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  #1  
Old 08-09-2013, 12:39 PM
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Exclamation BMW 4.6 is poor acceleration and hesitation

Hi I have a 2003 4.6is
When I first owned it, a year ago, everything was fine and it was very fast
Recently the EML came on and the power seemed to gradually disappear
All 8 spark plugs were changed and 3 bad coils were diagnosed and these were changed
Now the car idles very smooth, but when I try to accelerate it is very slow/hesitant
I can overtaken by a 1.4 vw golf!
The car can't go above 4000rpm (it struggles to get there)
At approx 4000 with pedal fully depressed I start to hear a rattling from engine area
Occasionally I feel the full power of the car trying to come through, but not often now
Any ideas? Your help would be really appreciated
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  #2  
Old 08-09-2013, 01:41 PM
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you need to pull the codes in order to narrow the issue...

firt things coming to mind, the engine is suffocating for air, or starving for fuel, or getting wind blown between the ears - vacuum leak...

start with air filter - make sure if it is good..

then go to MAF - check to see if it works - unplug it while engine is idling, if idle stumbles, and then recovers, the MAF is probably good, if idle remains the same, the MAF is a toast. keep in mind, the MAF problem can be intermittent

Check the rubber boot feeding air to the throttle for cracks and tears, it can introduce unaccounted air into the system

check the throttle body for gunk

those are simple things...

then, listen for whistle of a vacuum suction

test for vacuum loss as per many posts here...

test for fuel pressure

and don't rule out that there can be other coils that are shorting out to the chassis under greater load...


your best bet is to pull the codes using BMW specific software and NOT a generic code scanner...
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Old 08-09-2013, 01:55 PM
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Thanks for your reply
MAF is brand new but I will check the other things you mention
Any idea what the rattling noise from engine could be?
It's not there at idle or if you gently increase the revs, it starts to appear as the engine struggles at approx 4000 rpm which made me think is there any variable intake on this car? It's a 4.6is and I'm fairly new to x5s (this is my first one)
Occasionally he revs manage to reach 5000rpm without the rattle, but less often these days
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Old 08-09-2013, 01:58 PM
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does the rattle sound like "marbles in a can"? how many miles?
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Old 08-09-2013, 02:01 PM
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though I have never been lucky enough that the cause was as simple as replacing the air filter, except one time when it was soaking wet. You don't need to check it, just pull it out and start it up and see if it runs better. As long as you don't drive may miles or through a dust storm, and I don't recall hearing about any in England, it very unlikely it will hurt anything. Hope that is your problem.
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Old 08-09-2013, 02:06 PM
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The rattle sounds to me like a metal flap that is rattling but I guess perhaps a can with a marble in it may produce the same noise. Originally the noise wasn't there when the car used to accelerate fine. As the problems with the torque/power/acceleration have increased, I have noticed the rattling starts as the car gets to 4000rpm then struggles. Occasionally the rattle is not there and the car will go past 4000rpm. The rattle is from the engine area but never rattles on idle. Rattle only starts to appear as engine struggles at approx 4000rpm
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Old 08-09-2013, 02:08 PM
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Car has done approx 100,000 miles
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Old 08-09-2013, 02:10 PM
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Quote:
Originally Posted by bcredliner View Post
though I have never been lucky enough that the cause was as simple as replacing the air filter, except one time when it was soaking wet. You don't need to check it, just pull it out and start it up and see if it runs better. As long as you don't drive may miles or through a dust storm, and I don't recall hearing about any in England, it very unlikely it will hurt anything. Hope that is your problem.
Thanks for the advice, I think the air filter is clean but I will remove it and try your suggestion in about an hour
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Old 08-09-2013, 02:12 PM
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Were the diagnostics from reading computer codes or I'm guessing that's what it is so I am going to replace that part and see?

I assume the new MAF was an attempt to fix the problem?

Can you post a video of the 'rattle' by making it happen one time?
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Old 08-09-2013, 02:14 PM
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search and read on the symptoms of the chain guide failure - while i don't think it is the case, but read up about the issue to rule it out... with this problem, the failure will be CATASTROPHIC, to the tune of a new engine... once you ruled that problem out, carry on with the rest of troubleshooting...

try to pull the codes, they can steer you in the right direction
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