Hello everyone!
I recently purchased an Audi S5 V8 4.2, 2010 B8 model. I've a problem which no body was not able to figure out the reason so far.
Car is running fine, no CEL, no engine codes, I've only oil leak which I didn't need to top it up after 2000km, so far oil level is still around 75%. Problem is; car is "shaking" in idle. Idle RPM is 600, sometimes goes up around 800 but problem exists regardless which gear, ac on, ac off, etc. And the tachometer is stable, it's either 600 or 800 rock solid, never moves. One more strange thing is, when this happens, I open the internal cabin lights and lights start "blinking", you can clearly see the voltage changes, whenever I push gas pedal and increase RPM slightly, light get brighter and also "shaking" disappears. Means, car is not able to produce enough power in idle, I've no idea why..
So far we did;
* Engine mounts replacement (due to this behavior, didn't help)
* Spark plugs replacement (due to this behavior, didn't help)
* Ignition coils replacement (due to this behavior, didn't help)
* All fluids change (engine oil, transmission fluid, steering fluid, coolant)
* Radiator replacement (was leaking)
* AC compressor replacement (was not working properly)
* Suspension arms replacement (was having some broken bushes)
* Throttle body cleaning
* Rear oxygen sensor replacement (was having fault code)
* Smoke test to check leaks (no visible leaks, didn't help)
* Alternator & Battery check (everything is normal)
Strange thing is, when I increase RPM slightly, at 1000 RPM, all this "shaking" behavior directly disappears. Also when I'm accelerating or during standard drive, there's ZERO issue with the car. Fuel trim is fine, no weird sound, basically no issue at all after 1000 RPM.
Having this oil leak makes me think about a vacuum leak, but as I've mentioned, there's no leak during smoke test.
And not having any engine code is making things even harder.
I'm currently in Dubai and unfortunately mechanic's knowledge here is also very limited. I'm waiting for your valuable inputs, thank you in advance!
Sounds like the voltage regulator in the alternator may be going out. I'd replace the alternator anyways if it hasn't been. Those ignition coils draw a lot of power, and if it can't keep up with the draw, it can run weird, especially at idle. The alternator is putting out the least power at idle vs 1,000 RPM+. The brightening headlights is a classic sign of a bad regulator. Those German cars are so computerized they can run on nearly dead batteries and alternators and not tell you there's an issue until it's completely dead.
I had a 1994 BMW 540i in 2013 that was still running on a battery that was manufactured in 2006. The only reason I figured out it was bad is the security system that I reconnected nearly killed the battery in the 8 hours I was at work. It set the alarm off, even though the car was running with the programmed key. I replaced it and it worked with no issues the remaining time I had it.
Hey Justin, thanks for the comments! We tested alternator and the voltage was 13.7, also the battery life was showing %97. Hence, they said nothing is wrong with alternator. But also I read some comments that healthy alternator should provide around 14.2V. So I'm confused now if I need to change or how else I can control it..
The alternators output will vary based on the load it's under and the charge on the battery. Your headlights and blower motor on max put it under the most strain. A full battery reads around 12.7 volts. Technically, as long as the output is higher than the battery voltage, it's charging the battery. Do you have a multimeter handy? You can check the voltages by running the car with the headlights and such on, and touching the leads of the meter to the battery terminals. Rev the engine, the voltage should stay about the same at higher RPM and lower. If the voltage goes higher and the headlights brighten, that's a good indicator of a potential problem with the alternator, or there may be some other problem in the electronics. Those Audis are extremely complex cars.
+1 to a faulty alternator.
Swap it with a known good alternator & see what happens. If it fixes it, you know immediately that you need to buy a new alternator.
Thank you guyz for all answers, I'll definitely work on the alternator.
One more addition, today I scanned the car in idle and saw that "long term fuel trim" was -15.6% and below is the total picture;
SHRTFT1 0.8%
LONGFT1 -15.6%
SHRTFT2 -2.3%
LONGFT2 -5.5%
O2 Sensors voltage is around 0.5 - 0.7 V
MAF is around 5.5 - 6.5 g/s
Fuel Rail Pressure 2540 kPa
BARO 99 kPa
Cat temps around 500 C degrees
VPWR 13.45 V
As far as I know the reason of negative fuel trim is vacuum leak/un-metered air going into engine. If LONGFT1 stands for Bank1, that means, I've a vacuum leak here and maybe I need to change intake manifold gasket for bank1?
Also I checked voltage in idle, blinking happens only when the voltage is more than 13V. As soon as it drops around 12V, lights are less bright but no blinking, car is more stable. When I open AC, voltage goes higher, hence rough idle is more noticeable. I think this is also directing us to alternator/voltage regulator in this case..
Guys finally I've some idea at least and I feel like we're very close to find solution!
Vacuum leaks are easy to find, spray carb cleaner around the engine bay and listen for an idle change. Pay attention to where the spray hits, and look where the idle changes.
Have you checked your air filter or cleaned the MAF sensor?
We checked the air filter, it was OEM lifetime air filter and we cleaned it. When it comes to MAF, technician said, if there was an issue with MAF, we would have appearing code during the scan, that's why they didn't clean... Also i tried to disconnect it to see if the car is running better without MAF (with stock ECU adjustment), car started running veeery rough instead and I had parkbrake malfunction, EPC, ESP lights on directly. As soon as I connect MAF back, all those disappeared.
Also for air filter, I think at some point this car was having performance intake kit, since some plastic parts was missing where we connect filter cabin to front grill. I'm not sure if that makes sense or no (maybe they did some adjustment after placing it, I don't know)
We did smoke test to check leaks but I think spraying carb cleaner is better option?