Why does my car run rough sometimes?

titaniumx3 · 22 Dec 2017 at 20:44

I've had this issue to some extent with the last three cars I've owned but its the most pronounced with my current car a BMW E92 330i (N53 engine). When I start the car cold in the morning, it runs beautifully, buttery smooth and tight, and the engine/exhaust makes a clean howling noise. However after it's warmed up or I start it again after driving for a while (say 20 min), the entire character of the car changes, the engine now feels rough when accelerating, and makes coarse exhaust noise. The effect is worse when I am driving slowly in town and even makes a rattle/grinding noise when accelerating from lower rpms. It sounds a lot like a diesel van. It's hard to say if the actual performance is being affected, but it certainly does not feel as good to drive, nor does it sound half as good.

Now the bizarre thing is that this does not happen when it is wet outside. When it's raining the car feels consistently smooth. I can even take the car for a wash and it will make all the problems mentioned above go away, until it has warmed up and dried again of course.

I've taken the car to multiple garages and they all say they don't notice any problems and that the car is working as it should. Some of them say that this is normal for BMW cars. I don't know if this is true as this is my first BMW. I've looked into possible issues with the DMF, clutch, propshaft mounts, but nothing is conclusive and all the garages tell me all these items are in good condition. It's honestly driving me mad at this point, as I really enjoy driving the car when it's working smoothly.

I would appreciate some advice on this. Is this a common problem? What is causing it?

dholdi · 22 Dec 2017 at 20:47

Do you live somewhere with a warm climate ?

titaniumx3 · 22 Dec 2017 at 20:49

dholdi said:
Do you live somewhere with a warm climate ?

No I live in the UK.

dholdi · 22 Dec 2017 at 20:52

titaniumx3 said:
No I live in the UK.

Ok, I would take a guess at fuelling being incorrect if it runs better when its cold.

Scania · 22 Dec 2017 at 20:56

Try replacing the MAF (mass airflow sensor) with a known good one, I’d put money on this being the culprit.

My old 328i e36 did exactly this, replacing the MAF cured the rough running - and transformed the car.

titaniumx3 · 22 Dec 2017 at 20:57

dholdi said:
Ok, I would take a guess at fuelling being incorrect if it runs better when its cold.

I've always use either Shell regular unleaded or their vpower stuff, it makes no difference, same issues. Unless you mean something else?

Armageus · 22 Dec 2017 at 20:58

is it fine at full throttle (i.e. rather than just gentle acceleration)?

During cold start/full throttle the engine runs "open loop" monitoring rather than relying on Air flow and Lambda/Catalyst sensors. As above MAF sensor, or even a dodgy temperature sensor could cause this.

dholdi · 22 Dec 2017 at 20:59

titaniumx3 said:
I've always use either Shell regular unleaded or their vpower stuff, it makes no difference, same issues. Unless you mean something else?

Yes, I meant fuel to air ratios.
As Scania and Armageus have pointed out these are determined by the MAF which could be out.

titaniumx3 · 22 Dec 2017 at 21:05

Scania said:
Try replacing the MAF (mass airflow sensor) with a known good one, I’d put money on this being the culprit.

My old 328i e36 did exactly this, replacing the MAF cured the rough running - and transformed the car.

I did read about this once, but didn't think it would cause such a huge change in how the car runs.

One other thing I've noticed is that when the car is running rough, if I rev the engine at idle, it makes this weird 'swirling' or 'swishing' noise after each press of the accelerator. It's hard to explain in words, but it's like air is leaking from a pipe or something. I doubt this is what is actually happening, but it kind of sounds like this.

sovietspybob · 22 Dec 2017 at 21:07

Don't know about modern BMW's but in some cars you can disconnect the MAF and see if it drives better as it'll force it to run open loop. You will get a dash warning light but it might be worth it to see if it is the culprit, i did this with my ancient Impreza while chasing a poor idle.

titaniumx3 · 22 Dec 2017 at 21:07

Armageus said:
is it fine at full throttle (i.e. rather than just gentle acceleration)?

During cold start/full throttle the engine runs "open loop" monitoring rather than relying on Air flow and Lambda/Catalyst sensors. As above MAF sensor, or even a dodgy temperature sensor could cause this.

Yeah, the higher I rev the engine, the less notice able it is. I also can't notice any difference when cruising on the motorway, it's just when driving in town.

Jonnycoupe · 22 Dec 2017 at 21:11

titaniumx3 said:
I did read about this once, but didn't think it would cause such a huge change in how the car runs.

.

Measuring the air flow into an engine is critical to work out how much fuel mass to then inject.

titaniumx3 · 22 Dec 2017 at 21:16

Is it is easy to change the MAF sensor? Where is the best place to get a replacement part? I'm a bit useless when it comes to car maintenance.

dholdi · 22 Dec 2017 at 21:21

Generally speaking it will be a jubilee or similar clip either side and an electrical plug connector.
Try your reg in ECP and see what it comes up with.

HoneyBadger · 22 Dec 2017 at 21:22

Enter your reg on www.eurocarparts.com

Dr Who · 22 Dec 2017 at 22:55

Clearly the issue is the Germans configured it for typical British weather?

titaniumx3 · 31 Dec 2017 at 03:01

Hello again. Sorry for reviving this thread, but unfortunately I still have the problem. I took out the MAF sensor and cleaned it, but it made no difference, and it was pretty much spotless when I took it out anyway.

I still have no real clue as to what the problem is, but I suspect it is an engine problem as I can notice a definite change in the engine noise even when revving in idle. I also feel that performance may be degraded when its in this 'rough' state. I also know that it has something to do with temperature and humidity, car runs smooth when it is wet outside.

Another thing that I've seen happen is that, when the car is running rough, if I turn it off, wait a minute or two and then restart it, it's running smooth again. I did this yesterday and after restarting it, the car ran smoothly for remainder of the journey (about 35 miles), plus it wasn't wet or damp outside, just cold.

I'm not sure what to do, other than take it to another garage. Anyone know of somewhere good in or around Essex?

titaniumx3 · 31 Dec 2017 at 03:04

[Ignore]

Phil W · 31 Dec 2017 at 07:38

Cleaning the MAF doesn't always work if it's kaput, replacing is the only way.

I'm sure I've read about vacuum leaks on 330i's too on the PCV circuit, worth looking into?

Armageus · 31 Dec 2017 at 07:55

If you haven't already got one, it's worth picking up a Bluetooth OBD2 dongle, and a copy of Torque or similar for your phone.

With that you can monitor the various sensors, and may be able to spot the issue straight away (e.g. if there is no or intermittent reading on one)