'84 300SD rough idle at stoplights (in D) but not in park (P) - PeachParts Mercedes-Benz Forum

clackity · #1 09-27-2014, 12:29 PM

I lurked this site for quite a while when I was an OM617 wannabe, but only just got my first Mercedes diesel with 288k miles a couple weeks back, and have what I hope isn't a stupid question.

Searching the forums, I see a lot of reference to rough idle, but not rough idle specifically at stops. When my 300SD warms up to 80+, and I come to a stop while still in drive, it seems like the engine is trying to jump out and hop away. In park, however, it idles just fine.

Other possible related issues are that the tranny seems to shift a little harshly sometimes and not others, and goes from 2-3 too soon at times. Both of which I understand might indicate a vacuum issue (will be getting a mityvac!). Also is sort of a dog off the line (even for a diesel), but acceleration is fine (again, for a diesel) past that.

My intended approach is this:

1. start going over the vacuum system(s)
2. clean banjo bolt and maybe adjust ALDA settings (free)
3. valve adjustment ($6 gasket)
4. Check rack damper (free) and possibly replace ($70 with gasket)
5. check engine mounts (though that's more a symptom than a cause, right?)
6. Maybe try diesel purge or start pulling injectors (doesn't feel like injectors, though).

I'm wondering if I'm barking up the wrong tree here, or if I should just go straight to #4 or ??? Basically, I'm hoping someone might have had similar issues with the car at idle while in drive, but not while sitting in park, and can tell me how they fixed it. Any ideas, however, are appreciated.

barry12345 · #2 09-27-2014, 01:05 PM

Valve adjustment on an unknown is very important. Even more so with your complaints .

leathermang · #3 09-27-2014, 02:01 PM

What he said..
plus... do just one thing at a time... and test.... otherwise you do not know what fixed your problems...

leathermang · #4 09-27-2014, 02:04 PM

Engine mounts are not necessarily just a symptom.... AND.... does your car have an ' engine dampener ' ... a shock absorber on the driver side of the engine ?

clackity · #5 09-27-2014, 02:38 PM

I guess I already knew I should deal with the valve adjustment first - also noticed a bit more blowback than I'd think is right, and the PO said "the valves are hydraulic," meaning he certainly never did an adjustment. I have to admit that the fuel armature up on the valve cover has got me spooked, but I suppose I need to bite the bullet...

I'll also check on that dampener, leathermag. Would go look right now, but wife and toddler son are out and about. Maybe I did too good a job convincing her of the safety, comfort and fuel economy of these old beauties. More likely we just share good taste...

Thanks!

leathermang · #6 09-27-2014, 02:48 PM

Fuel Armature...
not sure exactly what that refers to....
but the fuel pump... where all the metal fuel lines Start from....
do not touch anything on that until you research it....
FSM warns about touching it...

clackity · #7 09-27-2014, 03:28 PM

Quote:

Originally Posted by leathermang

Fuel Armature...
not sure exactly what that refers to....

That whole manual "stop" lever and fuel throttle linkage thingamabob with the ball and cup connections that sits across the valve cover. Am I seeing complexity where it isn't? Looks like a "gumption trap" if you know what I mean.

patbob · #8 09-27-2014, 03:57 PM

Rack damper bolt might need adjustment or replacement with the new version.

rscurtis · #9 09-27-2014, 08:15 PM

Save your money on the engine shocks. Their only purpose is to reduce engine shake and stress on the mounts when the engine is shut off.

clackity · #10 09-27-2014, 10:32 PM

It was almost dark by the time I had a peek. It does appear that there is a vertical strut down there, about even with the center injector. Also looks like the IP damper bolt is gold-ish, which hopefully means adjustment rather than replacement.

Also, I'm ordering a valve cover gasket and plan on tackling the adjustment ASAP.

Thanks, all. Guess my hope of there being a separate, hidden "stoplight idle adjusting knob" or similar was a pipe dream.

masc243 · #11 09-28-2014, 08:53 AM

Things to do:
1. Drip timing; timing that is too retarded (less than 24 degrees start of delivery) can cause a lope at idle.

2. RPM; If RPM is very much below 700, you may see some idle issues. Bring it up to 750 and idle will improve. This tends to only mask the real problem by raising the course idle. The true idle is set in the IP with the idle governor (which is not accessible with ip installed in motor). Careful on the course idle. You don't want to bring it up too far where you will be unable to manually stop the motor with the shutoff lever under hood.

3. Injector nozzles; dirty and or unbalanced injector nozzles can cause idle quality issue. (Greazzer is the guy on this forum who can check, rebuild, and balance your injectors at a reasonable cost).

4. Injector line pipe connections and delivery valve plunger/spring. The pipe connection has a tiny orifice/snubber that if clogged can affect idle. Pull each one when you drip time it and soak in some pb blaster for a while.

5. Valve adjustment and compression can also cause, but i think others already mentioned these.

6. Some will say timing chain stretch/ elongation can cause. I agree that it can, but would do the above items first.

7. Fuel pressure; FSM says to stretch the return spring to 27mm. Fuel pressure pulses rather violently and is difficult to monitor pressure on a shop built gage setup. Also remember that the returning fuel provides cooling to the IP. If you suspect fuel pressure issues, you can simply buy a new check valve spring combo for 30 bucks or so. Or you can adjust the spring to 27mm as others have done with success.

8. Of course the main filters, but often overlooked is the tank screen filter. If you suspect it could be causing an issue, you can swap the input and return lines which will allow the engine to feed off the unscreened return plumbing. if engine idle quality improves, the tank screen is likely clogged.

9. Lots of folks will blame air intrusion into a fuel line. I just can't wrap my brain around how air intrudes into a pressurized line. Only area it is unpressurized is from the tank screen to the lift pump. If that line had an issue, you would likely see wet fuel soaked areas. After the lift pump, it is under pressure.

Items are in no particular order; just a brain dump of things i've learned over the years.

Mike

vstech · #12 09-28-2014, 09:51 AM

Step one. Inspect your drivers side motor mount. It is IMPERATIVE that it is tall,and undamaged! A bad mount here will not only show as bad vibrations and shake at sleep under load, but it can beat a hole in your frame! Also, it can take out your motor, if it allows the power steering belt to contact your oil cooler line.

Step two. Adjust your valves then go for a drive ov at least 30miles, then adjust the valves again.

leathermang · #13 09-28-2014, 09:52 AM

Quote:

Originally Posted by clackity

....... a separate, hidden "stoplight idle adjusting knob" or similar was a pipe dream.

No, there is one.... but forum rules do not allow us to tell you where it is until you have a certain number of posts and oil in your driveway........

vstech · #14 09-28-2014, 09:55 AM

Quote:

Originally Posted by leathermang

No, there is one.... but forum rules do not allow us to tell you where it is until you have a certain number of posts and oil in your driveway........

Yup, sorry... Only us over 10,000 post members are privy to the sensitive info of simple easy remedies on these cars.

clackity · #15 09-28-2014, 09:31 PM

Thanks, all.

Well, guess I've got a good weekend and evening list, and I've got my little oil puddle started. Hey, I was warned that buying an old Mercedes would keep me off the streets!

As for the 10k club, I suspected as much. I'll look forward to the 10,000 mark, then. I suppose you also get to learn, conclusively, which mechanic-in-a-can really makes your car run like new for another 250,000 miles, and which is potentially engine-damaging snake oil?

Oh, and how to permanently repair your whole vacuum system in 30 seconds with a can of hairspray?

Awesome! Can't wait...