I was just wondering how common it is for these engines to have some roughness while idling after a cold start, even after letting the glow plugs glow for some time after the light goes out. However if I step on the throttle a little and bring up the rpms to about 1000 it's smooth and if I hold it there for say about 20 seconds and then release the pedal then it usually stays smooth. Is this common? Or do you get smooth idle without throttle right from the beginning? I wonder if I have low compression in one of my cylinders. All my glow plugs are new and working, I had valve adjustment done a few months ago, and use Redline about every other refill.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Yeh, I wouldn't worry about it until it doesn't smooth out after it warms up. Remember, diesels use heat to ignite the fuel, and a dead cold engine, even with hot glow plugs, is not gonna run real smooth until the pre-combustion chamber heats upa bit. My engine is pretty fresh, and on real cold mornings it takes about 15-30 seconds to settle down. An older engine with more wear is probably gong to take a bit longer. (I know my old 220D's did) But what you have sounds right in line with how mine acts. (Same engine)

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past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
'93 Ford F-250
'96 Corvette
'99 Polaris 700 RMK sled
2011 Polaris Assault
'86 Yamaha TT350(good 'ol thumper)

People have asked this question in the past, and no one seems to know exactly what causes it. Of coure, a good valve adjustment and fresh glowplugs always might help, but my car is in a perfect state of tune, and it will hiccup for about 5 seconds in the morning (if its cold out). It otherwise runs perfectly, so I don't worry about it.

There was someone who mentioned that they solved this problem in a w124 diesel by replacing washer/seals that lie underneath the check valves in the injection pump. These valves keep the pressure in the lines between the pump and injectors. If the seals leak, the pressure in the injection line bleeds down, which means the pump has to build pressure in the line before it will pop off the injector, and all this may case the hiccup in the morning (pressure bleed off overnight).

I don't know much about these seals, so if anyone else does, please chime in.

GregS
'84 300D
'90 300CE

Could this also be a failing fast idle electronics control circuit (probably not the correct name, but hopefully someone will understand what I am talking about, the circuit that gives the fast idle on a cold engine to keep it from stalling, not only diesels but cars such as Cadillacs have this circuit, btw)?

Anyway, my point is my unit is completely failed, I am thinking of buying a new one, but for now I have to use the accelerator pedal to bring rpm up to 1000 to warm up the car, or it hiccups then stalls. So could this circuit be partially failed, not giving the full fast idle? I don't know enough about the circuitry, but just wanted to throw this idea out there.

I don't think that the OM617 turbos have a cold idle (fast idle) circuit?

My car isn't exactly smooth either. However synthetic oil, and valve adjustment definitely helps cold starts. I just smoothes out faster and the idle is 'stronger'

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2008 BMW 335i Coupe

My 85 300td does the same thing and I'd like to get a clear understanding of what is happening. I've posted on it in the past but haven't got a satisfactory answer. I did replace the glow plugs but no difference. It starts right up at first crank even at 10 below zero. Once in a while it will do it when warm also but that is fairly rare.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

These cars don't have any cold-start devices as far as I know. Non-turbo Mercedes diesels had a cable lever on the dashboard that turned up the idle speed, but no turbocharged Mercedes diesel has this.

Anyhow, I gave all the knowlegde on this issue that I have. It would be interesting if someone out there pulled the lines off, replaced the check-valves, check-valve springs, and any seals. That's too much trouble for me, but I would like to hear if this helps the cold start hiccup.

GregS
'84 300D
'90 300CE

Greg is right about the fast idle. The turbo cars apparently have none. I even asked a Mercedes mechanic why the older diesels have an idle adjustment knob but the turbo's don't and he said "it's typical Mercedes practice, when something works really well they decide to change it." It's really weird that these cars don't have a fast idle or the good old adjusting knob. Every car I've driven in my life has a fast idle. But then again this is my first diesel and they might be different in this aspect.

I haven't confirmed this but the severity of hiccups also seems slightly related to fuel. I'm getting the impression that with fresh Redline in the tank the hiccups go away sooner, and today I had an opportunity to do 2 cold starts without fresh Redline in the tank and I had the hiccups constantly for a long time (again only if not pressing the throttle) until I decided to start driving. But I repeat I haven't confirmed this. I'm actually considering for my next refueling to add a quart of fresh engine oil to my fuel tank to see if it changes anything. I assume Redline is similar to engine oil so it shouldn't hurt anything, and besides there's been a lot of talk recently about alternative fuels, though I'm not willing to put in anything more than a quart.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

After some more studying I think there are 3 main explanations for hiccups:
1. Insufficent fuel due to low initial fuel pressure, air in the fuel system or bad injectors
2. Low compression not allowing the fuel to combust
3. Insufficient cetane of fuel

#2 seems less likely to me because if my hiccups are caused by low compression, then why do they go away if I give the engine a little throttle?

