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Registered 2000 SLK200
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39 Posts
Discussion Starter #1
Doing a diagnostic check today and found on live data that I have a missfire recording, the results are, cyl 1 =0, cyl 2=1, cyl 3=1 and cyl 4=34 but increasing, by the time I stopped live data the reading for cyl 4 had a reading of 43, increasing the revs didn't seem to increase the speed of the missfire count but I could feel a slight drop of revs on idle and the count increased accordingly.
I did fit new plugs when I got the car last year and they would have done only about 3k miles so I suspect the coil on cyl 4 is starting to give trouble, would this missfire count not have generated a engine fault code
IMG_20201015_163713.jpg
 

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Super Moderator (UK)
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Not an expert, but often see them suggesting to swap the coils over and see if misfire moves.
If it does, you have your answer.
 

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Registered 2000 SLK200
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Discussion Starter #3
Thanks, I'll have a go at that, car seems to run OK so not too worried about it for the time being but can't leave it too long. I'm a bit surprised it didn't register a fault code for the missfire
 

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Premium Member 2002 R170 SLK V8 5.0
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Your Misfire counter suggests Plug / Lead Coil number 4, it won't register a Misfire unless it does something like 10 plus per 5 seconds or some-such, that is where, as you have done, live Data comes in handy ....................

Good Diagnosis so far ;)
 

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Registered 2000 SLK200
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39 Posts
Discussion Starter #5
A bit more on this, I'd say someone has been doing some investigation on this already as number 4 coil has been marked
IMG_20201016_115058.jpg

Next was to move 4 to 1and recheck the live data which resulted in no missfire on any cylinder, then with the diagnostic still connected I drove it the 2 miles around the block to make sure it was fully warmed up, it still didn't register any miss, then things became a bit strange, with the car stopped in Drive and the brake pressed no miss was being registered But when I put it in Park then the miss counter started, it registered over 80 misses in a couple of minutes on cyl 4 still. Being a bit confused by it at this stage I restarted the live data again, again it started to register the miss so I drove it again, no matter how I drove it including wide open throttle it didn't register any increase in the miss counter until the car was put in Park which then saw a steady increase in the miss count on cyl 4
Confused as to why the miss only registers in idle in Park, diagnostics still measures other things while driving so I don't think it's that
 

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Super Moderator (UK)
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Well, it stayed in the same place, so not coil methinks.
Are the leads original? They'd be 20 years old.


Just a note on plugs.
We had an incidence of new plugs failing (cracks in the ceramic I think).
Maybe inspect and swap 4 with A.N.Other if it looks good.
 

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Registered 2000 SLK200
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39 Posts
Discussion Starter #7
Thanks, no traditional plug leads on this just coil on plug, as you suggested, checking and moving the plug will be next. Any idea why the miss is only registered when the car is in Park
 

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Not a clue. Maybe some idle switch kicks in.
@Dave2302 Thoughts? I know you're not a 4 potter, but theory should be the same?
 

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Premium Member 2002 R170 SLK V8 5.0
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3,793 Posts
@Dave2302 Thoughts? I know you're not a 4 potter, but theory should be the same?
Now you know what "should be" is the mother of dontcha :ROFLMAO:
V6 wise to V8 wise, kind of ...............
4cyl wise with COP's and 4 Cyl with 2 Coils and 4 HT leads are totally different Wiring and s/w in ECU's ;)

What does it do Misfire Counter wise in Neutral and Reverse ?

Is it actually Misfiring ?

Do you get a Code or CEL ?

What drew your attention to this ?
 

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Registered 2000 SLK200
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39 Posts
Discussion Starter #10
What drew my attention to it is probably too much time on my hands, I wouldn't have any reason to check it only I was doing the diagnostics for the lateral sensor and just continued on with the diagnostic on live data.
No check light or fault code but there is a slight stutter on idle and this coincides with an increase in the miss count, I'll swap around the plugs and try it again
 

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Premium Member 2002 R170 SLK V8 5.0
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3,793 Posts
Yep, got ya, let me know, I'll look in later today, please check what it's doing in N and R as well, I might have a theory as your car is Auto ;)
 

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Premium Member 2002 R170 SLK V8 5.0
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I'm getting worried now, as I've just sat down for lunch (15:45 hrs) and I've had a lightbulb moment which could be an expensive one for your Car John ........................

First up, I'm no gambler but I'm gonna bet that the misfire counter goes up in N & P but not in D & R ...................

We'll await the results of Johns testing and if that is the case I will go further and get into ECU Strategy :) and Cylinder Compressions :(

We know someone has had a go at this before, I see you've had the Car about a year, so might not be that bad ......... yet ;)
 
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Registered 2000 SLK200
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39 Posts
Discussion Starter #13
If you were a gambler you'd be a winner.
Missfire count increases on cyl 4 on idle when in Park or neutral but not in drive or reverse, I'll attach all the screenshots of the engine live data, this is with the original cyl 4 coil moved to cyl 1 and cyl 4 plug moved to cyl 2
IMG_20201016_151235.jpg
IMG_20201016_151142.jpg
IMG_20201016_150951.jpg
IMG_20201016_150944.jpg
IMG_20201016_150938.jpg
IMG_20201016_150929.jpg
IMG_20201016_150922.jpg
IMG_20201016_150915.jpg

More to follow
 

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Premium Member 2002 R170 SLK V8 5.0
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3,793 Posts
OK John, sorry I'm just off in but in the morning (not too early mind as Sat is lay in day), I will explain what I think and why, in the meantime can you get hold of a Compression Tester and do a Compression Test on all cylinders, I think you will find number 4 is lower than the rest :cry:

