CLK320 Rough idle - spark plugs diagnostic value?

[Dizz]

Hi I have 2004 V6 CLK320 which has a rough idle, sometimes it’s barely noticeable and othe times the car shakes (but never stalls), I usually slip it into neutral whilst sitting at the traffic lights as this helps smooth things out.
This started a few months ago so I had it serviced and had new spark plugs fitted but afterwards the problem was still the same.
I have had someone plug in a diagnostic today which showed no fault codes whatsoever but he was thinking it might be a coil pack related problem
He was able to watch the spark plugs on the screen and said that number one and number six cylinders were acting differently to the rest but wasn’t sure what the value should be (plus or minus) can anybody help me with this figure and what it should be if the car was running normally.
Thanks.

 

[clk320x]

Have you ruled out engine/gearbox mounts & 100% it’s a misfire/rough running engine?

 

[Dizz]

Have you ruled out engine/gearbox mounts & 100% it’s a misfire/rough running engine

I suggested engine mounts but the mechanic said he didn’t think it was that, is there an easy way to check?

 

[clk320x]

I suggested engine mounts but the mechanic said he didn’t think it was that, is there an easy way to check?

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[Dizz]

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Ok thanks for that video, but if the mounts were short wouldn’t it vibrate badly all the time?

 

[Dizz]

just did the same test and my engine doesn’t move like the other chaps did.

 

[grober]

take out no1 and 2 spark plugs and compare their appearance- check they are the correct heat range and gapped appropriately

 

[Dizz]

take out no1 and 2 spark plugs and compare their appearance- check they are the correct heat range and gapped appropriately

The problem occurred before the plugs were changed so I’m inclined to think it’s not the plugs, need to know what the plugs should be doing on the diagnostic screen, two plugs were showing -18 while the rest were around 0.0 and fluctuating +/- 3

 

[Tuercas viejas]

Conduct a smooth running test by graphing or looking at the numerical scales/wave form patterns on a scan tool engine both hot & cold running .
Second what are the O2 sensor doing upstream and what is STFT & LTFT figures ?.
Look at the rear O2's! (fairly static or bouncing about?)

Are they fairly static or bouncing around especially when you do a snap throttle test to WOT. ,and remember the rear O2's do the final fuel trim adjustments as well as doing the Cat effy monitoring with I/M240 E controls .
If you see a Long Term Trims (LTFT;s) in negative numbers, then the ECM is taking away fuel i.e. something is rich at idle either on both banks or maybe just one side .

Equally if the trim is in the +ve % age side then its adding fuel because of a weak mixture (vacuum leak perhaps) AND don't rule out the brake servo (booster having a hissy fit) if the vacuum tap off is on one bank and you have abnormal figures on that bank being a Vee engine.
This stuff is what makes me get out of bed in the morning!
Some have accused me of dabbling in the automobile black arts, like tuning a 4 Strombie carb'd vee 12 Jag .S
Tuercas viejas

 

[Dizz]

Conduct a smooth running test by graphing or looking at the numerical scales/wave form patterns on a scan tool engine both hot & cold running .
Second what are the O2 sensor doing upstream and what is STFT & LTFT figures ?.
Look at the rear O2's! (fairly static or bouncing about?)

Are they fairly static or bouncing around especially when you do a snap throttle test to WOT. ,and remember the rear O2's do the final fuel trim adjustments as well as doing the Cat effy monitoring with I/M240 E controls .
If you see a Long Term Trims (LTFT;s) in negative numbers, then the ECM is taking away fuel i.e. something is rich at idle either on both banks or maybe just one side .

Equally if the trim is in the +ve % age side then its adding fuel because of a weak mixture (vacuum leak perhaps) AND don't rule out the brake servo (booster having a hissy fit) if the vacuum tap off is on one bank and you have abnormal figures on that bank being a Vee engine.
This stuff is what makes me get out of bed in the morning!
Some have accused me of dabbling in the automobile black arts, like tuning a 4 Strombie carb'd vee 12 Jag .S
Tuercas viejas

Wow thanks for the great reply but that’s way beyond my understanding, think I’m going to need a mechanic who really understands what he’s seeing on the diagnostic screen and can work out what’s going on with this misfire.

 

[Jobsworth]

Having now been drawn to this thread I’d be inclined to look for a vacuum leak first I think.

 

[Dizz]

Having now been drawn to this thread I’d be inclined to look for a vacuum leak first I think.

Last year I had a broken bracket to the mass sensor causing misalignment and unmetered air, the result was very similar to what I have now with the rough idle, that was problem was repaired and they also replaced some vacuum hoses as they were not in good condition, the garage also resealed the rocker cover breathers.
What else could cause a vacuum leak?