PeachParts Mercedes-Benz Forum - View Single Post - Help needed: 1990 350SDL quiet on cold strart, but loud engine noise after

Maxbumpo · #6 10-16-2015, 11:11 AM

dino67, start simple/cheap/easy before getting expensive/complicated.

I'm pretty sure your car has the "after-glow" feature, which energizes the glow plugs immediately after the engine starts, and keeps them on for a short period of time according to engine temperature (the colder the engine, the longer they stay on). The noise when quiet (while the after-glow is working) is what the engine should sound like normally once at full operating temperature.

As already stated by others, at 210k miles the injectors are likely tired and ready for service. When the glow plugs are on (about 1200 deg F), the injected fuel is burning nicely. But if the injectors are tired and not spraying a good pattern, the fuel is not igniting cleanly hence the "nailing" sound that you describe.

If you have the right tools and some experience working on cars etc, removing and replacing the injectors is not too hard, but you will need a torque wrench and the factory service manual. The intake manifold needs to come off, which means there are two gaskets that need to be replaced, and you will also need new fuel return hose and new heat shields to install under the "new" injectors. While you have the intake manifold off, this would be a great time to replace the glow plugs (if one or more fails in the next 100k miles, the intake manifold may need to come off to replace it). Only install Bosch glow plugs.

If you can let the car sit for a week or so, you can remove the injectors, mail them to Greazzer (list member) and he will clean / test / set the opening pressures, and then mail them back to you.

If you can't afford the down-time, buy some Bosch rebuilt injectors or get a set from a junkyard, install the new or send the old to Greazzer (or send the new the Greazzer for test / set), and then install a set that are tested and the pressures properly set. The injectors need to be balanced (opening pressures set to within a 5 bar range) to get a smooth idle.