Finally...Combining TWO 1985 300D's - PeachParts Mercedes-Benz Forum

Diseasel300 · #11 04-24-2018, 09:14 PM

Timing chain stretch causes the cam timing and injection timing to lag the crankshaft. At first it causes no real noticeable problems, but as the stretch increases, the retardation of timing begins to show up as a reduction in power, white/grey smoke especially at idle, and elevated operating temperature due to the late injection timing. If the chain is REALLY worn out, you can sometimes hear it slapping inside the timing cover. Definitely a situation you want to avoid! If allowed to go long enough and assuming the chain doesn't break first, you'll eventually have the pistons hit the valves with catastrophic results. Not to mention the chewed up sprockets in the timing pathway.

The easiest way to check chain stretch is to pop the valve cover off, line up the timing pointer on TDC, then turn the crank until the timing marks align on the camshaft and cam tower then measure your "stretch" in degrees on the crank damper. If you're >5˚, you should consider an offset key and budget for a chain replacement. If you're >7˚ or if an offset key is already installed, strongly consider a chain replacement in the very near future. If you're >10˚ stop driving the car until you get it repaired.

While you have the cover off, grab the chain on the cam sprocket and see if you can lift it off the sprocket. If you can, the tensioner isn't working properly or the chain is very seriously stretched.

For occasional lumpiness/bucking I'd be looking squarely at accelerator linkage rods being out of adjustment and the rubber fuel lines under the hood. All of them. If they're old, or if you haven't personally replaced them, use some 7mm fuel line and NEW fuel injection style clamps and replace the lot. If you have the old style "white handle" primer pump on the lift pump 86 that and get the updated one-piece replacement. Leaky hoses or leaking primer handle will let air into the fuel system and cause all sorts of unrefinement.