AMGtech's Crazy Diagnosis: Engine Porn Edition

Now with more blue balls! Why blue balls you ask? Because it’s not a very exciting engine, not even an AMG. In fact it’s a diesel. With a whopping four explodey-holes. Although, this being oppo, you might prefer that to a raucous V8. This particular OM651 engine (2.1 liter) came in with a simple chirping noise from the serpentine belt. No big deal, right? RIGHT?! Boy was I wrong...

Backside of an upside down engine

Simple chirp. Let’s take the belt off and see if the pants fall down. Wait... No... See if the chirp goes away. So off the belt came. But the chirp remained the same. Fine. This particular engine has a crankshaft pulley with a one way roller/sprag clutch built into it. Similarly, the OM642 diesel engine has the same style set up on its alternator pulley, and those sometimes make noise. You may also remember from my last crazy diagnosis post that a crankshaft pulley sort of burned me before. I was not going to let that happen again. It’s also important to note that the front crank seal rides against the crankshaft itself, not the pulley as many engines are set up. So off comes the crank pulley. Fire her up. No dice. Chirp still there. What the hell?

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Next step is to pull out the old ticker-listener, the trusty doctor’s tool, the handy dandy stethoscope! Time to pinpoint this noise and gather some basic information. First slowly rev the engine to see if the noise is rpm-dependent. Which it is, however it goes away above 1200rpm, or at least can no longer be heard over the sound of exploding dinosaur blood. Then pinpoint the noise to a specific location if possible. So a few of us took turns listening to various areas, top to bottom, side to side, front to back, since noises can easily be interpreted differently from one person to the next. Diagnosis by consensus FTW amirite? After messing around with that for what seemed like far too long we all agreed we could only hear the chirp from the front lower section of the engine, but couldn’t decide what the actual faulty component was.

So, logically, the next easiest step was to remove the front crank seal and run the engine as briefly as possible that way. Done. Noise still present. Believe it or not I had actually seen this exact problem, a squeaky crank seal, on a CLA a few months prior. Looks like it’s going to be an internal engine noise (like you didn’t see that coming). Time to get Mercedes involved, they need to be aware of this. Right now.

Anywho... Capture a sound file, take a few pictures, let them know what we’ve done so far, then wait for them to get back to us with what they want us to do next. We’ve already decided the engine needs to come out and get torn down, but that’s a drastic step and there’s always the possibility they know something we don’t, maybe they’ve seen this before. Finally they get back to us. They’ve never heard of something like this on this engine so they ask us to remove the front cover. The front cover on this engine is quite small because all of the timing components are on the back of the engine, which is another thing that makes this noise so strange, there’s almost nothing on the front that can cause any noise because there really isn’t anything there except for a main bearing. Maybe that’s it?

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I’m a master decorator of metal cakes! Pretty black icing!

Pictured above is the front cover just before final reassembly. The black stripe is sealant, like RTV, but Mercedes own stuff and it’s seriously a gazillion times better than anything else I’ve ever used. That stuff is how the majority of components of the majority of our engines have been sealed for the last decade and it usually only leaks when someone messed up the application process while working on it. The front crank seal is pressed into the round bore and is maybe just over three inches in diameter.

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So enough about sniffing glue. Front cover is off. Can’t see shit aside from one edge on the #1 main bearing. But I can tell it hasn’t spun, so that’s good. Can’t even see into the oil pan. That was a useless step, as it turned out. Oh well, had to be done.

Next they wanted me to detach the transmission from the engine just to make sure the noise was in fact in the engine and not just transferring through from somewhere else. Ugh. This was going to suck. Keep in mind, the transmission needed to remain connected electrically for the engine to start because it’s control unit, which is inside of the trans, takes part in the security “handshake” when attempting to start the vehicle. So I do that, very reluctantly I might add, with the starter very loosely mounted in a very nerve-wracking way because that’s the only way it would work.

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BLANG BAM CRASH BLANG DING BING LOUD NOISES SPARKS FLYING FUCK ME WE’RE ALL GOING TO DIE KILL THE ENGINE MEOW!

Shit! Apparently the engine and trans were just off-center enough for the flywheel bolts to just barely touch the snout of the torque converter while rotating. Line it all up and hold everything in place with some scrap wood. Look it over, no damage, whew. Let’s try this again. Luckily this time no crazy noises. Oh, except for that chirp. I’m shocked. If you could see the look on my face you would know just how shocked I am. Guys, I’m shocked alright, just believe me. Really.

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Finally after song and rain dance, the German gods bless the engine removal and tear down.

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This particular engine cannot be mounted on the rear as you would with most engines because that is actually the timing cover. So you have to remove a ton of components, like the turbos. Dos turbos to be exact. All from the sides and use a special engine stand to mount to the sides of the block.

The turbos in question
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Yes, there are two there, cute little things aren’t they?
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That big plate in the top right, next to the lift post, is the timing case cover from the back of the engine block. One cool thing about this engine is the timing set up. As you may have noticed, it uses a lot of gears rather than chains or belts. It does have one chain, but it’s short for an overhead cam engine and only goes from the injection pump to the camshafts.

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So many things had to come off I started running out of room to store them all.

Now back to your regularly scheduled programming. If I remember right from high school English class, this is where the orgasm kicks in. Or do they call it the climax? Whatever, same difference.

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So engine is coming apart, oil pan off, timing cover off, valve cover off, windage trays off. Everything that matters is visible. Everything looks perfect. Rotate the engine by hand, but even with the injectors out and no compression building I can’t generate nearly enough speed to duplicate the noise. Damn it. Better keep taking things apart for a closer look. Here is where a little product-specific knowledge comes into play. This particular engine uses what is referred to as a Lanchester balancer to smooth out engine vibrations. It’s basically two counter-rotating balance shafts mounted near the crankshaft, actually just below and off to either side. These shafts run the length of the engine and have three roller bearings each; center and both ends. I had been suspecting one of these bearings for quite some time already, but as you can see they’re not very easy to access.

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You can clearly see one of those shafts in this last picture. These shafts and their bearings are actually built into the main bearing cap assembly, which is all one big unit. Didn’t take a picture of that though, sorry, it by itself is classified and I’m not Edward Snowden. Or I forgot. You decide.

Unbolt and remove said main cap assembly, referred to as the Lanchester balancer assembly by ze Germans. Now That those shafts aren’t meshed with anything else they will spin freely. Rotating them over by hand I don’t feel anything wrong at all and there doesn’t seem to be any excessive play. Luckily the drive gears are bolted to these shafts. So I take my cordless, slap on the appropriate socket, and give ‘er hell. Sure enough, the driver’s side front roller bearing emits the exact chirp noise we’d been chasing all along.

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Press out the old, replace all six bearings, both shafts, and both gears. Press the new ones in. Slap it all back together. And done. Noise gone. All that for a stupid little noise. But to be fair, these engines have been exceedingly reliable since they were released here in the US and no one that I’ve heard of has ever had to replace one or do more than replace fuel system components because it was filled with gas.

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