Odd engine RPM dip when pulling up (only once)

[pillow]

Hi everyone,

First off, car is a CL600, 2007 with M275 engine.

I am having a bit of an odd issue that I am struggling to pinpoint to something specifically. The scenario is as follows:

I set off in the morning no issues. My morning journey has few points where I come to a complete stop, and where I do it's usually very brief. After about 5 miles of driving I come to a set of lights which is the first place I stop for any prolonged period of time. Every morning I stop at these lights, about 3-4 seconds after coming to a stop (car returns to idle smoothly and sits there for several seconds) and then out of nowhere a very sudden dip/rpm stumble. This literally lasts a split second. When this happen the instrument cluster needles dim slightly, and the UB voltage on the cluster drops (I need to confirm whether it drops before the RPM dip or after). Generally after this happens once it does not happen again on a journey. I also haven't experienced it happening on my return journey home later on in the day.

In my investigations I have been looking at the alternator output (as this is where I suspect the issue lies). Generally when running it will vary between 13.5 to 14.2 (usually staying at 14.2 but occasionally dropping to 13.5 before returning up).

I have noticed something interesting however. If I quickly press the throttle a bit (by which I mean I press it very lightly, but quickly) the rpms obviously increase a bit before returning to idle but the UB voltage slowly drops to high 12s before returning up to 14 or so. However if I rev the car normally it stays around 14v. See the video attached. https://photos.app.goo.gl/GKkEBmmcYfwAuBx28

I've considered things like vacuum leaks, clogged PCV valves but other than this rather odd random dip the idle seems smooth enough (between 610-630 in park, and 570-590 in D or R). The is a very slight suction when taking the oil filler cap off (which I believe is normal due to the engine vacuum).

If anyone has any pointers, suggestions or more diagnostic approaches I could take please let me know.

Edit: I should also point out there are no misfire codes and also the misfire counter does not register any misfires at all on any cyclinders when this occurs.

 

[Deletedmember130310]

Air con pump kicking in ?

 

[pillow]

Thanks for the suggestion. I managed to rule that out by turning climate control on and off when idling and listening for the tell tale Air con pump sound. That had no impact on rpm. This also meant I could check whether it was the radiator fan being actuated but again that spun up without causing any notable change in rpm.

 

[pillow]

Well here is some data to look at. The first column is the time, second column is the Voltage (Control Module), third column is RPM, 4th column is fuel rail pressure, 5th is Throttle Position, 6th column is intake manifold pressure.

As you can see (particularly in the morning journey) that the car returns to idle when stopped and is nice and stable but the voltage plummets from 14+v down to 11.8v. You can feel a stumble but it seems from the data the RPM doesn't really drop (at least not long enough for the logger to pick up a drop)

The fuel rail pressure, intake manifold pressure and throttle position all seem to be stable when this happens so I don't think it's a dirty throttle-body, vacuum leak or bad fuel delivery.


This only happens once per journey when the car starts from cold. I have only ever noticed it on the first drive of the day and it doesn't seem to happen when the car is cold. Only about 10-15 minutes into a journey. This is perhaps the reason it is baffling me so much. Naturally no codes stored on any modules. Does anyone have any other ideas?

 

[pillow]

Just updating this post in case others have a similar issue.

The cause of the dip was a system self test of the secondary air system pump. The pump was actuating for about a second when stationary after a cold start and after 7 minutes of driving. I believe this is due to the OBD I/M readiness tests and will happen after every cold start until it is able to complete a full test cycle.

I couldn't be bothered to wait for this so I have disabled the secondary air system self test with DAS.