Weird Acceleration

Motohip · 05-13-2012, 11:24 AM

I have a 09 128I with 70k miles with the N51. I've only noticed this twice and both times it was in fourth or fifth gear accelerating to about 60. The car accelerated like I was pushing on the gas, then letting off slightly, then on, then off and so fourth, but very slightly at the rate of 60 per minute. Any one else have this problem? I've tried to replicate it but it hasn't happened again.

Dackelone · 05-13-2012, 12:08 PM

I would scan your car for codes. Maybe its your "drive by wire" or maybe even a slight vacuum leak causes a "lean surge" condition. Carbon buildup can also cause this surging under light load(s).

Jut out of curiousity how old are your spark plugs? You might start there. I know they are good for 100K but that doesn't mean you should wait that long to replace them!

RPM90 · 05-13-2012, 07:03 PM

As Dack pointed out, check for any codes.

Given your 70K miles, it may be time to get your fuel filter replaced.
At high engine rpm fuel requirements are higher. If your fuel filter is getting near clogged it could give you the symptoms you are describing as enough fuel isn't getting through at the proper rate.
At start up and low rpm you could be getting the proper fuel flow, as demands goes up, more flow may be slightly impeded and thus causing what you have.

If it is the fuel filter, over time this condition will get worse and worse, and at some point even at part throttle you may get this power stutter. When the condition becomes bad enough and becomes too lean the ECU will throw a code. It may be very limited at this point and you're feeling a slight stutter that is not enough to throw a code.

Other things to consider are your spark plugs. At 70K miles I would have replaced them by now. Take them out and check their condition as well as gap. If the gap, due to wear, gets too big then the spark can get "blown" out by the compression if the spark is not strong enough, that too can cause a stutter.

If this condition is new, did it occur within the current tank of gas, or did it start with a new tank? If it's recent, you could simply have some moisture/water in the lines or some "bad" fuel that may have too much moisture. But, that should present even with low rpm, but still it would be a good idea to get some fresh fuel in there and try a bottle of
"heet", which is a moisture remover.

Some other things would be coil packs, and coil wires. High rpm also needs a hot spark and if a pack or two is bad it may not be giving enough juice to fire the plug/s properly. With this a code should get triggered.
Plug wires can go bad as well, and should throw a code as that is a harder thing to find unless you go ahead and do a complete tune up and replace plug wires when you do the plugs and fuel filter.

The biggest item would be the fuel pump. Granted you don't have a high pressure unit like the turbo engines, but fuel pumps can go bad as well. An intermittent pump can be hard to diagnose unless it fails to a point where the air/fuel goes very lean and a code gets triggered.

I'd start with the easiest things, and I listed them in that kind of order.
Good luck with your diagnosis. Post when you find the issue.
It'll be good reference for others with higher mileage 1's.

One more thing regarding vacuum leak, I've encountered vacuum leaks a couple of times, but generally it presents even under idle with an unsteady irregular idle where the rpm goes up and down a few hundred RPM. If you've done a test for this type of leakage it's very easy to do, but be careful.
One easy and safe way to do this is with a can of compressed air using a long flexible hose. As the engine idles, spray some air around the intake areas and note if the engine idle goes up or down. No leaks, no change. If idle changes up or down then it's likely an intake leak.

Motohip · 05-15-2012, 09:50 PM

Thanks guys for the advice. I'm not sure if the spark plugs have ever been replaced. I'm not sure about the maintenance history. I think it just started with this tank of gas but I might be wrong. I keep trying to replicate it, but it hasn't happened again. There are times when I accelerate hard, that I feel like it's doing it, but it's so fast that it's hard to tell. I'm gonna check the plugs this weekend.

On a side note I cleaned my MAF sensor and dusted out the crap in my air filter and at the same time I reset the throttle calibration and transmission adaptation and it drives and shifts a lot better.

