Bad Idle when Hot
 

My '82 300SD has developed a weir quirk. It starts fine, has good power, especially with the Dawes boost controller, and normally idles great except when very hot. I don't mean the coolant temperature is hot, it could be a little over 80C like it was this morning, but after the engine has made a good run down the freeway.

Using this morning as en example, she starts and runs fine around town all the way up to operating temp. I get on the freeway and do 80-85 for about 25 miles to work. Getting off the freeway, it's loping badly, temp gauge reads about 82C. If I sit and let it idle for a minute, maybe less it depends on how hard I've been driving it, the idle smooths out to normal. If I just stay around town or avoid high speed (>75mph) freeway runs, it idles fine. It only does this when I've been cruising fast for a while.

I think compression is Ok though I haven't tested it. It started right up in the mountains this weekend at about 10 degrees and 7000 feet and has plenty of power. My dad's 300D lost compression and this car acts nothing like that one.

I never have to add oil between oil changes. Oil rings seem Ok at least.

I get about 21 mpg, 22 at best.

I have no idea when the injectors were last changed/cleaned.

I adjusted the valves about 5K ago. (That help her run much better at the time)

Replaced the fuel filters about 2 months ago.

There are no fuel leaks that I can see.

I tweaked the alda up 1/2 turn initially when I installed the boost controller but backed it off to about 2/3 due to smoke. It didn't seem to help the smoke. This car was never a bad smoker before and I don't know if installing the boost controller made it worse, my girlfriend drives this most of the time.

Anyway, I'm leaning toward a bad injector but have read here about the IP needing some seals or the likes?

Any help diagnosing would be appreciated.

Thanks,

Frank

Elripster,

I may be that the rack adjustment screw in the injection pump needs to be adjusted or replaced.

MY 2 300SDs run smoother when slightly warmed up than hot.

P E H

Could you give me a quick run down as to how to do this? I have that CD/manual so I should be able to put parts to names.

Thanks,

Frank.

I believe the adjustment screw refered to is called the fuel rack dampener. It is located on the rear of the IP, between the IP and oil filter.
There is a bolt there, with a tighening nut on it. If it is silver colored it needs to be replaced. If it is gold tone, it may need replacement, but it most likely you can adjust it.
To replace, first get a new rack adjusting screw. Then loosen the nut and turn the bolt all the way out. Install new bolt. To adjust you turn it in, with engine idling, until the idle smooths out. Trick is not to over adjust. Rev it and when RPM's fall, the engine should not stumble. If it does the adjuster needs to be backed off a bit. If you do get it over adjusted, you won't be able to get any power. Just pull it back into the yard and let it out a bit.
I have an 81 300SD. Had same symtoms as you descibe. Found it had a silver dampener bolt there. Replaced it and hot idle smoothed right out. The gold dampener bolt was a redo by Benz. Silver dampeners were faulty. Most have been changed out by now. But you have to ask yourself. Do I feel lucky?

I hope this helps..

The part's called Idler Adjuster Pin

D2030-17407
That is fastlane's part number. Somebody posted the MB part number but I couldn't find it...

Here is that pin on the pump...

BTW, anybody want to buy an Injection Pump? It's quite a paper weight and I'm running out of storage space for these things :p

Holson,

Hello, I would like to buy the ALDA if it works and you are willing to part with it. What would you want for it?
Thanks, Adam

Holson, Old, thanks for the info. I'm going to look into this Sat or Sunday.

One question, could a maladjusted idle adjuster pin cause, or have anything to do with getting poor gas mileage?

I get 21-22 and that seems a little low? I would figure more like 23-24. I do use the Power Services addative.

Thanks,

Frank

Don't know that the fuel rack pin would affect your fuel economy much. But your story could explain it a bit..."do 80 - 85 for about 25 miles to get to work".
Do ya think? Na, I didn't either.

I definitely agree that high speeds will hurt the mileage. I normally don't drive that fast. Typically it's more around 75. Although, my girlfriend normally drives the car and she has been bragging about "putting the smack down" on other vehicles since I put the boost controller in.:D

I have tried staying around 70-75 for full tanks of gas on I5 but only got like 23mpg out of her at best. I hear of people getting more and start to wonder if there's anything I can do to improve things.