I hope not, but that's my gut feeling on this :(
 

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Registered 2000 SLK200
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39 Posts
Discussion Starter #16
Thanks, there's absolutely no hurry, car has only done 1000 miles since March, I've been thinking about getting a compression tester anyway so this might speed me up a bit getting one.
The car is driving fine but it does give a bit of a stutter at times while on tick over which I'm now hoping is no more than a clogged injector as the electrical side seems ok
 

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Premium Member 2002 R170 SLK V8 5.0
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3,793 Posts
Hi John,

Misfire counters on MB can be a real PITA, bain of my life at times, like a Diesel CDI reporting low Fuel Rail Pressure, can be absolutely bucket loads of different issues :(
That said yours is a bit more unique with this only in P & N malarky ;)

After the Compression test showing OK that would be my next move to swap #4 Injector to a different Cylinder ;)

OK, here is a theory if the Compression is down a bit .................

Anyone remember cars with Carbs and Distributor Ignitions ? ........................
Back in the day many Engines had a slightly low Compression, and they would miss a little on Idle but pull ok when driving ..................
With no fancy smancy electronicamabobs we would simply turn the little idle screw on the Carb and up the idle speed a tad, misfire gone (for a while) rinse and repeat :)

Sadly we can't do that with Engine management systems like these.

The ME ECU has a piece of it's thousands of strategy lines that when you select D and R it bumps up the Idle RPM to overcome the extra load that the Torque Converter now puts on the Engine, when the Trans has it's Reverse or Forward Clutche(s) engaged ................

That does several things ..................

It now gives the Engine a little more Air and Fuel, and advances the Ignition Timing slightly, it also increases the load on the Engine and the Cylinder Pressures rise as there is now a bigger BANG inside the Cylinders .............

So the ECU's monitor s/w detects a Misfire from measuring the Ionic Current drawn by each Spark Plug via its Coil and counts them, if it counts a certain number of misfires in a certain time period it will shut down that Cylinder to protect the TWC and pop an actual Fault Code, and MIL.

Yours is obviously not quite at that point yet, but it is detecting Misfires, which is your advanced warning together with the little stumbles on Idle.

If you eliminate the Plug and Coil, you are left with the following issues .........................

Injector, cant see how P & N would affect that (unless it is over fuelling a touch on unloaded idle) still worth swapping for elimination purposes as its easier, but do the Compression Test first as you don't want to waste your time and efforts flogging a dying horse (cylinder)

Air Leak, worth a shot seeing if there is a Vac Pipe, Breather or a Manifold Gasket leak near to the Inlet Port of Cyl 4, but usually an Air Leak would also show a few Misfires on other Cylinders, less misfires the further they are from the leak.

Low Compression on that Cylinder, which makes it harder to fire, and also, if it was Rings / Piston or Bore wear issue, that slight increase in load in D & R would help the upper Piston Rings to seal as there is more Cylinder Pressure pushing them outwards and thus they seal better .......................

Look for any signs of Oil on #4 Spark Plug, but I'm not really expecting that ;)

A note for owners of other SLK's with 4 cylinder Motor that only has 2 "Siamesed" Coils, rather than COP's , some or all this may well not apply to your Engines, I've not researched it, but the COP's 4 Cylinder Engines are similar in this strategy to the M112 V6 and the M113 V8's management wise and thus I have half a clue with this ;)

Disclaimer .................... Of course it has been known that I might be totally wrong 🤣 ....................

But right now, and in the absence of a more knowledgeable answer I'd go with the above tests and procedures to eliminate a bunch of stuff ;)

HTH :)
 

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Registered 2000 SLK200
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39 Posts
Discussion Starter #18
Thanks for all your help, I'll have to wait till next week to get the compression tester, driving it today it runs OK but if I put it in Park at traffic lights then the stutter appears but if I keep it in drive with the brake pressed then there's no stutter.
I will try doing a vacuum test, then if that doesn't find anything I'll move on to the injectors, a thought in mind though is that this miss wasn't noticed until the last few days, I'm thinking that it may be the petrol that I filled up with around a week ago
This car has a pretty low value here so I don't want to spend lots of money on it but at the same time it was bought for use as an autumn / winter convertible while my mx5 gets used during spring/summer so I'd like to be able to use it without the fear of major bills.
Thinking of major bills, while going through the previous owners receipts looking for any parts that may have been replaced I found a bill from a main dealer, I didn't think the 2.0 kompressor takes 7 litres of engine oil at a cost of €109.90 + vat
Again, Dave, thanks very much for your help
 

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Premium Member 2002 R170 SLK V8 5.0
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I'm not sure about the 4 Cylinders, but most MB's take 7 litres plus, some of the CDI's take 11 lt o_O

I don't know what the Euro is doing vs £ but IIRC last MB that had 7 litres here in my workshop would sell out for circa £80.00 plus the Filter ;)

You could try @jbanks15 's fix, which believe it or not does work more so when the issue is Fuelling related ......................

Bung a bottle or 2 of Techron or comparable Fuel System Cleaner through it ;)

BTW, I'd use the SLK in the Summer, and reserve an MX 5 for winter use, well actually I'd sell the MX 5 and get another SLK for winter use 😈
 

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Super Moderator (UK)
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Hunting the web looks to be 5.5 litres for 200 in 2000.

1.5L seems small for a flush, but may have charged from a standard list.
 
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