What would cause engine misfire at low revs + high load/throttle?

February 26, 2018 4:47 PM

My 2011 BMW 328i (N52 straight-6 naturally-aspirated engine) shudders and loses power if I give it lots of gas at low revs (less than 2500rpm). What are the most likely causes?

I've got a service appointment at the dealership tomorrow morning, but the car is out-of-warranty so I want to go in armed with as much knowledge as possible to steer the mechanics in the right direct and minimize unnecessary costs.

This doesn't happen at high revs regardless of the load, nor at low revs and low loads. Only the combination of low revs and high loads results in the shuddering. Also, no warning lights have come on whatsoever throughout.

I'm assuming that this is just a misfire, and hoping that it isn't knock, so I've continued to drive the car gently for the last few days. One hunch I had was a bad batch of fuel, as it started happening the day after I filled up at a local gas station that I never frequent. But I ran the tank dry and filled up again with premium, high octane fuel from my local gas station that I trust, and the problem still persists.

What gives?

posted by wutangclan to Travel & Transportation (8 answers total)

Hard to diagnose - check engine light? Any codes?

Most likely is that at low loads the car is going to either a high lean or high rich state because it can't accurately gauge fuel/air mixture. This could be because of an O2 sensor (relatively cheap) or a throttle body (mucho dinero) or ECU (now we're talking MAJOR dollars).

I don't think it is coils (high load and high revs would be more likely to show an issue), it could be crappy fuel maybe but you seem to have removed that, or even a water temp sensor giving a spurious reading and the car is operating on a wrong fuel mixture as a result.

So it's impossible to diagnose from here because it is just 'something in the fuel/air control stage' which is most of the major engine control systems and sensors. So it is also impossible to 'steer anyone in the right direction'. It need diagnosing, which means checking for codes (there will likely be some, even if the engine light is not on) and that may be easy enough. It may have thrown an O2 sensor or throttle body code and it takes ten seconds to diagnose or it may be longer to work out.

Good luck, is all I can say, basically. It doesn't sound particularly complicated to diagnose, from the vague description, but no guarantee it won't be expensive.
posted by Brockles at 5:21 PM on February 26, 2018

My first caveat is listen to Brockles first because he knows his stuff and I'm just a hobbyist, but does it happen dependably whenever you open the throttle wide at low RPM, or does it only happen after you've been driving around for a bit and have maybe already been in the wide-throttle/low-RPM state a few times? I'm wondering if you're lugging the engine a lot by habit, meaning that you're giving it a larger combustible charge at a point where the engine isn't making much torque, causing the drivetrain to resist more against the ignition wave and thereby heating up pistons and cylinder walls to the point where timing might be affected.
posted by invitapriore at 5:33 PM on February 26, 2018

You should get a code reader :) Normally Autozone or similar will loan you one.

The DISA valves are a suspect. See https://www.eeuroparts.com/blog/7336/disa-valve-rundown-symptoms-replacement/

It should be easy to disconnect the DISA valves and then see if the problem goes away. Checking the coils and plugs will probably take too much time tonight.
posted by pdoege at 6:56 PM on February 26, 2018

Thanks guys.

There's no Check Engine Light or any other indications at all. I don't have the equipment to check codes ... won't be able to buy one before tomorrow night.

The shuddering / power loss happens all the time, though to a lesser extent after the engine has warmed up. It also happens less if I progressively squeeze the throttle rather than stab it, but beyond a certain point (let's say >60% throttle) it happens regardless of how quickly the throttle was applied.

DISA valves are not applicable in my case. Only the 2006 330i came with a 3-stage intake manifold -- 2007-2012 328i N52 engines came with the single stage intake. Some googling indicates it might be an issue with the VANOS solenoids. But yeah, Brockles is right: it could be anything based on so little info.
posted by wutangclan at 7:35 PM on February 26, 2018

Have you visited any BMW forums to search for similar issues or ask your question? I ask because BMW owners are often super nerdy about their cars (I’m married to one, we have a vintage 2002 and a newer sport wagon) and my husband combs these forums for maintenance suggestions.
posted by padraigin at 8:16 PM on February 26, 2018

My E46/M52 did something similar, it turned out to be a busted crankshaft rotation sensor.
posted by JoeZydeco at 10:00 PM on February 26, 2018

I think it's a torque converter or transmission problem.
posted by bz at 10:58 PM on February 26, 2018

So I checked the vehicle in for service. Despite my skepticism based on Brockles advice, they said that most likely it's coils & plugs -- $1000+ parts & labour to change everything. However they said they'd do a full diagnostic first and get my authorization to do any specific work. Let's see what happens!
posted by wutangclan at 9:06 AM on February 27, 2018