1996 Mercedes E300 Diesel, belt vibrations, rocking tensioner

Hi all,

I have a problem that I am absolutely stuck on and I'm hoping someone smarter and more experienced than me can give me some guidance.

I've got excessive movement in the serpentine belt seemingly caused by excessive movement in the belt tensioner. Vehicle runs good but has some minor engine vibration at idle which seems fairly normal to me. The belt has a slight squeak and a vibration as it more or less flops up and down between the water pump and power steering pump.

My first thought was stretched belt, bad tensioner, or some excessive drag from an accessory. This is my personal vehicle, so I generally replace any possibilities as a matter of preventive maintenance. I replaced the belt, tensioner spring, tensioner shock, and the water pump. The water pump turned slightly rough but all other accessories are smooth. No help. Same issue.

My next therory was that perhaps a small amount of air was getting in the fuel and causing a slightly rough idle. I saw a small amount of air in the lines I replaced all the fuel lines and filters which did not fix the issue.

Other things that have been replaced recently: Engine mounts, fuel shut off, fuel pump, fuel thermostat.

I usually don't ask for help if I get stuck, but I have run out of ideas. Any thoughts, ideas, or suggestions would be highly appreciated.

I would be happy to answer any clarifying questions and/or provide video + slow motion video of the issue.

Thanks all.

Not much experience of these diesel but I would have a good look at the crankshaft pulley which I would assume has a rubber torsional vibration damper. I don't Mercbut BMW diesels of this era have regular issues with them causing rough idle.

If I were looking at a BMW common rail diesel I would want to run a diagnostic and look at the pids for RPM per cylinders and injector corrections, but I don't know what diagnostic tool options are available for diesel Mercedes of this vintage,

Andy,

Thank you for the response. I have an Autel DS708, but for some reason, I am unable to read any data pids from the engine module. Using the 38 pin diagnostic connector, I can scan for codes and see ECU indent info, but I have no other functionality. I can see all other modules and have bi-directional functionality. Using OBD2, I can see about 8 engine Pids none of which seem to be very helpful.

I'll do some research and see if I can find someone with a similar problem due to crank pulley. I took a slow motion video of the crank pulley and saw nothing out of the ordinary.

Thank you for your response.

It is one of those situations where one make forums can be helpful for your drive belt issue just give any advice a sanity test.

Any gradual increase in engine roughness at idle as the mileage has built up is going to be down to either low compression or injector.

Just an update, the problem has been solved. It turned out to be a faulty new tensioner shock. The shock was off brand. A new Mercedes brand shock fixed it.