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Current 2017 GLE350, 2007 S550, 2002 S430, 1998 ML320 Deceased 74 240D, 92 400E, 97 E420, 13 GLK350
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1,915 Posts
Discussion Starter #1
Thanks to information on the forum, felt confident about troubleshooting an engine misfire. Could not find all the information in one thread for my 'orphan' engine type, so I thought I would post results with key words for future searches.

Had a rough running engine and CEL on my 1997 E420. The Check Engine codes were P0300 (random cylinder misfire) and P0307 (Cylinder 7 misfire). The misfire caused the engine to shake like it had nine cups of espresso.

Haven’t changed motor mounts yet, which probably added to the engine vibration. Like any good DIY’er, cleared the codes in hopes that all would be forgiven. Then, the engine vibration smoothed out after restart! Too easy – took the car for a test drive and CEL / vibration back within a mile.

The E420 model has the M119 eight cylinder engine. Since I last changed the plugs (Bosch super) at 111,000 miles, and the car now has 144,000 miles, I knew that I was at least due for a spark plug change out.

In pulling the spark plug gallery cover, noticed that a colony of ants had set up shop, and were mostly congregated around the coil for cylinder 7. Of course they were all fried. Looks like they got in through a tiny crevice for the gallery cover. Inspection of the wires did not indicate any insulation chew through, so I lucked out on the ant colony. Out comes the vacuum and away goes the colony.

Removal of the coil from the plug was too easy. Turns out the spark plug boot had degraded over time and lost its elasticity. Inspection of the plug indicated a larger than expected gap, and carbon on the electrode. There was also brown ring of discoloration at the bottom of the plug’s insulation. (This discoloration was present on all other spark plugs - see photo with cylinder number 2 spark plug boot pulled back.)

I should have switched the coil from 7 to another cylinder to verify the check engine fault followed that coil, but based on forum history simply ordered a new one from AutohausAZ for $71. Also got 8 new Bosch (Super Plus) and 8 spark plug boots. Two day shipping was 11 bucks, for a total of $212. The Bosch plug box stated 21 ft-lbs to torque the plugs, I also used motor oil to lightly oil the threads and gasket seating surface.

Cleared the CEL, changed the plug, boot and coil for cylinder 7, and the engine ran smooth. Finished changing the plug and boot for the remaining cylinders, and all was good with the world. :D

Merry Christmas
 

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97' E 320 //////02' C240
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88 Posts
Thanks for the write up mine is an inline 6 but I have the same symptoms I am gonna check it out!
 

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1999 E300 TD
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2,045 Posts
Great job, but just another reason to own a DIESEL------they love ants, bees, wasps, squirrels and snakes. No major under hood stuff, except keeping the sea gull S off the roof!!
 
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