By the way, I just read that fuel additives that contain engine oil as the active ingredient may foul injectors, so it's probably not a good idea to add a quart of oil to my fuel like I was planning. I think I'll try 2 bottles of Redline next time I refuel. Theoretically it should improve the idle because it'll increase the cetane of the fuel and thus make it more combustible. Reason #3 could very well be the culprit.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Not to argue what you guys have already said, but this may be a factor. I think you're all talking about 616/617 engines that have this problem. And on those engines the glow plugs turn off as soon as you start the engine. Mercedes changed that on the newer ones, I think they started with with 603. They have a circuit that keeps the glow plugs on for a while after engine start. Solves the stumbling problem, but we end up with shorter glow plug life.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

Rick, that's a highly possible factor. In fact I'm quite certain that if my glow plugs kept on glowing after the engine starts (assuming that I don't already have this feature like you say) the engine would run smoother. I notice the hiccups don't start until a few seconds after the engine starts, most likely as the glow plugs start cooling down too much.

What was the first model year of the 603 engine?

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

1986 I think. The OM603 is a 6 cylinder iron block aluminum head engine. Your 82 300D has an OM617 all iron 5 cylinder. I've read here about an aftermarket glow plug relay that could be fitted to your car to make the plugs stay on longer. Don't know any details, but you could run a search for it.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

This thread started me thinking that my CD did the same thing. Hicupps for about 10 sec. and then is smooth. The last 3 days I've turned the key 2x and then started. All 3 times it started without any hicupps. I'll keep on trying for a few more days, and then without turning the key 2x and see whay happens. BTW, it's about 50 degrees here at night, if that has anything to do with it.

Looks like new glow plugs for me.

Craig

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1994 E420, Pearl Black/Black. 2.82 rear diff., AMG front spoiler, painted lower half. SOLD

1972 & 1974 BMW 2002tii's.

Rick, there are aftermarket kits....both Beru and Bosche make them.

A short while back, there was an excellent thread on the mbz.org diesel list. Here are some pertinent posts that I think others may find interesting.

"Wray's post reminded me that I hadn't shared my recent upgrade success with
my fellow listers. Last month I installed a fast-glow AND afterglow system
in my '87 300TD. I installed a 1995-sty MB glow plug relay, which has the
180sec after glow, and I put in 6 BERU GN-858 fast-glow/long glow
plugs--they're specifically designed for afterglow. Since this relay has an
additional pin that connects to ANOTHER! coolant sensor in my head, I have
constant 180sec afterglow all the time because the relay isn't getting a
"reading" from it to regulate how long of an afterglow to give. The result
is not only fast starts but almost instant warmup! I intend to install the
add'l sensor in my head--there's actually a hole/plug for it already
there--as soon as I get the time but since it's now cold I'm actually
enjoying the 180sec of a smooth-running cold Diesel! If anyone's thinking of
installing the fast glow system, I highly recommend going the next step and
adding the afterglow feature. All modern Diesel have it and it's an easy
upgrade... at least for the OM603/124 cars."

"Disconnect the violet wire from the GP relay. (I carefully pried open the 4-pin connector and removed it, and taped it
up.) This will hide the start signal from the relay, and will extend glow to the full timeout duration."

"Kabel, I've been doing some research on this subject. First, if you purchase a
Bosch update kit-yes the plugs are the same. Downside, seems like they don't
last(I'm told) so you will endup having to put a newer style plug in the engine.


"Based on what I've been told(will call MB this week), the late model 2.5 has a
similar glowplug relay. The MB relays have 2 temp sensor and vary the
afterglow time with temperature. "


"For ppl which are considering general gp replacement, I would opt for Beru vs
the Bosch(kinda like Knetch/Mann vs Bosch airfilters). A similar opinion seems to be stated with VW's."

"I tried jim's number 1 solution and was impressed with the performance.
prior to the #1 addition, I would have to glow/start/die the motor 3-4
times in the morning.
This morning I cut the violet wire back from the connector and started the
car on the first try.
It ran a little rough, but better than before, It didn't die as it did
prior. It also had about 10 times less smoke.
I then spliced wire onto both pieces of violet wire and fed them out
beside the console.
I connected a test lamp to the glow plug and confirmed, When starting the
car the glow plugs stayed lit until the relay timed out.
I also confirmed if I connected the wires together the glow plugs went out
upon starter being used.
I am going to get a toggle switch and connect the two wires to it. This
way I will have a cold start and warm start position.
I suspect this configuration will actually extend my glow plugs life, as
they will not need to be energized 3-4 times each morning, even though
they will be used for a longer time. I suspect the time-out in the relay
has taken some of the glow plugs life characteristics into its design.
Great work Jim, if you hadn't tried this, I would have probably
procrastinated for another few weeks before I got around to it!
Thanks (my neighbours thank you too)"

To the authors of these emails, I hope you don't mind me posting them. If you do, I'll gladly pull them from this posting.

Chris
'85 300SD

I just thought I'd update this thread. Last time when my car had hiccups during cold idle, it was after checking and doing the obvious: valve adjustment, adding Redline to the fuel tank, checking all glow plugs, and letting them glow well after the light goes out before starting the engine. The thing that finally fixed it for me was raising my idle speed slightly. I set it to 800 rpm warm which turned out to be about 750 cold. No more hiccups.

__________________
2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL