you hear detonation.

You can damage it without hearing the detonation. Personally, I would siphon it out down to a gallon or two, put the 87 into the wife's minivan, and drive gently to the gas station to refill with premium. You could try driving gently. I'm told it's possible to drive a car for an entire tank of gas without ever getting it into the boost, but I'm not sure I believe it. I've certainly never managed it. :)

This is what I would do. There is no way I could drive like a granny for an entire tank with a car like that.

z31maniac wrote:

BrokenYugo wrote: I highly doubt the ECU is incapable of pulling the timing back enough for it to live through a tank of crappy gas, engineers keep that kind of thing in mind when designing something that will be sold with a warranty to the general public. Driven gently it will probably just get crappy mileage on the next tank of good fuel as it brings the trims back to normal.

NO there is a limit to what modern ECUs can pull as far as timing. The more you retard timing the higher you send EGTs. And keep in mind. The EVO X doesn't have a "modern" ECU by today's standards, it's already more than a decade old design.

Even a decade ago, the exhaust models were really good- so they will do a decent job keeping the exhaust temps from getting too high, even when the spark retard is higher due to knock.

Still, the module may be old, but the software can be updated with better code- it's been long enough.

I would hope that Mitsu ran the car hard with regular fuel just to make sure it does not harm itself. We would, many times over.

I just know from the BRZ scene that there is actually a limit to how much the timing the computer will be allowed to be pulled. EVen using an aftermarket tune.

In reply to z31maniac:

I'm sure there is. But if Mitsu didn't at least set that to regular pump fuel, they are fools. This isn't the first or last time that someone will put regular fuel into the Evo. Some of the other times will be totally intentional.

easy enough to stick a tube down the filler hose and siphon it out. Not like you are going to waste it.. I would drain it as much as you can get out.. like down below a quarter of a tank, then gently drive to the nearest Sunoco and put 94 in it

alfadriver wrote: I would *hope* that Mitsu ran the car hard with regular fuel just to make sure it does not harm itself. We would, many times over.

Mitsubishi also had a reputation of voiding warranties for the thinnest of reasons.

Given the level of engineering I see "in the field" on Mitsubishis vs. Fords, I would not make any assumption that they test to the same level that Ford does.

If you got hard on boost the lower octane MAY cause detonation, but then the knock sensor would detect it (well before you could hear it) and retard the timing. Now you will have lost single digit HP at peak!

Drive it and don't worry unless you planned on taking it to the drags on that tank of gas.

I'm going to suggest RTFM. Every car I've owned that cared about premium, and every manual I've ever read has had something in it about this eventuality. I bet your manual does too.

RTFM? Sure that ain't "RTMF"? As in "run the mother ####er" Lol.

In reply to Trackmouse:

Probably. I think that's what they all said: If you have to use regular, put as little in as possible. Don't load the engine much. Fill the remaining space with the correct grade as soon as you can.

Look at this as an oppertunity to practice your hypermiking skillz and see what kind of mileage you can squeeze out of it..

But if you do accidentally get into the boost, i'm sure the ecu will pull timing and limit the amount of boost to keep things safe.

I wouldn't worry. Just take it easy for a tank.

In other news, I filled up a boat from a 5 gallon can the other day with one of these. It's just a marble (literally a cat's eye glass marble) loose inside a brass fitting and a piece of hose. Gave it 5 or six shakes and it siphoned the gas. Blew my mind...how did I not know these were a thing, and why don't I own one?

Shaker Siphon

Edit...I don't endorse trying to siphon the gas out of your Evo, or draining the tank. Just take it easy.