300D Rough Idle when HOT "Diesel Purge" did nothing - PeachParts Mercedes-Benz Forum

skibum13 · #1 01-28-2006, 01:17 PM

I have an '84 300D with over 220k miles on her. the girl is having problems idling. she starts right up in the morning (glow plugs are good) and idles at about 800 rpm without too much roughness (normal for a diesel when cold). but once she gets up to operating temperature, the idle speed increases to about 900 rpm (not sure if that's normal or how to adjust that) and she idles really rough. i have to give her enough gas to keep the RPMs above 1000 to make it bearable.

here's what I've tried: i cracked open each of the fuel injector lines. three of the cylinders weren't providing power. bought one new injector and installed it; no effect. I swapped a "bad cylinder" injector with one that wasn't bad; the "bad cylinder" remained where it was (which I'm pretty sure means it's not the injector). Bought two more injectors just in case and replace two of the "bad cylinder" injectors; it ran WORSE, so I put the old injectors back in.

Here's the interesting part: I ran an entire bottle of Lucas Fuel Treatment undiluted 100% through the engine and the girl ran as smooth as a baby's butt! But as soon as I switched it back to diesel she ran just as rough as before. I got my hands on some Lubro Moly Diesel Purge hoping it would do the trick. While running one bottle undiluted 100% through the engine, the girl ran WORSE! And it ran the same as before afterwards.

I also tried adjusting the rack dampener screw, but it didn't make any noticable difference adjusting it all the way down to about 1/4" out. I'm at a loss for what to try next. My guess is that the Injection Pump is bad and needs to be calibrated/replaced. One mechanic I talked to thought it was just the low cetane rating of truck stop gas that I use that is causing the rough idle. I doubt it, but I got some cetane boost to add to the diesel to see if that makes a difference.

Can anybody think of anything else to try before I start trying to get the injection pump off? Thanks for your help!

TheDon · #2 01-28-2006, 01:26 PM

ok i rember someone told me this here when i first got my car.. dont always think its the biggest most expensive part... when was the last time you changed fuel filters?

JimSmith · #3 01-28-2006, 01:30 PM

I would check the engine mounts and (I believe your car has) the engine shock absorber. Do a search, lots of information and instructions. Relatively easy job to do, just messy because by this time there is typically an oil leak somewhere and everything in the area of the mounts is covered in black stuff. The mounts and the shock are a "system" and they are sensitive to idle speed - more so as they age and change their natural frequency. I would also check the valve clearances and possibly the transmission mount as well. Good luck, Jim

Brian Carlton · #4 01-28-2006, 01:37 PM

Welcome to the forum.

Just about every person who condemned the injection pump found out that the pump was fine and other "issues" were the cause of their problems. The pumps are normally bulletproof and they don't fail in the manner you described.

Before doing anything radical to this engine, can you kindly tell when the basics were done and by whom:

1) valve clearance
2) injection timing
3) camshaft timing

Now, these items may or may not be the source of your problem. But, I do think you need to verify them and correct them to specs before continuing with troubleshooting your issue.

RAYMOND485 · #5 01-28-2006, 01:45 PM

1984 300d Turbo 136k
Run Purge Direct In To The Prefilter And Replace 2 Ea Filters When
Done Diy, Pull 5 Pin Plug On Glow Plug Relay Box, Check For 1 Ohm Or Less, Grd To Ea 5 Pin, If Open Plug Replace, Auto Zone $11.00 Ea .
I Had Power Loss It Was A Fuel Hose Clamp At The Rear Of Fuel
Tank, Good Power After Fuel Stop Than Loss Power 1/2 Mile, Remove The Egr Valve And Use Hand Vacuum Pump And Carbon Cleaner And Use A 12 Inch Brush To Clean The Intake

skibum13 · #6 01-28-2006, 02:34 PM

Thanks for the fast replies!

The valve clearances were done by a local shop here less than 20 miles ago. They said they couldn't adjust the timing until the rough idle was worked out. I saw a tutorial on adjusting the valve clearances (after I paid $45 for the shop to do it) and I've seen tutorials for replacing the timing chain, but I haven't seen anything on adjusting the camshaft timing and/or injection timing.

I have also read that it could be the engine mounts. But why would it run perfect running Lucas Injection Cleaner? Wouldn't worn out engine mounts cause a rough idle then as well?

I could also ask the same questions regarding the timing: why would running Lucas have an effect on the timing?

Fuel filters were both replaced recently (less than 300 miles) and I filled up the secondary screw-on filter with injection cleaner when I replaced it.

Is there any test I can do to rule out that it's not the injection pump?

JimSmith · #7 01-28-2006, 04:12 PM

Engine mounts are easy to check. It is a visual examination. If they are collapsed they are done. If the other stuff you used is not fuel, per se, it won't have the same bang per cylinder firing and it might have a significantly different ignition point as a result. Different idle woud be due to different fuel. Jim