Need a little idle help please

[John300SDTurbo]

Hello,
A few months ago I aquired a 1983 300SD Turbo and I couldn't happier with the car. The car has over 350,000 miles on it, averages 34mpg plus, and runs like a dream. Recently I've had a quirk pop up that I can't quite figure out. At idle, in gear or out, the engine vibrates much more noticably than it did originally. As soon as I touch the pedal it smoothes right out and runs perfectly. I change the oil and all filters as recommended. Any and all sugestions or ideas would be appreciated.
While I'm here, how do you test a glowplug and where is the infamous glowplug control relay located and can it be be tested?

Thanks in advance for the help.

 

[grasmere]

Sounds as simple as the idle is slightly below optimum and hitting 'resonance', check the idle speed / increase it slightly

 

[NormanB]

Hi John and welcome

As Ian says it may be idling low - but there may be an underlying cause so I would hesitate to just 'wind it up'.

I know little about diesels- but there is a forum on your side of the pond (where there are loads of MB diesel nuts) where there are many experienced and knowledgable contributors, who have cars just like yours. You can find it here - I would recommend you sign up and post there too.

Also - I do not know what records you have on the history of your vehicle but at the age and mileage of yours, if the engine mounts have not been changed, they would almost certainly benefit from new ones - if they are the original ones they are most certainly shot and will be transmitting engine vibes to the body.

Anyways, please let us know how you get on with this problem - and good luck.

 

[John300SDTurbo]

Both of these options (motor mounts and possibly "winding it up") have crossed my mind and will I probably end up doing both if needed starting with the mounts. Reply from mercedesshop.com on my side of the pond (thanks for the link Norman) suggests changing glow plugs. I'll probably do this to. As for "winding it up", I've got to beg ignorance on how to go about it on this thing. I've never seen such a throttle linkage system in my life and I've got no idea where to start if I want to idle it up a touch. MB may well do things the proper way but dang if they don't make it complicated.

 

[Ian B Walker]

MB may well do things the proper way but dang if they don't make it complicated.

You have noticed then

 

[NormanB]

Both of these options (motor mounts and possibly "winding it up") have crossed my mind and will I probably end up doing both if needed starting with the mounts. Reply from mercedesshop.com on my side of the pond (thanks for the link Norman) suggests changing glow plugs. I'll probably do this to. As for "winding it up", I've got to beg ignorance on how to go about it on this thing. I've never seen such a throttle linkage system in my life and I've got no idea where to start if I want to idle it up a touch. MB may well do things the proper way but dang if they don't make it complicated.

John

I would have thought glow plugs were more of an issue with starting than stable and correctly set idle.

Keep pressing the 'diesel weasels' on mercedesshop - they will produce the right answer I am sure.

 

[Dieselman]

It's probable that the idle damper screw has worn out so the idle wanders slightly up and down. This is a bolt that screws into the pump and literally acts as a damper.
If not this then as has been said the idle may be too low. The idle isn't adjusted by the cable lever but by a stop on a bypass lever. If the bypass is set too low then the engine is being starved of fuel. To a certain degree this is normal as the only way to control a diesel is by fuel reduction. This is why diesels sound crabby at idle.

Firstly run a strong dose of injector cleaner through the old dear, as you may have a poor injector causing cylinder imballance.

 

[John300SDTurbo]

Ok, this is where I really start to look like an idiot. I can take a small block chevy apart and put it back together with my eyes closed but I'm lost on a diesel motor. Where is the idle damper screw and how do I tell if it is worn to the point of replacing? BTW, I did as you suggested and added a full bottle of injector cleaner to a little less than half a tank of fuel. It may just be the cetane booster but it is idling a bit better. I'd still like to check the damper screw to be safe. I don't know anything about past service on this car and I want to treat her right.

 

[John300SDTurbo]

Ok, this is where I really start to look like an idiot. I can take a small block chevy apart and put it back together with my eyes closed but I'm lost on a diesel motor. Where is the idle damper screw and how do I tell if it is worn to the point of replacing? BTW, I did as you suggested and added a full bottle of injector cleaner to a little less than half a tank of fuel. It may just be the cetane booster that was in the injector cleaner but it is idling a good bit better. I'd still like to check the damper screw to be safe. I don't know anything about past service on this car and I want to treat her right.

 

[John300SDTurbo]

Thanks Dieselman, the injector cleaner seems to have done the trick.Having never owned a car with fuel injection I never even thought of something that simple. The car is purring like a kitten (or as close as a diesel can get to purring) after it warms up.

 

[Dieselman]

Nice and easy.

This usually describes maintenance to a diesel, especially an MB.

Forget anything on the pump if you are unsure, and if it's running well.

The reason why you noticed the uneven idle so much is because yours is a good old five banger so is naturally unballanced, exacerbated by one cylinder being low.

In reality a diesel block and head is the same as a petrol. The only difference is that there are no throttle flaps and the engine speed and output is controlled soley by the quantity of fuel.
The pump meters and times the fuel injection, the injectors spray it. Smoking or knocking is nearly always injectors...so keep them clean with injector cleaner and lubricant. The pump should outlast the car.

Now relax and enjoy the drive.

Ps. If it starts from cold forget the glowplugs.

 

[John300SDTurbo]

Ok, I'll forget about the idle damper screw. You talked me right out of that one. I think I am gonna have to change the GP control relay though. My GP light is not coming on anymore and I've checked the glow plugs....they ohm out as good (between 0.7 and 0.8 ohms). The fuseable link in the relay is good (replaced just to make sure) and I've got voltage on the #1 pin (I think that's the right one) in the 4 pin connector in top of the relay but no voltage to the GPs. When I cut the switch on I can hear the relay energize and then, a second or so later, cut off. I guess it's possible that because of where I live and the outdoor temp right now(lows in the 70's and highs in the upper 80's to low 90's) that the relay is deciding it doesn't need to heat anything up. What's your thinking on this? I hate to keep coming up with questions but like I said in an earlier post, I'm a complete newbie to diesel motors and I'm still trying to comprehend a motor that runs without electricity. I am grateful for your help and insight.