Tuesday, July 5, 2016

Balance shaft failure on Mercedes V6 engines

Your check engine light comes on, but the engine in your car seems to run fine.  What’s the worst it can be, you ask?  That’s never a good question, because the worst it can be, is usually pretty bad.  Perhaps a new engine, or even a new car.

In this blog I'd like to look at the dread P1200 and P1208 codes, and what they mean.  

Mercedes has a long history of building reliable and long-lived engines.  That’s why it came as such a surprise, when we found engine lights coming on from internal wear.  This is what Mercedes technicians refer to as “the balance shaft problem.”



 The balance shaft is a weighted shaft Mercedes installed in V6 engines to improve smoothness.  The shaft is driven by the timing chain, which also turns the camshafts in precise relation to the crankshaft rotation.  Therein lies the problem. 

The timing chain and sprockets are like a bicycle chain and gears.  As long as the chain stays in place, the gears all rotate in unison and timing integrity should be maintained.  Unfortunately, that hasn’t happened.  The Mercedes timing chains stretch with age, so a chain that started out 36 inches long in 2005 might be 36.5 inches long today.  The chain sprockets wear too, and when the chain sits deeper on the sprocket the sprocket effectively shrinks in size, which has the same effect as stretching the chain.

In both cases, the cam timing becomes retarded, and that ultimately sets the 1200 and 1208 codes.  That sort of thing always happened with timing chains but it wasn’t a problem on old time motors because the timing relationship between crank and cam didn’t need to be so precise.  Older engines didn't generate cam retard faults because they didn't monitor cam timing. That’s all changed as auto engineers push to get every bit of power from ever-lighter and smaller engines.  One of the ways they do that is by varying the cam timing with adjuster mechanisms on the camshafts.  The adjusters can compensate for some wear, but when the wear moves beyond the compensation limit, a check engine light appears.

In extreme cases, the engine will actually begin to skip, or lose power.  If you live in a state with emission test (most of the USA) the car will no longer pass the annual smog test. 

We often see these cars after their owners have visited the dealer and been stunned by a $6-9,000 repair estimate.  They can’t believe a car that runs so well (all that’s wrong is that light!) needs such an expensive repair.  And they don’t understand how that could happen, on a high-end car like theirs.

Two explanations have emerged for how it happens.  The first is that the alloys used in the gears were too soft, and they wore out prematurely.  The second explanation says there had to be a lubrication failure, for any metal parts to wear out.  That puts some or most of the blame on engine oil – either the wrong oil was used in the motor, or it wasn’t changed often enough.

Mercedes says they’ve addressed the “soft gears” with new improved parts.  They sell all the internal parts you need for the job in a single kit. They feel that is a permanent repair, and I hope they are right. Even so, we encourage owners to minimize the lubrication risk by changing oil at 7,500 miles (not the 10,000+ miles Mercedes originally suggested) and insisting on the correctly rated oil, like Mobil 1 0-40.  But that only helps once the engine is fixed.  And that’s a project.

Balance shaft replacement requires engine removal and extensive disassembly. These photos show a typical Mercedes sedan with engine removed, and then the progressive teardown of the engine to reach the affected parts.  This is a week-long job in most cases.





To do this repair the engine is set on a stand.  The lower oil pan is removed for access, and then the front end rear covers come off the engine.  At that point the timing chains are exposed and the balance shaft (which runs through the motor, front to back) can be removed.





The photo below shows a new balance shaft being inserted into the block.





When the engine is pulled from the car we encourage owners to look at all the ancillary parts, as this is the time to replace them pro-actively.  Items to check would be belt idlers, motor mounts, water pump, and others items that are prone to wearing out. Also look at maintenance – are the plugs and air filters fresh?

When the engine goes back in the car it should get fresh Mercedes coolant and fresh synthetic oil.  While some shops re-use old fluids those services are part of every repair at Robison Service. 

There’s one more possible complication I should mention.  That is the possibility of sludge.  If the engine’s service was badly neglected there may be sludge in the motor, and there’s no good way to clean that out short of complete overhaul (a very costly job).  That is a serious problem, and if found, we generally suggest engine replacement.

(c) 2016 John Elder Robison
John Elder Robison is the general manager of J E Robison Service Company, celebrating 30 years of independent Mercedes-Benzrestoration and repair in Springfield, Massachusetts.  John is a longtime technical consultant to the Mercedes clubs, and he’s owned and restored many fine German motorcars.  Find him online at www.robisonservice.com or in the real world at 413-785-1665

Reading this article will make you smarter, especially when it comes to car stuff.  So it's good for you.  But don't take that too far - printing and eating it will probably make you sick.


1 comment:

HarvVAG said...

Why would owners even bother to do the repair on a W203? They are basically throw away cars at this point.