Diesel misses when first started - PeachParts Mercedes-Benz Forum

777funk · #1 01-11-2007, 11:12 PM

Hi guys,
I have a 300d (silver beaut!) that tends to miss a little bit when I first start. I think all the glow plugs are good. I test continuity in each. But it just sort of takes a second to get smoothed out (about 20 seconds of 2k rev). After that it's totally fine. Also, if I plug it in it starts smoothly. If I start it again within the next hour it's fine. Etc. basically if it's warm it's smooth. If a cold start (even on a 60 degree day) it's rough at first. Is this normal? Should I do anything about it?
Thanks for the advice!

PanzerSD · #2 01-11-2007, 11:19 PM

Well, the only thing that could possibly cause a rough start at cold/yet mild temps is one or more faulty glowplugs, or a leaking primer pump. If you have to rev it to 2k to get it to smooth out I'd check to make sure your primer pump handle is screwed down. mine had shaken loose once and those are the exact same symptoms I observed.

Oh, and valve adjustment might cure it...so I've been told...

BioPOWER · #3 01-11-2007, 11:52 PM

Sounds exactly like glowplugs. My E300 has one bad GP, so when I start it in the morning it runs on five cylinders until the sixth one comes on, then it smooths out. If you're sure it's not glowplugs it's probably an air leak into the fuel system. I'm not familiar with the 617 IP system but on my car I can look for small bubbles in the clear lines in the morning to tell me if there's a leak.

PanzerSD · #4 01-12-2007, 12:45 AM

Isn't that what I said?

captainmonk · #5 01-12-2007, 06:21 AM

Am I the only one who thinks when you own older diesels in the winter time that it is normal to have them start a little rough and smooth out within a minute???????? I have owned over 30 different diesel engines in many different applications none like the cold
I suppose the key word above is "little" and what that is to everyone If your GP light comes on your glow plugs are ok at least in my SDL
838 more miles to 400000

777funk · #6 01-12-2007, 06:48 AM

Captainmonk is probably right about the word 'cold'. I agree with everyone else that the other two things can also be a cause. But plugging the car in fixes the problem. It always starts fine when plugged in. This means that temp is the problem. However, is this normal. It sounds real bad everytime I start if not warm. I know diesel depends on temperature to operate. But it seems like it shouldn't be like this. Of course it's an old car. But wondering what I can do to fix. Also, maybe compression??? I don't know but I thought this could be it. What should compression be on a 617?

nhdoc · #7 01-12-2007, 07:45 AM

Try glowing it twice and seeing if that helps...the glow plugs may be all working but sometimes they don't heat all that well and the relay cuts them off too early...as an experiment try running the GP cycle once, then turn key to off and glow again until the light goes out for the second time, then start...see if that helps.

Also, a prior post mention valve adjustment...that can also help cold starting. If you don't know when the last time they were done you might consider this.

Low compression can cause it too, but I'd look there last after completely ruling out GPs and valve adjustment as the cause.

lutzTD · #8 01-12-2007, 07:49 AM

Quote:

Originally Posted by 777funk

Hi guys,
I have a 300d (silver beaut!) that tends to miss a little bit when I first start. I think all the glow plugs are good. I test continuity in each. But it just sort of takes a second to get smoothed out (about 20 seconds of 2k rev). After that it's totally fine. Also, if I plug it in it starts smoothly. If I start it again within the next hour it's fine. Etc. basically if it's warm it's smooth. If a cold start (even on a 60 degree day) it's rough at first. Is this normal? Should I do anything about it?
Thanks for the advice!

mine did this until I did a valve adjustment. they were off by a mile.... now it starts when cold and is very smooth. It also got rid of my blowby problem and increased my power noticably. if you havent done a valve adjustment you should do it at least so you know when it was last done.

#9 01-12-2007, 08:04 AM

Quote:

Originally Posted by captainmonk

Am I the only one who thinks when you own older diesels in the winter time that it is normal to have them start a little rough and smooth out within a minute???????? I have owned over 30 different diesel engines in many different applications none like the cold
I suppose the key word above is "little" and what that is to everyone If your GP light comes on your glow plugs are ok at least in my SDL
838 more miles to 400000

Not normal, but also not unusual. If everything is correct it should not run rough, even when cold. The last time mine started doing this it was also a weak glow plug (about 10 or 20 ohms) that did not show on the the light. After replacing the glow plug, it starts and runs smoothly (no noticeable shaking), regardless of temperature.

JimmyL · #10 01-12-2007, 09:38 AM

When you test the glow plugs you aren't looking for continuity, you are looking for low resistance, .6 to 1.1 ohms or so. Also, some folks have measured the resistance of their plugs, and they checked out OK, but still didn't "glow" very strong when they removed them for testing.
Since it works fine when plugged in, I think that rules out a fuel leak. Properly adjusted valves will help of course, but I'm not sure if it could be that based on starting fine when plugged in. We are assuming you have adjustable valves, but you didn't give the year of your 300D. We are assuming M617.
Good time to update the sig.

777funk · #11 01-12-2007, 10:06 AM

I've done the valves... No smoke when warm. Runs good instantly when plugged in. Just a few misses when started before it gets warm if not plugged in. Also, I've tested the glow plugs and they all measure around 0.8 ohm dc resistance. By the way it's the 617 turbo. It's an 84 300d. The plugs are new as of a few months ago. I think the previous owner used bosch plugs. Could it be that there is carbon built up and he didn't ream the plug holes when he replaced them??? Would this do this?
Thanks,
Nick

babymog · #12 01-12-2007, 10:21 AM

I know mine's a different engine, a 602 with afterglow, but in the owner's manual it says that a rough start cold is normal and to rev the engine until it runs smoothly.

I'll check my 603 manual today, I'm curious whether it suggests same.

I didn't at first subscribe to reving the engine cold, prefer putting it under load by putting it in reverse as soon as there's normal oil pressure (seconds), but the realization that a missing cylinder is fuel spraying onto cylinder walls is what leads me to follow the recommendation.

When plugged in I have never had a rough start, but when sitting out in a parking lot it sometimes does. Rare as mine is mostly a fair-weather ride, but waiting a little while after the light goes out as some on this site suggested seems to cure that also for me usually (hasn't been in sub-10F).

777funk · #13 01-12-2007, 10:52 AM

I've got a 6.2 Suburban and it's rough as heck when cold started so it may just be a diesel thing. Doesn't seem like the new ones do this though... I duno.

juanesoto · #14 01-12-2007, 01:04 PM

Maybe injectors wear?? If the injectors are weared out, the spray pattern in not very accurate so that might cause the rough idle until the pre-chambers heat up. I mean, if the injectors are not capable of pulverizing (not sure if its written this way) the diesel, you'll need hot chambers to vaporize the diesel so it will properly burn into the cylinder... Just a thought

777funk · #15 01-12-2007, 02:25 PM

That may be it... it's got 180k so it could be right.