Slightly rough idle on warm engine only? - BMW M5 Forum and M6 Forums
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post #1 of 15 Old 5th April 2019, 02:51 AM Thread Starter
Andrej89
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Slightly rough idle on warm engine only?

Hi! Would like to ask for support & advices.

On my BMW E60 M5 since I have bought it (1,5 years, almost 30k km driven already) I have an issue with slightly rough idle, but only on a warm engine. The issue is more noticeable in case the engine idles a lot e.g. in a traffic jam. Possible to feel then some vibration as well.

Idle control valves have been replaced, but nothing has changed. Throttle bodies have been cleaned as well. Vacuum leaks have not found.

No trouble codes identified.

Some days ago have noticed that MAF sensors show different intake temperatures on bank 1 and 2 i.e. on a hot engine the difference is sometime 5-7 Celsius. Have tried to switch the MAF sensors between banks, but ISTA shows same date i.e. MAF sensors are probably fine. Could it be a MAF wiring issue or such difference might be simply because the air is really warmer in some parts under the hood?

Unfortunately, did not have a chance to check if MAF sensors should different temperatures on a completely cold engine.

I understand that in theory different intake temperatures on banks may cause some rough idle due to different fuel mixtures. Could it be true?

Mass air flow values on idle are shown by ISTA as quite identical on both banks. About 18-19 kg/s if I remember correctly.

Any advices where I could dig further? Do not really feel any effect on performance from this slight rough idle issue, but still would like to finally solve it.

Thanks.
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post #2 of 15 Old 5th April 2019, 11:40 AM
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Vanos solenoids can cause idle problems, if you got software test them. I had very annoying idle problem in all temperatures, after replacing charcoal filter under rear wing problem gone. During replacement I faund one o ring incorrectly installed on pipe going to filter so very likely that cause air leak upsetting idle. That was one of those rare cheap fixes after nearly half year of searching to resolve idle problem

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post #3 of 15 Old 5th April 2019, 07:24 PM Thread Starter
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Thanks. Glad to hear such cheap fix stories

Vanos solenoids have replaced already. Have also recently changed fuel pumps, fuel filter, fuel pressor sensor and charcoal filter. As preventive maintenance.

Within next days plan to replace main oxygen sensors. They seem to be still factory installed and might be related to rough idle.

Another "theory" is unsynchronized throttle bodies.

The main question is why the issue is visible only on a warm engine and gets more noticeable the longer the engine idles e.g. in traffic?
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post #4 of 15 Old 5th April 2019, 07:37 PM
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You also mentioned some vibrations, what kind vibration you got? On idle only or maybe in some specific rev range?

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post #5 of 15 Old 5th April 2019, 07:54 PM Thread Starter
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Only on idle. The rest is totally fine.

Vibration like from misfiring, but very moderate. However, no codes and fuel mixture, ignition timings are fine.

That moderate vibration is also not constant. On one traffic light you may feel it, but when you reach the next traffic light it may almost disappear, although on tachometer you still see +/- 10-20 rpm rough idle.
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post #6 of 15 Old 5th April 2019, 09:14 PM
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Change your gas cap. Go to Amazon and order yourself one before you do anything

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post #7 of 15 Old 6th April 2019, 07:01 PM Thread Starter
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post #8 of 15 Old 7th April 2019, 08:44 AM
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post #9 of 15 Old 7th April 2019, 05:04 PM Thread Starter
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Thanks! Carbon filter next to fuel tank have changed recently. How can can a gas cap make such an effect? The gas cap does not let the gasoline fumes out or what? Will change it, although btw there is no “sucking” effect when I open the gas cap after driving.
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post #10 of 15 Old 9th April 2019, 06:25 PM Thread Starter
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Have replaced the gas cap - the issue is still there. However, subjectively I can say that after replacing fuel pumps, filters (including active carbon), pressure sensor and gas cap, idle has become more stable and becomes slightly rough on a higer oil temperature. And moderate vibration gets noticeable only after really longer idling. Therefore, the whole story should be somehow linked to temperature i.e. the higher the heat is around, the more the issue appears.

Plan to replace main oxygen sensors during the weekend. Will be useful in any case as currently there are still the ones from the factory.

Any other ideas?
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