All you smart diesel benz guys, help!!! 300CD runs like crap!!!! - PeachParts Mercedes-Benz Forum

beena77 · #1 02-23-2010, 09:40 PM

I just bought this really clean and nice looking 1983 300CDT for a good chunk of change.How many miles it has is everyones guess.Too bad I did not test drive it before i bought/shipped it!!!!
Here is the problem-

It is very hard to start , idles very rough, hard to keep idling when cold (S. Cal cold, not Montana cold) but revs up smooth

Blows out tons of white/gray smoke when cold which smells like unburned diesel

Little blow by. Idle smooths out and smoke subsides once it has warmed up.
Valves were properly adjusted, all filters replaced.
Have not yet done a compression test.

Where should I start? Anyone have any ideas?

Previous owner claims son put a couple of gallons of gas in it by accident and it has been running rough when cold since. Supposedly they purged the fuel system and ran additiver in it etc. I frankly do not believe anything the guy says...

Thank you !!!!!

layback40 · #2 02-23-2010, 10:06 PM

injectors?

Jimmy Joe · #3 02-23-2010, 10:13 PM

sounds just like my car when she had 3 dead glow plugs...

vstech · #4 02-23-2010, 10:14 PM

gray smoke and diesel smell equal one of two things if during a warm running engine... well three.
1st. injectors not spraying a pattern that can burn
2nd. low compression from bad valves
3rd...flat cam lobes preventing adequate air for the compression in balance with other pots.

if it only smokes when cold, it COULD just be glow plugs.
it could also be low compression, injectors, and or cam...

tdelaurier · #5 02-23-2010, 10:34 PM

Id remove and check injectors - I think hot burning gasoline will hurt them. Id also check IP timing. If its hard to start and your glow plugs check out ok -- Id say timing. My timing was way off - difficult to start and smoked quite a bit until I corrected the timing. Timing options are all over this forum - try em all - I did.

rcounts · #6 02-23-2010, 10:48 PM

Quote:

Originally Posted by beena77

It is very hard to start , idles very rough, hard to keep idling when cold (S. Cal cold, not Montana cold) but revs up smooth

Blows out tons of white/gray smoke when cold which smells like unburned diesel

Little blow by. Idle smooths out and smoke subsides once it has warmed up.

When you hear hoofbeats, look first for horses - not zebras.

90% chance (with that combination of symptoms) that at least some of your glow plugs are toast. Easy to diagnose and fix. Unplug the GP harness (2" squarish plug with one 1/2" cable going to it on the driver's side fender well), you can follow the wires from the glow plugs to the connector.

Next connect one lead of your Ohm meter to a good ground (anywhere on the engine block will do) and insert the end of the other lead one at a time into the sockets 1 through 5 in the female half of the plug (the end not screwed down to the fender well). Set your Ohm meter to read in the 1-10 Ohm range (or the lowest range available). Each socket is one of the glowplug wires so the meter should measure less than 2 ohms for each socket. If any read higher than that, then that glow plug is bad.

They are easy to change. Just need a 10mm wrench to remove the nut that holds the wire on the plug and a 13mm (or maybe a 12mm) wrench to remove the plugs from the head.

Its a lot easier to change them if you remove the injector lines though. That is easy as well - the injector line nuts take a 17mm wrench. Loosen all 5 of them at the pump, then loosen all of them at the injectors - they come off in two sets and each set is held together with metal clips that bolt together - one set of two lines and one set of three lines.

After you swap the bad plugs - or better yet all 5 plugs - with new ones, reassembly is the reverse of disassembly...

IF all 5 plugs are good (and I'll bet 5 bucks they aren't - based on your symptoms), THEN its time to look at some of these other things - like compression, injectors, timing, etc.

JimmyL · #7 02-23-2010, 11:38 PM

Quote:

Originally Posted by rcounts

When you hear hoofbeats, look first for horses - not zebras.

90% chance (with that combination of symptoms) that at least some of your glow plugs are toast.

Now that is an answer from somebody paying attention and having a good grasp of the subject matter. Nice job!

Several others had it too, but I just like how this post started out.

leathermang · #8 02-23-2010, 11:48 PM

If it persists after the glowplugs are checked and the hole reamed of carbon buildup.....
the next most common smoke upon cold startup.. or after a little sitting even if hot is the valve stem seals needing replacing.

TchTchr · #9 02-24-2010, 12:04 AM

Make sure to ream glow plug holes or your new plugs will die prematurely (I think this is a must - my 2 cents). Diesel911 sells a nice one. Also, use a ratchet type box end wrench; it makes the job ALOT quicker and with it, you don't need to remove the injection lines.

rcounts · #10 02-24-2010, 10:56 AM

Quote:

Originally Posted by TchTchr

Make sure to ream glow plug holes or your new plugs will die prematurely (I think this is a must - my 2 cents). Diesel911 sells a nice one. Also, use a ratchet type box end wrench; it makes the job ALOT quicker and with it, you don't need to remove the injection lines.

Reaming is a good idea, no doubt, but if the plugs pull out without any resistance once they are unscrewed, then reaming is optional IMO. Basically only necessary if the carbon build up is making them fit tight in the holes.

On the gear wrench, again, I agree - it makes the job easier. However, I've found that even with gear wrenches, I had a heck of a time getting my big sausage fingers in behind the injection lines. After fighting with one for around 10-15 minutes and dropping the nut that secures the GP wire (it fell behind the IP and was a BEAR to fish out of there), I decided to take the lines off. With the lines out of the way each plug took about a minute or two - instead of ten or fifeen. Taking the lines off and putting them back on is about a 10 minute job, so for me it saves WAY more time than it costs - making it time well spent IMO.

Now if you're one of those with l-o-n-g, slim, spidery hands, it might be different. But for those of us with square, blocky hands and short, thick fingers, taking the time to pull the injection lines is definitely worth it. Plus you don't end up tearing open your knuckles on the sharp corners of those injection line clips...

dadette123 · #11 02-24-2010, 11:07 AM

Quote:

Originally Posted by rcounts

Taking the lines off and putting them back on is about a 10 minute job, so for me it saves WAY more time than it costs - making it time well spent IMO.

I agree, at least for the first time, to take the injector lines out. Like most things, once you've done it a time or two you find which method/shortcut works best for you.

One thing I have learned when putting the injection lines BACK on is to loosen the line clips a bit AND hand start each line at the injectors and the pump before tightening any of them. IMO, it helps seat the fitting.

brownrice78 · #12 02-24-2010, 04:01 PM

bad glow plugs are the most likely culprit. change them.

rcounts · #13 02-24-2010, 04:30 PM

Quote:

Originally Posted by dadette123

One thing I have learned when putting the injection lines BACK on is to loosen the line clips a bit AND hand start each line at the injectors and the pump before tightening any of them. IMO, it helps seat the fitting.

I've never loosened the clips, but starting the nut on both ends of every line before tightening any of them is definitely the thing to do - thanks for making up for my oversight to mention that ...

MattBelliveau · #14 02-24-2010, 05:25 PM

Has anyone mentioned whether or not it runs better once at the proper temp.?

toomany MBZ · #15 02-24-2010, 05:28 PM

The OP mentions "when cold".