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Discussion Starter · #1 ·
OK, so today I was going around a corner in my 99 slk 230 and all of a sudden my engine began to sputter a bit, like it was misfiring. The check engine light came on, and sure enough when I hooked it up to and OBDII scanner, I got a P0304 error code, misfire in cylinder 4. (is cyl 4 the one closest to the windshield or closer to the front bumper?)

Dealers were closed so I went to pep boys and got the closest thing to OEM they had, single prong autolites...not better than OEM I know, but I was hoping to get me through the weekend until my local dealer opens up again. Well, replaced them, still the exact same problem. The only other 2 codes I have are pending codes, not fault codes, one for p0300 (general misfire) and p0130 (o2 sensor, usually comes up with misfires). So I'm sure that the problem is solely the misfire in cyl 4. Since the plug didn't solve the problem, is my culprit likely just a bad wire?

(because if it were a bad coil pack, then I should have two misfiring cyls, right?)

Thanks so much everyone!
 

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Discussion Starter · #3 ·
By back you mean the one by the windshield? I know it sounds stupid, but depending if you are sitting in the driver's seat or under the hood, back can me either side...thanks!
 

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2001 E55 AMG
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sorry the phone rang.near the windscreen. front of engine is where the belts are.....pull the lead off and put a spare plug in,rest it so it touches metal (don't hold it!!!!) see if plug sparks when you start the car.
 

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Discussion Starter · #5 ·
By lead, I assume you mean to take the ignition coil off of #4 and rest it against something metal to see if there is spark?

(thanks so much for your help on this....with no dealers open now and the closest one being almost an hour away, your information is all I have to work with?
 

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Yes,is #4 the one directly under the pack? can't picture it at the mo....Use a spare spark plug inserted in to it and watch for a spark....If no spark you can swap the packs which should then move the miss to another cylinder. If it's the lead side swap the leads(wires) Chances are it's just the rubber boot that connects to the plug. Hope you get the jist of what I'm trying to say?
 

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Discussion Starter · #7 ·
that's a good idea to use a spare plug to test the spark....I'll try that tomorrow in addition to swapping the coils. I think I get what you're saying. This is the first car I have owned that has this kind of setup...so I'm still getting used to having only 2 wires and 2 coils for 4 cylinders. If I find that the coil is dead, do you think I can rest assured that is the problem?
 

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By doing what I suggested you are finding out if you have a coil problem...if the miss changes to another cylinder after swapping then it's either coil or rubber boot between coil and plug (plug cap).........If you have a spark on your first test with a spare plug then you may have to start looking further afield ,like the injector...if need be swap two injectors..again if the miss changes cylinders..faulty injector...but stay with making sure you are getting a spark to #4 plug first....like I said don't touch it gives off a right belt!......
 

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1998 SLK 230
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as above...

im guessing you have a pre face lift slk (< yr 2000). if so you have 1 coil pack for 2cyls. it uses a "wasted spark" system. post face lift got 1 pack per cyl for better emissions..

cyl 4 is closest to the windsheild, cyl 1 is at the front of engine where belts etc are. 10min job take coil packs out and check them for cracks etc and if they are covered in oil. common issue with the SLK is oil leaks around the seals there and it could have some issues with the coil pack. so no harm taking them out and looking at them. take the rubber boot off the bottom of the coil pack and check there also...

you can check the resistance of your leads/coil pack plug. take them off and get a multimeter. they should be around 2 Ohms. if higher, replace.

also swap the rear coil pack with the front one and see if the code moves to the front cyl. if so its coil pack...... does this happen under load or randomly (and cannot reproduce all the time?)
 

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The other week someone brought his car in after he had put new plugs in and new coil packs,and other bits and bobs.He couldn't understand why it was still missing,what he didn't do was change the rubbers between packs and plugs (why is a mystery) you could see brown marks on the ceramic on the plugs where it had been arcing,and the bottom of the rubbers had little rough bits where they had perished...........Worst thing is he took them off the old packs and put them on the new ones.
 

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Discussion Starter · #11 ·
I didn't even think to check the boot but that's a good point. I had a 3 series with a crack in the boot that caused me 2 weeks of diagnosing grief..I'm hoping it's just the boot then...they are relatively easy to come by, right?
 

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Discussion Starter · #12 ·
Well I followed your advice...I swapped the ignition coils to cyl 2, and the misfire went from 4 to 2. I swapped out the boots, but kept the bad coil on cyl 2, and the misfire was still cyl 2. I switched them again, (with the swapped boots too)and put the bad coil on cyl 4 with the boot from the coil on the working cylinder. And the problem went back to cyl 4. So it looks like you were right in that my coil is bad, and not giving any juice to the plug...

I'm running out to buy a coil now...hopefully that solves the problem. But why would the coil randomly go bad? I inspected the coils and boots for scorch marks or cracks, and nada....look perfect. Thanks so much for your advice....and hopefully in a few hours this will be behind me...
 

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Discussion Starter · #13 ·
Thanks for the sage advice and support you guys, I replaced the coil (and boot just in case), and everything works normal again!!! I never even thought to swap out the coils as simple as it was....and that suggestion ended up saving me a trip to the dealer tomorrow, a day off of work, and a quote of $680 to diagnose and repair at the dealer. Ended up being 103 for the coil, and 6 for the boot! 15 minutes after I got home, it was all put together and good to go!
 

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Glad to hear it's fixed....just shows how much the dealers make...this site is great for learning AND saving money:thumbsup:
 

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The other week someone brought his car in after he had put new plugs in and new coil packs,and other bits and bobs.He couldn't understand why it was still missing,what he didn't do was change the rubbers between packs and plugs (why is a mystery) you could see brown marks on the ceramic on the plugs where it had been arcing,and the bottom of the rubbers had little rough bits where they had perished...........Worst thing is he took them off the old packs and put them on the new ones.
I did the same thing, put new coil packs on the old wire set. Could this cause a coil to go bad? With the new coil packs, I had the p0304 and p0130 code, I switched the coil packs and the code went away. A few months later I just have the p0130 code.
 

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