Check engine light on

[Hacexd]

So I know I might get judged really badly for this but I need assistance. Like an idiot I filled up my tank with 91 octane because there were no other options since I was driving out of town (I live in a very rural area). I didnt get any issues initially and the car drove fine. However, today the car misfired and my check engine light is now on and is staying on. Could that be because of the fuel or would it be an issue with the coil packs? 

 

Please help. 

[89CAL]

What car?
What have you done anything to it recently?

Limited info

You can bridge two of the wires in the diagnostic plug and it will flash a fault code I believe

[Hacexd]

It's a 4 door skyline r34 GT-T. The check engine light came after a day that I fueled up with 91. Revved the engine to about 5k and it misfired and the engine check light came on. 

[Duncan]

running on 91 is perfectly safe assuming you were just pottering around. If you floored it up a hill and noticed but ignored the sound of a small rock rattling in a tin then you've likely f**ked your engine.

Is the car idling and running OK? Do you have 98 in it again now?

It's most likely not related to the fuel, but you still need to find out what the error code is.  Any local mechanic could put a scanner on it, or like Callan said there would be a tutorial on here somewhere on how to get it into diagnostic mode to tell you what the error is

[Hacexd]

I did floor it a bit. not completely and it was not uphill. The car misfired and since then I've only been driving really gently. The car runs fine for now. I haven't put 98 in yet because im still on a quarter with the 91 fuel. If i am right, when it goes to half, I could fuel up with 98 then to make it closer to a 95? I might give the diagnostic tutorial a go and see what pops up. 

[Duncan]

Yeah, I'd fuel up at 1/4 tank with 98 and yes you end up with the average octane so that will be OK.

I've been caught a few times without high octane when I'm out and about and it's fine. Actually back in the day when I was out at Alice Springs for 6 months my old 33 ran on only 91 and it survived OK....you only need the whole octane rating at maximum torque / maximum load. So if you aren't flooring it up a long hill in 4th at full boost it's fine.  Aftermarket tunes are almost always less tolerant to make more power but factory is pretty safe.

[Hacexd]

So a quick update. I drove to work this morning and drove it gently again as i did for the past few trips. The engine light check went off on its own and hasnt appeared since. I'll keep on driving it gently and maybe after putting 98 in and waiting for a day or two, ill try to floor it and see if it misfires. 

 

That's my issue, i live in katherine and to get premium fuel here is quite hard. Only a few places provide it but after this has happened I definitely will ensure that I always have premium fuel in. 

[niZmO_Man]

I'd say invest in a tune for 91 RON fuel (does Nistune do R34s? I Haven't checked).

[Hacexd]

I can still get 95 most times as long as I'm in town but if in the future I find it hard to find 91 fuel on the way I would definitely consider tuning it to 91. 

[Duncan]

If you can control yourself it's safe enough....its just hard to be good all the time

The quick option is to lower the base timing by 2-3o, that should make the factory tune safe on 91. Just need a timing light to do it correctly. Of course it won't be as crisp in the midrange but at least it will be safe on low octane

[Hacexd]

So update again. after two trips the engine light came back on. I stopped the car. started it again and drove for around 200 metres and it went off again. This has already happened twice today so I'm quite concerned. I did the self diagnostic as well and it came back with code 55 meaning everything is fine. Also topped up with 98 just before. How long would it normally take for the fuel to mix properly and make a higher rating? Not willing to floor it yet just in case I get more misfire and damage the car. 

Edited December 22, 2017 by Hacexd

[blind_elk]

The fuels will mix immediately.

If you are  overly worried, you could pop into your local auto parts store or servo and get a bottle of octane boost and add that to the tank.

[Hacexd]

I guess maybe Im having other issues then. The car still seems to misfire when boosting. Engine light hasnt come back on from three trips that I've done after fueling up.

[Hacexd]

Also if that may give more information, my boost gauge works on and off. Could that be a potential reason why my car hesitates on boost? 

[Duncan]

well, that sort of depends whether it's actually on boost when the gauge doesn't work or not

a missfire can stop the engine running well enough to create decent boost.  Alternatively, an electrical issue like a bad earth, dying alternator etc could also cause a misfire. There's just too many reasons to really guess without more information.

Is your current situation that you have mostly 98 on board, you drove it 5 times yesterday, you haven't had a MIL for the last 3 trips, and it missfires? And where specifically does it misfire, approx which revs? And does it misfire if you free rev it or only when you are driving it properly under load?

If it is just light missfire in the mid range under load it is most likely a spark plug or coil pack issue. start by pulling the plugs out and checking their condition and gap. If the gap is over 0.8mm gap them down to that and see how that goes.

[Hacexd]

I might try to bring it to a mechanic on wednesday and see if they can find anything because as you've said, there are a lot of possibilities here. I filled up 98 when I was around half a tank on 91. No I haven't had a MIL for the few trips that I've done and yes it still does misfire. It probably misfires around 4.5-5k revs i would say. It only misfires when driving it properly under load. I do have to rev the car quite high to make it happen as well. The car just hesitates for a bit when it reaches the revs I specified and then after a few seconds it gets a sudden surge of acceleration like it should.

 

I was actually thinking spark plug or coil pack as well but at the same time, I just talked to the previous owner (I had the car for a week now) and I asked him if he changed the fuel filter and he did say that he doesnt recall when he changed it. So i might get the fuel filter changed as well and see the results. 

The only issue with all of this is knowing exactly what's going wrong and I hope the diagnosis will find out the specific problem. 

Thanks a lot everyone for your help as well. 

[Chris32]

I had this once after switching fuel brands, all 98

I’d been running on BP for ages and then out some Mobil 98 in, and got the check light after giving it a bit

The code it threw up was the O2 sensor

It did it a few times but haven’t seen it since. That was about 4 years ago

[Hacexd]

Did your car end up hesitating as well when boosting ? 

[Hacexd]

So quick update again. Ended up getting a fuel injector cleaner. I put it in the fuel tank and fueled up with 98 again and dropped the psi a bit on the turbo and car is running fine. No more hesitation on boost at all. Potentially just needed to be fueled up again I believe. I would also like to help everyone for your help on this matter. I spent a whole week worrying about what it could be and reading about it day and night on every forums lol. All solved though.