Somewhere along the line she started to smoke a lot more. Even well before the boost contoller. I'm just not sure why.


Frank

I wish I could tell you what the fuel economy wiht my 81 300SD is, but it has been so long since I checked I doubt it would be accurate now. Maybe I should. Last check was with the old engine still in there. I was almost measuring the fuel use against the oil use. Happy to say that even at with its last breath of life the old turbo still used a little more diesel than oil. Just a little more.

I have been getting 29-30 MPG since I can remember on my 1983 300SD. About 75% highway and 25% city.

Wow:eek: How do you do it? How fast do you usually drive? That's a huge difference between our cars. Mine has about 240K miles, and seems to have good power. I don't see any leaks but I could check again. I've done things like adjust valves, change filters, K&N air filter, boost controller....and while power is way up, mileage has not improved. I'm starting to wonder now if injectors are bad, maybe IP timing is off due to belt stretch, etc.... I can climb the mountains north of LA with no problem at high speeds if I wish, I just use a lot of fuel.

Anyway, thanks again.

Frank

I am not doing anything abnormal. If you look about 5 posts down you will see many in the 26 to 31 range.

Does someone have pictures of the old and new rack damper bolts? I don't think I can tell if the bolt in there is silver or gold.

What changed in the design? Or was changing the color the key to improving hot idle? :)

Sixto
91 300SE
87 300SDL
83 300SD

I just replaced mine on my W123. Can't say I saw any real difference between the two except that the old one seemed a bit worn on the working (internal) end. It did solve my hot/rough idle though. It took a little tinkering to get the adjustment right.

Yeah how much do you want for the pump? Bring it to Connecticut if it's not sold.

adamb,
I sent you a PM regarding the pump

btw turbodiesel, that's not a bad idea.
I'll take it along with me. A nice weight in the trunk :p

I dont really know.. I wanted to get $40 for it... but didnt sell for the longest time (should've tried e-bay) but now ... $35..
just wanna get rid of it.

No guarantee that it works... the seller told me it works perfectly.. but I never tried it myself as my mechanic thought of something else for my injection pump troubles

Holson,

Neat picture. I heard a lot about the rack adjustment screw but I never knew exactly whew it was located.

Why is the rack adjustment screw exclusive to the 5 cylinder engines? I have had many MB 4 cylinder Diesel engines and they all idled steady. Vibration, yes, but a smooth vibration.

P E H

I kinda wonder too. Maybe it's the odd-number of cylinders? Isn't the I-5 just intrinsically less well-balanced compared to the I-4?

My first ever 240D experience was when I looked over an 82/83 240D Automatic. The car had 310k mi.

It definitely sounded different to my 617. The 617 sounds ... like a 617. It has a weird off-beat 'rythm' to it. I could tell apart a 240D from a 300D on the road just by the engine sound...

The 616 also seemed to transmit more of that 'steady' vibration to the chassis. However I think it was that particular car's engine mounts that were just bad.

If I'm not wrong the earlier non-turbo 617's didn't have the rack dampener either.

kinda makes me wanna talk to the designer of these 'amazing' motors...

P.E. Haiges

That rack adjustment screw is not found on all five cyl diesels. Not sure which came with it, but I don't have it on my non turbo 78 300D.

I just ordered one of these magic bolts for my shaky 1983 300sd. What kind of tools will i need to get this done. Will the affects of tightening/loosening the bolt be instant or will i have to let it settle, then adjust?
thanks

12 and 14mm I think I used to adjust mine. Easy job to do, mine will idle when cold now :D

I haven't gotten the new bolt in the mail yet, but my mechanic thinks it's the timing. He thinks it needs adjusting. Do you think that would make the car shake wile idling?

BTW you can tell if the rack bolt is bad or not by trying to compress it between your fingers.

If you can easily push it down, its bad.

If it starts to feel like its poking pretty hard into you and really pushes back...then its most likely fine. :)

BTW I used to have a REALLY bad hot idle...to the point that it would somtimes stall.

I ended up adjusting my IP delivery valves (not something a DIYer can do...best if a injection pump shop does it) and it smoothed out.

THe rough idle is sorta back...but thats cuase I never finished the adjustment. I was worried I was making it worse but actaully ended up getting much better!!!

Timing can do some...but that is easy to check.