05-13-2012, 11:24 AM	#1
Motohip Lieutenant 37 Rep 435 Posts Drives: 128i Join Date: Mar 2012 Location: Washington iTrader: (5)	Weird Acceleration I have a 09 128I with 70k miles with the N51. I've only noticed this twice and both times it was in fourth or fifth gear accelerating to about 60. The car accelerated like I was pushing on the gas, then letting off slightly, then on, then off and so fourth, but very slightly at the rate of 60 per minute. Any one else have this problem? I've tried to replicate it but it hasn't happened again.
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05-13-2012, 12:08 PM	#2
Dackelone European Editor 10539 Rep 22,992 Posts Drives: N54 e82 Join Date: Feb 2010 Location: Bayern, Germany iTrader: (1)	I would scan your car for codes. Maybe its your "drive by wire" or maybe even a slight vacuum leak causes a "lean surge" condition. Carbon buildup can also cause this surging under light load(s). Jut out of curiousity how old are your spark plugs? You might start there. I know they are good for 100K but that doesn't mean you should wait that long to replace them!
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05-13-2012, 07:03 PM	#3
RPM90 Major General 890 Rep 7,047 Posts Drives: 340i M-sport AT Join Date: Mar 2006 Location: Chicago iTrader: (1)	As Dack pointed out, check for any codes. Given your 70K miles, it may be time to get your fuel filter replaced. At high engine rpm fuel requirements are higher. If your fuel filter is getting near clogged it could give you the symptoms you are describing as enough fuel isn't getting through at the proper rate. At start up and low rpm you could be getting the proper fuel flow, as demands goes up, more flow may be slightly impeded and thus causing what you have. If it is the fuel filter, over time this condition will get worse and worse, and at some point even at part throttle you may get this power stutter. When the condition becomes bad enough and becomes too lean the ECU will throw a code. It may be very limited at this point and you're feeling a slight stutter that is not enough to throw a code. Other things to consider are your spark plugs. At 70K miles I would have replaced them by now. Take them out and check their condition as well as gap. If the gap, due to wear, gets too big then the spark can get "blown" out by the compression if the spark is not strong enough, that too can cause a stutter. If this condition is new, did it occur within the current tank of gas, or did it start with a new tank? If it's recent, you could simply have some moisture/water in the lines or some "bad" fuel that may have too much moisture. But, that should present even with low rpm, but still it would be a good idea to get some fresh fuel in there and try a bottle of "heet", which is a moisture remover. Some other things would be coil packs, and coil wires. High rpm also needs a hot spark and if a pack or two is bad it may not be giving enough juice to fire the plug/s properly. With this a code should get triggered. Plug wires can go bad as well, and should throw a code as that is a harder thing to find unless you go ahead and do a complete tune up and replace plug wires when you do the plugs and fuel filter. The biggest item would be the fuel pump. Granted you don't have a high pressure unit like the turbo engines, but fuel pumps can go bad as well. An intermittent pump can be hard to diagnose unless it fails to a point where the air/fuel goes very lean and a code gets triggered. I'd start with the easiest things, and I listed them in that kind of order. Good luck with your diagnosis. Post when you find the issue. It'll be good reference for others with higher mileage 1's. One more thing regarding vacuum leak, I've encountered vacuum leaks a couple of times, but generally it presents even under idle with an unsteady irregular idle where the rpm goes up and down a few hundred RPM. If you've done a test for this type of leakage it's very easy to do, but be careful. One easy and safe way to do this is with a can of compressed air using a long flexible hose. As the engine idles, spray some air around the intake areas and note if the engine idle goes up or down. No leaks, no change. If idle changes up or down then it's likely an intake leak.
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05-15-2012, 09:50 PM	#4
Motohip Lieutenant 37 Rep 435 Posts Drives: 128i Join Date: Mar 2012 Location: Washington iTrader: (5)	Thanks guys for the advice. I'm not sure if the spark plugs have ever been replaced. I'm not sure about the maintenance history. I think it just started with this tank of gas but I might be wrong. I keep trying to replicate it, but it hasn't happened again. There are times when I accelerate hard, that I feel like it's doing it, but it's so fast that it's hard to tell. I'm gonna check the plugs this weekend. On a side note I cleaned my MAF sensor and dusted out the crap in my air filter and at the same time I reset the throttle calibration and transmission adaptation and it drives and shifts a lot better.
	Appreciate 0 Quote