Rough idle, worse with A/C off - BMW M5 Forum and M6 Forums
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post #1 of 18 Old 6th June 2019, 10:40 AM Thread Starter
smithr
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Rough idle, worse with A/C off

My '99 M5 is idling rough at the moment and I noticed that it's a lot smoother if I switch the air conditioning on.

Any idea why that might be?

I'm thinking that the change of load on the engine could just be damping out the roughness or possibly the engine is just operating at a slightly different point in order to drive the A/C.

Could this shed some light on the cause of the roughness?

A little background: I also noticed that B1 and B2 LTFT are ~10% and 17% respectively as reported by Torque Pro, INPA is currently showing 1.11 and 1.18 'Adaptation value multiplicative'. These come down quite a bit after extended cruise on the autobahn. INPA is showing that the B2 cam timing is ~2degrees more advanced at idle than B1 which was also a surprise because I though VANOS should adjust them and then they should be equal?

I've checked for vacuum leaks and checked the O2 bungs for cracks.

Spark plugs, coils, MAF, Cam and Crank sensors, fuel pump, and VANOS solenoid valves are all relatively new. The car passed both the MAF and VANOS tests.

Any ideas appreciated, I will drop it down to the werkstatt soon if I don't figure it out.
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post #2 of 18 Old 6th June 2019, 11:07 AM
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The symptoms you're having are identical to mine a few hundred miles before the water pump failed catastrophically. It's probably unlikely that you're experiencing the same, but figured I'd mention it because it's so similar to what I experienced. I honestly don't have any good theories for why my pump failed that way, particularly since the A/C compressor and water pump aren't on the same drive belt. The idle has been rock steady ever since replacing the water pump.

Anyway, can't hurt to check your water pump pulley for looseness.

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post #3 of 18 Old 6th June 2019, 04:11 PM
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The idle speed is raised when the AC is on, likely why it smooths.
You have all the symptoms of a vac leak or bad mafs, but admittedly there are many other things that can do this. First thing I would do is electrically unplug the MAFs and see how it runs then how it changes over a day or two of driving.
Like the other posters problem anything failing that is driven by the belt could be causing this, might be worth pulling the belt and see if the idle returns to normal. If it does something driven by the belt is bad.
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post #4 of 18 Old 11th June 2019, 10:45 AM Thread Starter
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Thanks for the help. I looked at the pulleys and belts while idling and they look fine, I haven't run the car without them yet - is it safe to run the engine without the water pump running? not long I guess.

I'd unplugged the MAFs before but only long enough to tell that the idle was still rough. I now ran the car for ~50km with MAFs unplugged and no change. Again, is it safe to run that long without MAFs?
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post #5 of 18 Old 11th June 2019, 02:58 PM
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If you run with the MAFs unplugged you won't have air temp because it is in the MAFs. There are other things that can compensate. Safe ya sure as long as you don't beat on it, not sure that would be totally unsafe though.
Just watch the water temp with the water pump not being driven. You will know pretty quick if it is something on the belt. The only really hard one is the alt which could have an electrical fault and might try and produce full output at idle.
The rest of the items can be spun by hand and the resistance felt and some like the power steering would likely have symptoms.
For that matter you could watch the voltage on the OBC while driving and see what the ALT is doing. You should see a voltage drop while the rpm is heading for idle and it will recover shortly after idle is reached.
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post #6 of 18 Old 19th June 2019, 11:29 AM Thread Starter
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I dropped it down to the dealer since I don't have more time to work on this. Dealer says that one of the MAFs is defective and recommends replacing both (~600eur). Current MAFs are BOSCH 13621433566 units which I put in a couple of years ago when I was having issues with starting the car with a warm engine (it fixed the issue at the time).

I'd discounted the MAFs since they are relatively new, the 'MAF test' passed, and - crucially - unplugging them showed no change in behaviour.
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post #7 of 18 Old 19th June 2019, 07:24 PM
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I would not argue with them because they have the car to test, but that does not sound like bad MAFs. There should have been a difference when you unplugged them? How old are your O2s?

Last edited by Sailor24; 19th June 2019 at 07:24 PM.
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post #8 of 18 Old 25th June 2019, 09:27 AM Thread Starter
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Just got the M5 back after asking them to replace the MAFs and it does seem better, for sure the 'wobble' between 2-3000rpm seems to have gone and overall it seems much smoother. I'll reserve my full verdict until I've had more time to drive but so far I'm cautiously optimistic.

BMW said their tech had tested the MAFs on the bench and they were giving different readings which is why, even though there were no codes, they recommended replacement.
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post #9 of 18 Old 2nd July 2019, 10:37 PM Thread Starter
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The problem is definitely not fixed. Car feel smoother >2krpm with new MAFs but idle issue is still there, actually it seems to be misfire at anything <1500rpm. WOT from 1000-1500rpm is very jerky, then it smooths out around 1500 and really starts pulling around 2000.

Also noticed that last two days that the throttle was very jerky after about 10 minutes driving - happened at exactly the same spot on my drive to work, and when I arrived at work I pulled 2D and 36 codes with the Peake (In addition to a persistent 8F code that I haven't been able to resolve).

Almost inclined to think that this is an issue with the DME, no idea how to confirm that without a known good unit to swap. Will start at the beginning again and go through vac, fuel, spark, etc again. It's getting real tiring though hence the less than fruitful trip to the dealer.
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post #10 of 18 Old 3rd July 2019, 12:15 AM
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Have you ever replaced your fuel pump / fuel filter? If not - how many miles / Km on the car?

Simon
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