The car idles fine, until the ac or the heater is turned on. So my thoughts are alternator load or a little bit higher idle speed. In park with ac on with rough idle it's looks like 600-650 on the tach. Just got this today for my 16 year old daughter who wanted to wrench on a diesel benz.

Idling it up a bit should help. SD's are funny though; sometimes they will idle as smooth as can be and at the next stoplight they feel like a mixer.

Welcome to the forum. How about some pics of the SD.

I will get some posted tomorrow

If the A/C or heater is causing the issue, it's probably alternator load from the blower motor. You should be idling somewhere around 650 RPM in gear. If the idle speed is too low, you can expect some roughness. When's the last time the valves were adjusted? If they're too tight, you can expect a pretty lousy idle.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Factory idle spec is 700 ~ 750 RPM's when hot so first thing do the valves then a diesel purge then adjust the idle up a skosch, it's clearly too low now .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I just got the car yesterday, I will be doing the valve adjustment, diesel purge, all filters changed and all fluids flushed. Well I won't be doing it all lol. Daughter will be doing most of it. I will kick up the idle a smidgen and check all the mounts and the engine shock. Overall it's a clean car and the kid is happy. She is getting shocks for xmas and tires.

So here is a update, valve adjustment netted 7 valves too tight. I haven’t started it yet, I’m waiting for the air cleaner rubber mounts to come in. Then I will purge it

I’d check the rack damper adjustment and see if the turbo spins freely too.

Mine started stalling on hot summer days last year when I flipped on ac. I did a rack damper adj. and found the turbo gooped upwith oil. I just spritzed it with carb cleaner and it loosened up. Then a tiny tweak to the idle.

Good luck.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

Thanx for the update and kudos for raising a Daughter who'll do fine in life .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

There should be a sticker with the emissisons info on it. It has the timing, valve clearances and the idle speed range.

If there is over one hundred thousand on the odometer and you think it has the original Injectors they should at least be tested to see of the opening/pop pressure is OK and if the Injector Nozzles still have a good spray patern.

Anyone buying an old Mercedes ought to inspect the Oil Cooler Hoses, all the mounts especially the motor mounts and all rubber parts that can be scene. Like the Flex Discs, Drive Shaft Support Mount, Axle Boots and the Differential Mounts.

Ball Joint Boots also.

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84 300D, 82 Volvo 244Gl Diesel

There should be a sticker with the emissions info on it. It has the timing, valve clearances and the idle speed range.

If there is over one hundred thousand on the odometer and you think it has the original Injectors they should at least be tested to see of the opening/pop pressure is OK and if the Injector Nozzles still have a good spray pattern.

Anyone buying an old Mercedes ought to inspect the Oil Cooler Hoses, all the mounts especially the motor mounts and all rubber parts that can be scene. Like the Flex Discs, Drive Shaft Support Mount, Axle Boots and the Differential Mounts.

Ball Joint Boots also.

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84 300D, 82 Volvo 244Gl Diesel

OM616 & 617 engines had the valve clearance and IP timing sticker on the valve cover, I've only ever seen a few of them .

My '84 greymarket 30-TD still has most of it, I've tried to find new stickers using the part #to no avail ~ it's long obsolete and no one seems to reproduce them sad to say .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Check the car over really carefully if giving it to the daughter. There are a few things that are important on these old buggies. Going over the brakes and steering components especially. Brake hoses really old? Calipers and pads free and good? Brake fluid needs flushed? Many time I find the brake pads need removed and a good cleaning of any corrosion present and lubrication applied where needed. Proper lubricants applied sparingly. Although probably far more desired and needed in our northern area. Where so much salt is used in the winter.

Yeah we are going through everything on it. She pressure bled the brakes already and we checked the hose condition. Pads were good and the calipers are in spec. She bought new vacuum hose for the engine compartment and were doing a diesel purge this weekend. She is getting a injector rebuild set up with monark nozzles.... a gift from dad lol. Best of all she’s is a girly girl but is demanding she do the work to learn it.

On my 84 300D the emission sticker is up front on the upper Radiator Support on the left side.

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84 300D, 82 Volvo 244Gl Diesel