1500rpm hesitancy/oscillation/vibration

M

Mr Whippy

Veteran
 Harrogate
Hi everyone,

I've had a slight fault with my Z4 for years. It's always just felt a bit iffy just around 1500rpm, say lifting off the throttle a bit from full throttle it'd feel like it wanted to accelerate harder somehow.

But then it began exhibiting a more problematic trait, under high load in any gear where the car can't accelerate, at high throttle (full basically), at 1500rpm almost exactly, it'll get a bit of a wobble on. Maybe 50rpm either side you can feel it kinda jerking softly but at 1500rpm it's almost like someone is snapping the throttle open/closed at about 4-6hz kinda range.

Obviously I lift off before it goes crazy. Just using half throttle or dropping a gear, or accelerating through that range quicker makes it go away. Sometimes it doesn't even do it unless you really force it to do it in the perfect up-hill conditions in 5th/6th gear.

In lower gears it just accelerates through and never shows any problems. Sometimes it doesn't seem to do it at all.



I've spoken to an engineer who tunes engines and fitted one of these to an E30 and he said the mapping feels a bit flappy around 1400-1800rpm (lifting off but still feels to accelerate)... I think it's a trait of the engine generally but something is exacerbating it in my case.

My car is full BMW SH, on about 113k miles right now.

Apart from this weird issue the engine is strong, fast, clean (passes MOT emissions tests with secondary cats removed), does good MPG etc etc...



Was thinking maybe it's a DMF issue? Or VANOS?


Anyone else had similar?


Thanks

Dave
 
That's right around the conditions where you can get knocking (pre-detonation) even with fuel meeting the minimum octane rating. The engine has knock sensors that shut down the cylinder under knock conditions, causing it to feel like the engine is missing. That RPM is outside the power band, it's OK for cruising, but if you really need to accelerate, you need higher revs.

Much like the doctor said to the patient who complained "my arm hurts when I do this" (this is a better visual joke)... "Don't do that!"
 
Ah hmmm, I've been running it on 95 octane for a while now and it's been noticeably worse.

I used to buy BS7800 Sainsburys Super Unleaded (BP fuel apparently) but now I've moved all the local Shell/Tesco etc are stupidly expensive for super fuels, despite not even matching the BS7800 rating.


Looks like I need to head over to the Harrogate one to get to my nearest one.

I'll run a tank or so and see how I get on with it!



I agree on the driving conditions thing, it's easy enough to drive around but it's such a good engine it'll let you dig this deep and still pull ok for most UK road work. I just wanted to make sure it's not an actual fault that is very very slowly developing over many many miles :)

Thanks for your help!

Dave
 
Am I missing something here? Unless your seriously cruising why would you accelerate full load at such low revs (and uphill!!)? Not doing your poor engine any favours at all, not surprised it sounds/feels bad. Change gear, problem solved.

If it's worse with lower octane fuel then as above, your probably causing pre-detonation. Higher octane fuel is more resistant to det but it'll still happen if your putting major loads on the engine so forgot the fuel, change your driving style!
 
Changing gear requires effort when 'just cruising'... plenty of UK roads are just a 40mph procession these days and when you live in hilly areas that can mean slowing up/down a lot through the 1500rpm zone.

Given how advanced the MS45 ECU is and how generally excellent/thorough BMW's mapping is for this engine I did expect such issues to be 'covered' electronically, unlike what might happen on an older cable operated throttle where the engine has no choice but to operate in non-ideal conditions.


But if other people notice this behaviour and think it's just normal then I'm happy. I was just making sure it's not a problem here with my engine.


I suppose there is support of this issue given that in different atmospheric conditions it might seem worse/less worse. Ie, one evening it seemed really bad and then the next day where the conditions were quite different it was almost impossible to make happen even when trying.



I'd say that running BS7800 super will probably fix this issue because generally I'd only ever noticed it very very occasionally (a few times in almost 5 years), and it's only recently since running 95 normal petrol that I've really noticed it more!

I'll run the tank down near empty over the next few days and then get a fresh fill of Sainsburys super!

Dave
 
OK I ended up going to Shell and got their v-power... fairly reasonable price at the moment.

Problem pretty much totally gone now under all conditions.

It just sounds a bit deeper through 1500rpm and maybe the slightest sensation of lower output, and then it's pulling hard again.


Back to super-only operation again :D

Dave
 
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