Mercedes Bends

[unixronin]

Now in the days of the Nude Hamster, cymrullewes and unixronin had two chariots, one of the making of Dodge, and one of the making of Mercedes.  And it came to pass that the Mercedes did illuminate its Check Engine light, and begin to run most unpleasantly rough; whereupon unixronin, lacking the mystical means to extract from the secret heart of the Mercedes its fault codes, took it unto the Mechanic of a local service station, saying unto him, "Lo, my chariot aileth.  Canst thou read out the fault codes for me?"

And the mechanic did read out the codes, and did poke and prod at the engine, and did observe real-time mixture and other mysteries, and did pronounce, "Lo, it is a vacuum leak.  It is causing the EGR valve to malfunction."  And he did swap out an eight-inch piece of vacuum tube, and call the problem solved.

But this cure did not sit well with unixronin, who believed it not, suspecting an ignition malfunction.  For on this day, he had come to believe through repeated observation that this Mechanic did, in fact, attribute all engine problems of which he knew not the cause to vacuum leaks; and further, unixronin did not believe that the nature of the misfiring was consistent with a vacuum leak, nor with an EGR valve problem; for it occurred only when the engine was both hot, and under load.  And lo, the Mercedes continued to misfire when hot and under load, and indeed the misfiring became more severe, and did begin to occur sooner after starting the chariot, and did begin to occur even at idle once the engine was hot.

But unixronin procured for himself an instrument for the reading out of the ODB-II data, and secured a manual on CD for the Mercedes; but the manual was not of use, containing no useful information, for it was not a manual of instruction on the servicing and assembly of the chariot, but rather one mostly prescribing repair and replacement guidelines.  And unixronin looked upon it, and found that it did inhale with great vigor, and was not useful.

Nevetheless, unixronin persevered, and did find the location of the ignition coils beneath a cover atop the engine, and at length did discover the location of the third bolt hidden beneath the intake trunk, and expose the ignition.  Then took he his xenon timing light, and used it to test the ignition and see unto which cylinders a spark was being delivered; but the Mercedes was obstinate, and would not misfire until unixronin took it out for a fifteen minute drive, during which period it began indeed to misfire most mightily.

Thereupon unixronin brought back the chariot, and reconnected his timing light, and observed that indeed, the ignition coil supplying spark to the second and third cylinders was producing less than half the number of sparks as the coil supplying the first and fourth cylinders.

Thus came he forth from the garage, brandishing his timing light and triumphantly proclaiming to one and all,

"Vacuum leak, my ass."

Comments

[dakiwiboid.livejournal.com]

And there was great rejoicing at the discomfiture of the mechanic!

[janetmiles.livejournal.com]

And unixronin looked upon it, and found that it did inhale with great vigor, and was not useful.

This is where I fell over giggling.

I'm glad you have located the problem, and hope that you are able to repair it with some dispatch.

[unixronin.livejournal.com]

Should be able to, yes. Now that I've confirmed it is a coil that's developed a temperature-related fault, as I suspected in the first place, it's a matter of swapping out the coil, which is probably 15 minutes' work and about $100 for the replacement coil. Then the Benz needs new tires, and it's good to go.

[danjite.livejournal.com]

My ass has never had a vacuum leak. Just the opposite, in fact.

Based on my previous experience of mechanics, perhaps tis' better to go to another shop for work after showing the nice incompetent man how incompetent he is?

[unixronin.livejournal.com]

Well, I was only going there to have him read out the trouble codes for me anyway. Now that I have an ODB-II diagnostic terminal, I won't need to go to him, I can just read out what I need myself.

[genuine-snark.livejournal.com]

However, unless the guy was a complete douche, it probably was throwing the EGR code, which may have meant an actual vacuum leak/EGR issue.

What'd you find when you dumped codes?

Nice work on the coil, btw. Misfires that only happen when the engine's hot are usually coils, as you guessed. Without the timing light, the easiest bet is to get a junkyard coil, and swap each one out until it no longer misfires. But you knew all that. :)

[unixronin.livejournal.com]

The only trouble code is misfire on 2. There's also some indication of lean mixture on 2 and 3 at idle, but I suspect that's an effect, not a cause; the ECU is reducing fuel delivery because of too much unburned fuel from the misfires. If the problem was due to a stuck EGR valve leaning out the mixture, I'd expect the misfires to be worst at idle, when airflow through the EGR valve is large compared to flow through the throttle butterflies andd MAF sensor; but that's not happening, it shows up first and worst under load.

[genuine-snark.livejournal.com]

*nod* I wonder if it was actually throwing an EGR code when he scanned it (and subsequently cleared it). Ultimately, it doesn't matter, but I'm trying to wrap my head around someone seeing "misfire" and saying "aha! EGR!"

[unixronin.livejournal.com]

Yeah, that never made sense to me in the first place. I could see an EGR malfunction messing with the idle mixture, but not causing misfiring under load.

Frankly, I still think EGR is a really stupid idea in the first place.

[ehintz.livejournal.com]

The word of the Lord. Thanks be to God.