99CLK320lover

Question: How do I test the coil packs on a '99 CLK 320?

Problem: I have a '99 CLK 320 (M112 engine) with a miss at idle. It only misses at idle and there is no performance issues while driving. The miss causes the car to shake when stopped, once the vehicle has idled down to about 550-600rpm. It is always there and decreases with rpm.

Further Info: I have determined that the "b" spark plug for the front passengers side cylinder is not firing consistently at idle (I pulled the new plugs I installed and this one is black and smells like gas). The "a" plug for the same cylinder has a slight amount of carbon build up but the white part is still tanish in color. The rest of the plugs look nice and white.

When I bought the car I determined the "b" plug wire was not firmly connected and this caused arcing which deteriorated the insulation in the plug wire. I promptly installed a new wire and installed a new plug. Unfortunately the shaking at idle did not change in the slightest (I was sure this was the culprit). I also have checked the motor mounts and transmission mount.

I then checked to insure the plug fires at idle with a spark plug light. The quick blips of an orange light lead me to believe it is getting at least some spark at idle.

I have read many posts about people with idle problems. I think my problem is possibly the "b" part of the coil pack or in the wiring or ECU.

How do I check the coils? What resistance range should they have and what pins do I check? I can use a voltmeter. Please help.

mleskovar

You have six coil packs...measure the windings and compare. If you think you can follow the symptom by checking the plugs then swap coil packs and see if the problem moves to a different cylinder.

99CLK320lover

Thanks. I was planning on switching the packs, but I figured someone could tell me the MB manual way of checking the coils while I had the suspected one off. How do I check the windings? Do I compare the resistance from the wire terminal to a ground? Of the 3 wires, which one is what? I got .07 ohms to 1 ohm between the 3 prongs and 2.48Mohms from the block to the spark plug wire terminal. It is the same as on the other coils, so I figured there has got to be a right way of checking them.

mleskovar

A weak coil can give you the correct resistance readings but fail the high voltage (spark) test. If you're not getting a CEL and the failure is so consistent maybe it's a vacuum leak?

99CLK320lover

That is definitely one of the possibilities. I've switched the coils and am hoping that the fouled plug location switches as well. If not I'm guessing wiring for the "b" plug on the #1 cylinder is bad or something with the ECU. Does my car have a OVP?

mleskovar

OVP? I'm not familiar with the term and all I could find on search was an OVP relay is often replaced for problems on older model MBs. Yea, if that plug stays wet after a coil swap it's most likely the HT wire.

lkchris

While you're at it, check the plug wires.

99CLK320lover

I checked the plug wires when I replaced all 12 of the spark plugs. All had identical resistance and pasted a visual inspection except for the plug wire that had the insulation eaten away by arching which I replaced with a new one. I am really surprised that didn't solve my problem because I dug out a few ounces of green gunk and the part that contacts the end of the spark plug was pretty messed up. I'm guessing/hoping that the plug being disconnected for such a long time with the arching caused the coil to fail...

mleskovar

When you replaced the fouled HT wire did you measure the resistance of the new one? (grabbing for straws here :-) )

vettdvr

If the coil swap doesn't move the miss I would suggest a compression test wet and dry. This will determine if you have a leaky valve causing the miss at idle or rings. I know you don't want this but it is something you can check.

platt-deutsch

Fuel Injector ?

Ausmbtech

The ignition leads should be around 2000ohms (+-200ohms)
The ignition coils secondary circuit can't be measured because it had a diode in it.

The coils rarely fail, if they do then it's just no spark on one circuit, not a weak spark.

Most rough idles come down to collapsed engine mounts, old spark plugs (they dont have to be worn, anything over 5yrs is too long), failed air mass meters (MAF/AMM) and failed ignition leads.

Ignition leads are the hard ones to diagnose without an oscilliscpoe. Basicly you will get a spark but it's a very "dirty" spark and it causes the rough running to seem a bit random. Without an oscilliscope you can either buy 2 new leads and keep moving a good coil with the new lads around until it stops, get a mechanic with a scope to check them all for you or replace all of them.

astamir

i know its an old thread but having the same problem with my clk55 w208 model
any updates on this issue please?

tripper80

sames issue here with my 2013 c300 V6 3.5L. Intermittent misfire only on idle, plus i can smell the gas odour at exhaust. what weird is never get a CEL. I replaced all spark plugs with new (right model), no improvement.

no reason I change all 6 coils ,right?