1984 300D- Rough Idle - PeachParts Mercedes-Benz Forum

Noah300D · #1 04-29-2021, 03:15 PM

Hello everyone, I am new to the Mercedes diesel world. I am familiar with mechanical diesel, but this car is making me scratch my head a bit. I've owned my 1984 300D for about 2 months now, and its had an off and on rough idle/reduced power. So far, I've cleaned the fuel tank, replaced the strainer in the fuel tank, new rubber fuel lines, rebuilt lift pump and fuel filters. I have also lashed the valves back to factory specs. The car has had a bit of a fuel cleanliness problem- was very dark in color and murky when I first drained the fuel tank. Knowing this, I've added some biocide to the tank, and have been changing the filters out pretty frequently. Usually, changing the filters out is enough to get it running great again. However, this last time hasn't seemed to make much difference. The engine is shaking like a paint shaker, and is much louder than it was a week or so ago. She has 158,000 miles on her, and doesn't smoke at all- unless its very cold out. Any help or guidance would be great! Thanks in advance.

Maxbumpo · #2 04-29-2021, 03:28 PM

How are the engine mounts? Collapsed engine mounts can increase the perceived noise / vibration immensely.

Well you've done all the easy things I think, and is the problem still intermittent?

If this is a steady problem, I'd say you should get the injectors checked next, and after that would be a rebuild of the delivery valves on the injection pump.

Loss of power can also happen if the ALDA pressure feed from the intake manifold is interrupted. This can be caused by plugged banjo fitting on the intake manifold, overload protection valve plugged up, or leaking pressure line.

Diesel911 · #3 04-29-2021, 03:36 PM

Quote:

Originally Posted by Noah300D

Hello everyone, I am new to the Mercedes diesel world. I am familiar with mechanical diesel, but this car is making me scratch my head a bit. I've owned my 1984 300D for about 2 months now, and its had an off and on rough idle/reduced power. So far, I've cleaned the fuel tank, replaced the strainer in the fuel tank, new rubber fuel lines, rebuilt lift pump and fuel filters. I have also lashed the valves back to factory specs. The car has had a bit of a fuel cleanliness problem- was very dark in color and murky when I first drained the fuel tank. Knowing this, I've added some biocide to the tank, and have been changing the filters out pretty frequently. Usually, changing the filters out is enough to get it running great again. However, this last time hasn't seemed to make much difference. The engine is shaking like a paint shaker, and is much louder than it was a week or so ago. She has 158,000 miles on her, and doesn't smoke at all- unless its very cold out. Any help or guidance would be great! Thanks in advance.

Because of the highlighted in read it sounds like it is fuel related.

You need to have 1/2 tank of fuel to do this. Where the Fuel Lines are at on top of the fender well swap the positions of the Fuel Hoses. That bypasses your Fuel Tank Screen. If it improves your Fuel Tank Screen is restricted.

If there is no improvement don't remove the hoses but change the Filters. If that works then it could still be your tank screen is plugged because changing the position of the hoses bypasses the Fuel Tank Screen and draws Fuel from a different position in the Fuel tank.

Note this is for testing only.

The throttle linkage pivot on the Fire Wall often have plastic parts behind the bracket that fall apart or rubber parts the deteriorate.

Noah300D · #4 04-29-2021, 03:48 PM

Wow! Thanks for the quick response guys. The engine mounts themselves are alright, but I do know that my engine shock bushings are bad on the drivers side of the engine- they disintegrated when I was replacing the oil cooler lines. I will switch the return and feed lines and see if that makes any difference in the idle quality/power. The problem is pretty steady for the past week- banjo bolt on the intake is relatively clean, usually makes good power (enough to hit 100mph without too much struggle) up until recently. I just changed the pre-filter and main filter on Tuesday, so I don't think that's it. Oddly, it idles better when I first crank it up in the morning. Thanks for the help!

Noah300D · #5 04-29-2021, 04:27 PM

Wow, thanks for the quick response guys! The problem has been intermittent until a few days ago. Started running rough again, so I changed the filters (Tuesday). When the problem happens, the car idles rougher, trans shifts less smoothly and less power overall. Motor mounts are in ok condition, however my engine shock bushings are shot on the drivers side from when I replaced the oil cooler lines (replacing soon). I just went outside and switched the supply and return lines. Did not see any difference, still idling pretty rough. Thanks for the help!

dieseldan44 · #6 04-30-2021, 09:56 PM

another thing to try is the rack dampener bolt behind the IP. it can be adjusted and ultimately replaced. at one point (i think 84 ish) they upgraded from a 'silver' to 'gold' one which was an improvement for all previous models.

Noah300D · #7 05-02-2021, 09:09 PM

Hey Dan, I removed my rack damper bolt earlier today. The tip of the damper was visibly concave from wear, and the spring was pretty weak. It was screwed all the way in before I messed with it- hopefully a new one fixes it! It idles better when cold, maybe the spring is getting weaker when the engine heats up.