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      Yesterday, 01:41 AM   #1
MidLifeM3
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Drives: 2008 E92 M3 6MT
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stumbling/bucking when cold - SOLVED

I recently solved an issue that I've had with my E92 M3 since I bought it six years ago. The solution was not obvious (in fact happened by accident) so I thought I'd share my experience in case someone else can benefit.

The issue I'd faced is that when cold, espeically for the first 5 minutes or so after startup in the morning, the engine would be prone to stumbling, and those engine stumbles would amplify and oscillate through the drivetrain to result in the whole car bucking. For someone with four decades of driving experience in manual transmissions it made me feel like a fool who couldn't modulate the clutch and accelerator. One mechanic dismissed it as, "oh, those M3's are known to be cold-blooded." The problem disappeared once the car was warm.

I spent years fruitlessly looking for a solution. (and of course, as all those in this forum are familiar, emptying my wallet dealing with the various ailments of these engines with their eye-wateringly expensive parts). I replaced throttle actuators. I replaced spark plugs. I had the injectors cleaned and blueprinted - was definitely needed and improved general running but did not solve the cold stumble/bucking problem. I thought it might be related to erroneous temperature sensor readings throwing off the cold fuel maps so I replaced the temp sensor and MAP sensors. I replaced all the rubber seals and rubber hoses in the intake system to ensure no vacuum leaks. The OBD did not report any misfire errors. I replaced the engine wiring harness - which solved a different problem but not the cold stumbling/bucking. I thought it might be due to worn out rubber bushings in the driveline allowing too much driveline wind-up and I replaced all the engine, transmission and differential mounts. Zero improvement.

I had almost concluded that the whole thing must be attributed to poor tuning of the drive-by-wire system that had a lag time unfortunately similar to the driveline's natural oscillation so that any irregularity in the torque delivery (due to an engine stumble or even a road irregularity) could result in oscillation as the engine control tried to over-compensate for the deviation. This theory may still have some truth to it, but I was able to luckily stumble upon a fix to this issue.

I recently had the serpentine belt idler pulley bearings go bad (as was easily detected by the banshee shrieking noise emanating from under the hood ... ). While replacing the idler pulley, I decided to also refresh the tensioner pulleys and tensioners. The tensioners themselves (the part that applies the spring pressure, which is a separate piece from the pulley) are hydraulic. My car is approaching 20 years old with over 100k miles and I guessed, correctly, that the hydraulic damping fluid was probably all gone by now.
After I installed new pulleys and tensioners and belts, the cold stumbling/bucking is completely gone!

At first I couldn't understand why replacing the pulleys could make the car run better. Now my hypothesis is that the old tensioners, without any remaining hydraulic oil to damp their action, were the root cause of amplifying any small engine stumble. These belts are of course connected directly to the crankshaft and the tensioner springs are pretty stout so maybe they could exert enough force as the belt winds up and unwinds to be able to perturb the cranskshaft rotation, especially during clutch takeup from a stop when the engine is running at lower rpm. With the new hydraulic tensioners being fully damped, now the tensioner can't oscillate and the bucking is completely gone.

The one roadblock to recommending everyone with these older cars go out and proactively replace all their pulleys and tensioners is of course the cost. As with so many parts on the ol' Bavarian Money Pit, doing my full set of replacements (idler pulley, 2x tensioner pulleys only available integrated on the tensioner arm, 2x hydraulic tensioners, adn a few plastic dust caps plus new belts) cost me an unreasonable sum of money compared to any other car. The tensioners themselves cost about $150 each and the tensioner pulley+arm cost about $200 each. (plus the cost of a new upper radiator plastic tube since I accidentally broke off the fragile little plastic nipple while removing that to get access to the belts. But that was the cost of my clumsiness.)


Hopefully my accidental discovery may help someone else struggling with the same symptom.
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