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Originally Posted by DrFerry
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Re vacuum pressure sensor, is that not just used for brake assistance/servo control?
Quote:
Originally Posted by DrFerry
Octane is certainly one of the major factors that can affect engine hesitation, but carbon deposits are another. Hence, the fuel detergent additive package also becomes important as the engine and fuel system (injectors, valve seats etc) mileage increases. In the USA, a third party (AAA) which is similar to the U.K.'s A.A. did a fuel quality study. I've attached the .pdf report below and IMO it's well worth a read. Forum members are reporting S65 injector failures with resulting engine failures due to bore scoring or bent rods from hydro-locking - when fuel leaks into the combustion chamber. I merely mention this in case the hesitation you're experiencing is an early warning sign of injector failure due to carbon deposits or other types of fuel deposits (gummy varnish?) affecting the injector spray pattern. I recall certain YouTubers ferreting out which fuels were used in the AAA report - with the best one for detergent additives being Shell V-Power. It's part of the reason BMW recommends Top Tier fuels in the USA as well as Techron fuel system cleaner. But Forum users report Techron coats the S65 plugs. See attached AAA report.
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https://www.m3post.com/forums/showthread.php?t=1898330
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A very interesting report (I'll read the full report later), and whilst I certainly wouldn't want carbon buildup found in the lower tier fuels by the AAA, I don't see that causing hesitation. Power loss I could certainly see happening though, with it restricting air flow somewhat. And pinging if it happened inside the cylinders (assuming knock control couldn't stop it).
I wish the AA or RAC had done a similar test on UK fuels!
Are the same labelled fuels the same constituents in the US and UK??
Quote:
Originally Posted by Space Horse
Unfortunately for me in the US, the octane supplied in gas stations varies at what altitude they are. Since Octane in fuel is usually only for compression, and when you are at higher altitude you don't need the same octane to reach the desired compression due to the air being thinner. So, the entire State of Colorado has 85-AKI, 87-AKI, and 91-AKI. It is incredibly hard to find anything higher than 91-AKI in my state or really any State that is higher than 4500 feet. Once you are in a lower altitude State then the pumps go to 87-AKI, 91-AKI, and 93-AKI.
There is also the argument that since the tune is at a certain octane and compression, that increasing octane won't necessarily do much. I don't exactly agree with that. I would definitely use higher octane if I could. 
It is funny I have rented cars in the UK; I should know that your fuel is different. 
What is the mileage of your car? That can definitely rule in/out whether carbon build up is a concern.
Also, what RPM are you usually at when cruising on the Highway? For how rich these cars run stock IMO (My SC tune makes it worse) unburnt fuel over long periods of not higher revs (3000+) will definitely start clogging things up.
These cars also have pretty aggressive camshafts. Have you checked the Camshaft or Crankshaft position sensors? My friend had an XJ Cherokee (Jeep) that had a camshaft sensor go bad which made it rev terribly. (Not here to say anything Jeep makes can even hold a candle to the mechanical marvel that the s65 is  )
A camshaft in the wrong place in the wrong time frame can certainly affect what will happen when you stomp the gas. That could also explain it getting worse slowly over time. These already don't typically like low revs.
With how low the revs are when this is happening, might point to the cam being offset slightly when near idle. The engines probably mechanically fixing it and adjusting it to then freely rev (Redlining 4th is awesome  .)
I know it isn't possible for it to be that far off. However, engines have cam position sensors for a reason. |
Re octane and altitude, ah got ya, I did see that mentioned in the wiki, I guess you have the octane you need then. Unless you travel to a lower altitude state!
My M3's done 86k miles now. Although it does very low annual mileage with me, this is not compromised of many short journeys, rather it is infrequent use but nearly all longer drives.
e.g trips to a circuit, a trip to the Nurburgring each year (but bar once, not on it
), country drives typically lasting between 2-3+hrs (on Sunday I went for a 3 hr spin). So combined with doing 2 track days a year (so far, would like to do 3 at least!) I think this usage would keep carbon deposits low.
Also, as I said just above, I don't see how carbon deposits would cause hesitation, but I'm all ears if I've missed something there!
Re cam sensor, well aside from no fault codes, it starts fine (a symptom of one of those going bandy) and revs just fine too, right upto the redline (which in 4th is indeed fun btw!
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And it runs fine at low revs, although I don't normally cruise at less than 2k RPM, it will pull fine from 1500 RPM. Unless I snap open the the throttle at less than 1500, which is normally in 1st gear, sometimes 2nd. Then it hesitates for a second, and actually when it did it once on my sunday drive, it actually misfired for that second or 2 this time!
Oh, and on highways I typically do 70-80 in 6th, whatever revs that is.
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Originally Posted by Scrippy
I see all the typical ideas being brought up and I agree with them. I’ve had idle problems like hunting and stalling before and a new fuel vent valve fixed it.
I’ve also had 1 of 2 TA’s fail (120,000ish miles) and prior to it failing I had idle issues like hunting, stalling and hesitation with gentle throttle application from a stop. Just this week my final OE TA failed (281,000) and I had idle hunting, stalling and hesitation with gentle throttle application from a stop. I finally got a code for bank 1 TA. I’ve opened both of the failed TA’s and the first 5-7 teeth are worn. This is the start range where the TA wakes on ignition and opens the throttles for start and idle. So I will bet you have 1 or both TA’s shitting the bed on ya. Good luck.
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No idle or hunting problems here, just the hesitation at low revs and quickly opening the throttle.
Wow that's got to be a record mileage for an OEM TA!
lol.
As per my op, mine were changed for rebuild ones a couple of months before I got it, so have done about 12k miles. I think it would be very unlikely they've gone already....
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Originally Posted by tdott
What tune are you on?
I've seen certain tunes that did not throw a check engine light, but as soon as stock 240E/241E was flashed back, there was a CEL and misfire code that led to a bad coil causing a slight hesitation/shudder at low rpm. Sounds like it might be similar to your issue.
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It's on the standard tune/map AFAIK (as per the op), I guess I could check with ISTA?
I mentioned in my reply to Space Horse above, that for the first time it actually misfired during a hesitation 'event' on Sunday.
Thinking about it, I've had this kind of problem on other cars at work, and coils/plugs is a common cause! And often won't give a fault code where the misfire is brief!
I think what I'll do is check for air/vac leaks, and if I don't find any I'll change the plugs first (even though they've only done 25k miles). Then see how it goes.