How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Matt Your Own Question
Matt, Engineer
Category: BMW
Satisfied Customers: 21858
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
Type Your BMW Question Here...
Matt is online now
A new question is answered every 9 seconds

BMW 328i Runs Fine Briefly But Starts To Idle Rough.

Customer Question

2009 BMW 328i 2d06, start fine, runs excellent for 5 mins but then starts to idle rough. Codes are present: cylinder misfires 4,5,6, 2d06 2e85. Though they are cleared it runs great only for 10 secs.
I have also checked new plugs, coils mass air flow, checked for broken vacuum lines, cracked intake tubes, unmetered air, and so on.
What is the problem?
Submitted: 11 months ago.
Category: BMW
Expert:  Matt replied 11 months ago.
This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane or brake cleaner around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.

if the engine condition is the same then chances are the meter or the connection to it is faulty

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading

Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

Customer: replied 11 months ago.
I had a smoke test done. no leaks in intake. the smoke test also applies pressure into intake. intake system held pressure, which I can easily relieve by unscrewing the oil cap. I checked connectors, wiring. I disconnected the mass air flow sensor and looked at the data difference on the scanner. I have access to snap on verus. I sprayed the intake and joints with starter fluid anyway. no change no response. fuel pressure is great. car runs and drives great. if I clear codes and start car with partial foot on the accelerator the symptoms are not immediate. if I continue to drive in normal traffic, when I come to a stop light I can feel the misfires. the codes are only for cyl 4,5,6 now I'm also getting a code 2E85 for electric water pump communication. as soon as I clear codes. runs like a new again at idle. and a few mins later it will surge and the start misfiring. any other suggestions?
Customer: replied 11 months ago.
also I cleaned the injectors. it has 6 new plugs and 6 new coils. and mass air flow sensor.
Customer: replied 11 months ago.
thought id mention. when the fuel pressure gauge is hooked up. I turn key on and fuel pressure jumps up to 75. I start car after I clear codes and the needle is at 73 -78, I wait for it to act up and needle starts jumping between 60psi and 80 psi. I give it throttle and is stabilizes around 78-80 at 2500 rpm.
Expert:  Matt replied 11 months ago.

Ok that fuel pressure sounds fine ( a bit high if anything) so I'd next remove the air intake and clean out the throttle body of any baked on carbon

and also check the condition of the main earth connection to the engine ( is it clean and tight) and there should also be some smaller earth connections to the engein that run out of the engine bay wiring loom and are secured to the engine with ring terminals

Customer: replied 11 months ago.
took the throttle body off, looked great, no carbon. I'm puzzled to why the fuel reading was super jumpy when acting up. I checked the plugs, they were soaked/fouled(mostly cyls 4,6) cleaned them up. I'm wondering, if a engine or body ground is bad connection, why affect only cyls 4,5,6. I noticed there's theres a ground resistor/isolator on cyls 1,2,3. no ground /resistor/isolator on cyls 3,4,5. any more suggestions?
Expert:  Matt replied 11 months ago.


the other thing to check is for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

Customer: replied 10 months ago.
had exhaust back pressure test done. no leaks in exhaust.
Expert:  Matt replied 10 months ago.


OK in that case I'd check the engine wiring loom for the small earth connections that run out of the loom and are secured to the engine with small bolts and ring terminals

these are the specific ECU earth connections for the coils and injectors

Customer: replied 10 months ago.
ok so finally got done checking EVERYTHING. the connectors. how do I know the connections and wires are good.. the scanner has function test. we can kill injector pulse and see instant misfire on all 6 cyls. then by disconnecting 1 coil at a time and noticing a misfire change. disconnected connectors and checked for proper voltage and reconnected. I'm suspecting a runner inside intake manifold could be lazy or sticky. ANY SUGGESTIONS?
Expert:  Matt replied 10 months ago.


Ok thats a lot of thorough testing and I'd agree a sticking inlet flap could be the cause, and its not uncommon

this video showing how to access the flaps may help

one other thing that occurs to me is that this could well be a problem with the VANOS system if you have the parameters available on the verus tool can you monitor the target cam position and the actual cam position?

Customer: replied 10 months ago.
I acquired time on another running car for part check for next Wednesday. I will swap everything over and back if I have to solve this. I hope I'm not going to be charged because me and the expert got to learn/solve something we couldn't figure out so far.. . I'm going to swap 1 sensor at a time and retest every time to see what solves it.
Customer: replied 10 months ago.
btw the vanos, cams, crank, target and actual are all in sync. no other pending dtc
Expert:  Matt replied 10 months ago.


let me know how you get on

good to hear that the VANOS all seems in order

Customer: replied 9 months ago.
got a fusebox from the junk yard, a used oem battery from the same car, and so far so good. no codes. no misfires, no mass air flow codes. took all my returnable parts back. a backyard carburetor mechanic working on a rusty chevy figured it out and charged me $200 in total labor.