Help me diagnose jumping RPMs at idle. (With Video) - Mercedes Benz SLK Forum

SLK R170 General Discussion SLK 200, SLK 230K, SLK 320, SLK32 AMG

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#1 Old 03-17-2018
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Help me diagnose jumping RPMs at idle. (With Video)

From time to time, I will get little throttle blips when at a red light, or when stopped. Any idea what it might be?

Turn your volume up, you can hear it pretty strong at the beginning of the video.

https://streamable.com/y258a
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#2 Old 03-17-2018
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Now that's an interesting one! Park up. get the bonnet/hood open and identify whats clicking for a start.

Aircon clutch placing a load on the electrics, ECU compensates but the clutch has dropped out?

MAF sensor failing.

Camshaft position sensor / cam chain?

Just a few ideas.
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#3 Old 03-17-2018
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When the EVAP System self tests run you can hear the purge valve clicking. However, that should not have a lengthy impact on the RPMS, just a short one time blip. I'll guess the pedal sensor or the throttle valve. A scan with SDS or equivalent while its happening would be revealing.
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#4 Old 03-23-2018
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Quote:
Originally Posted by nimbusgb View Post
Now that's an interesting one! Park up. get the bonnet/hood open and identify whats clicking for a start.

Aircon clutch placing a load on the electrics, ECU compensates but the clutch has dropped out?

MAF sensor failing.

Camshaft position sensor / cam chain?

Just a few ideas.


I have had to clean my MAF quite a few times and I think it might be going out. Whats weird is that this issue is pretty intermittent. In the night of the video it was jumping pretty aggressively until I got going again.

When I turn the AC On or Off the RPMS do jump once or twice, but then it stops.

CPS/Cam Chain? I hope not, isn't that a pretty costly repair?

Next time it happens I will try to pop the hood and get a video.

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When the EVAP System self tests run you can hear the purge valve clicking. However, that should not have a lengthy impact on the RPMS, just a short one time blip. I'll guess the pedal sensor or the throttle valve. A scan with SDS or equivalent while its happening would be revealing.
I think the purge valve is fine, had a EVAP issue on another vehicle and this doesn't seem like it. I think you are on to something about the pedal or throttle as it does give a slight acceleration, but obviously I dont move because I am holding the brake down, but can clearly feel the jump and the see the RPM going up.

I also found this video, interesting as it hits all above points.

Last edited by Lepper; 03-23-2018 at 04:02 AM.
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#5 Old 03-23-2018
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I've never faced the failing pedal sensor but I do wonder if the car will idle with it disconnected....
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#6 Old 03-23-2018
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is the supercharger clicking on and off???

that will definitely change the engine load.

unplug the supercharger and see if it keeps doing it.
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#7 Old 03-23-2018
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is the supercharger clicking on and off???

that will definitely change the engine load.

unplug the supercharger and see if it keeps doing it.

How do you unplug the SC?

I read something that it could be the coupler in the supercharger making the clicking noise?

There are alot of Ebay rebuild kits that include oil, gaskets, coupler, and a hand pump.
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#8 Old 03-24-2018
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Quote:
Originally Posted by Lepper View Post
How do you unplug the SC?

I read something that it could be the coupler in the supercharger making the clicking noise?

There are alot of Ebay rebuild kits that include oil, gaskets, coupler, and a hand pump.
There is a little electrical connector just above the supercharger that you can unplug. You wont have any power, but it will still run fine and you'll be able to see if that's your problem.
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#9 Old 03-29-2018
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The noise seems to coming from the air oil separator/resonator/supercharger. Air intake housing was covered in oil
Is that supercharger valve suppose to open?
Just cleaned my maf and its running better, but I still occasionally get the rpm jumping.
Clogged PCV?
I have a facelift SLK so the AOS is different then the early models.





https://streamable.com/v8sb4

111 098 01 09/280 750 045 is the throttle body sensor and is directly attached to the supercharger. Could oil contamination make its way into this sensor? Makes sense as I get the jump during an extended pause in drive or park and is exacerbated by the MAF sensor. Or maybe its purely MAF.
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#10 Old 03-29-2018
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Vacuum leak? (Don't know about the clicking though - couldn't hear it....)
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#11 Old 03-29-2018
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Before one starts throwing expensive parts at a problem, and no codes are set, go for the simple stuff.
On 3 different cars, a unstable idle was caused by black carbon deposits behind the throttle valve (butterfly valve) in the throttle body. When you look into the throttle body you don't see it until you open it manually.
With the engine not running, open the valve, spray carb cleaner on a lint free rag, clean off the carbon on the valve and opening. Get as much as you can with the rag, then with a warmed up engine running, spray the valve intermittently so as not to stall the running engine.
Vacuum leaks would be next on the list.
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#12 Old 05-23-2018
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Well I haven't had any issue with this in several months. As long as I keep the ac on it doesn't do it. Weird
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#13 Old 05-23-2018
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i'd just connect Torque Pro and see what else is jumping (mixture, O2, throttle position, timing, MAF, MAP,...)


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