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Discussion Starter #1
Hello everyone! I'm very upset, today something strange has happened to my M5 and I can't fall asleep until I ask people about it. I could not find a clear answer to my problem so I decided to take the risk and post it here, I'm sorry if there's another discussion but I haven't found it. So here's my issue: Today I was short on fuel and there was no (100 octane) premium fuel available at the moment so I decided to get about 10l of the regular 95 because even 98 is hard to find in Prague. So then I drove slowly about 10km around, then stopped for a half an hour and after a break I started the car again and it noticed something... I ''blipped'' the gas pedal on Neutral and noticed the throttle is kind of.. jerky? like... in quick blips up to 2000rpm... and the idle is also rough, car shakes more aaaand sounds a little bit different, like a slight knocking, I can definitely tell it sounds more rough than yesterday... When I try to get the car rolling it feels weaker at 1500-2000rpm, like it wants more gas you know. I would say that compared to before the car almost feels like it's gonna stall at very low rpms when trying to move from a complete stop. Maybe not stall, but definitely not as smooth as with 100 octane premium fuel. I should also mention that the PO said he ran it for 2 years on premium fuel as well, so... I did use the same type of fuel for months, could it be that the car has still yet to adapt to the lower octane fuel? I've read that it has 2 knock sensors for that, maybe they're broken. I'm not really sure what I'm talking about here guys, but I would reaaaallly appreciate any kind of advice. After all, the car ran fine until I put regular petrol in it for the first time in maybe years? Is this how the car adapts?! About my car:

The car has presumably 140k kilometers (almost 4.5k done by me) Oil was changed about 2000km ago, went from Shell to Castrol..... found the broken drain plug so I replaced the whole lower oil pan+ gasket. It also smoked a bit so I've changed the upper hoses and both separators, doesn't smoke now! I got the chain tensioner + gasket ring in my M5 but haven't changed it yet, couldn't find any plastic bits in the oil pan when we replaced it, so I assume they're still ok for now?
 

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When was the last time the spark plugs were changed? Have you tried reading the internal codes and see if there is any misfiring?
 

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Discussion Starter #3 (Edited)
Thanks for the reply! Well, I think they're still original, I have a box of parts with both vanoses, head gaskets, valves and some other crap so I think they have not been changed. Nothing in maintenance history book... Unfortunately I don't have a code reader at the moment but I was told to buy the BMW 1.4.0 scanner so I'll get that ASAP and try to get the software running and look for codes. Yeah I know that now, without the codes we are basically just guessing here but sh*t, the car is supposed to run on 95 and this is when the car started doing weird things. The only thing I've done for now is I've drained the fuel tank, so at least something. And the worst thing is, I don't understand what I've done wrong, BMW says to run on RON 95-98 so what the hell? I had hopes that somebody is gonna tell me this is normal and the car needs to adapt but now I just can't stop thinking what if I damage the engine... Anyway when I find out what causes this I will definitely post a long *** text about it.
 

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Thanks for the reply! Well, I think they're still original, I have a box of parts with both vanoses, head gaskets, valves and some other crap so I think they have not been changed. Nothing in maintenance history book... Unfortunately I don't have a code reader at the moment but I was told to buy the BMW 1.4.0 scanner so I'll get that ASAP and try to get the software running and look for codes. Yeah I know that now, without the codes we are basically just guessing here but sh*t, the car is supposed to run on 95 and this is when the car started doing weird things. The only thing I've done for now is I've drained the fuel tank, so at least something. And the worst thing is, I don't understand what I've done wrong, BMW says to run on RON 95-98 so what the hell? I had hopes that somebody is gonna tell me this is normal and the car needs to adapt but now I just can't stop thinking what if I damage the engine... Anyway when I find out what causes this I will definitely post a long *** text about it.
It can run on 95 RON. If you are not running it hard, it won't matter. And blipping the throttle in neutral is certainl not running it hard. So, could be bad gas (look to fill up at another station, 98 preferred), could be clogged fuel filter (underneath car, below left front door), failing fuel pump, failing fuel pump "relay" (in the boot, right hand side, looks different than other relays, lots of info on this board). There is a thread something like "everything you need to know not to get stranded with a bad fuel pump relay" or similar. May be more difficult to research the site, since it was recently "upgraded".

Could also be plugs, but they will not go bad all at once. 1 bad plug would put you down on power, but it should be constant. Check also the coils, one per plug. Make sure no wires are pinched and everything is seated.

My best guess based on facts would be bad gas. Throw in some Techron or other preferred fuel cleaner, or something that absorbs water (Stabil) and add fresh gas from known good station.

Regards,
Jerry
 

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I also agree with Jerry. Right now there's really not enough information to narrow down the potential source. It could be bad gas, fuel pump, spark plugs, or any number of potential issues.

If you see any misfires codes in the logs, changing the spark plugs is a good start. If you plan to keep the car and don't know when they were last changed, it is good maintenance to change it anyway.

Your best bet is to start narrowing potential causes. The cheapest one is to 1) check the logs if you can or 2) run through this batch of gas, refill with a known good supplier, and add a bottle of fuel system cleaner like Techron or similar.
 

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Discussion Starter #6
Hey guys, so I have and update. The first thing that I've done was to suck the **** fuel out of the tank, both of them. Then the car was sitting for like 2 weeks. But a few days back I finally put in some premium fuel and decided to start her up. After like 5 minutes of idling it started to get ''smooth'' and it felt like the car was going back to normal but I had to go so I shut off the engine. Today I turned the engine on again and it seemed okay! I went for a drive, about 20 miles and it was just like before! I think it's gonna be okay. I still need to get a code reader though...
 
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