240D rough idle - Do I or don't I have rack damper bolt? - PeachParts Mercedes-Benz Forum

#1 10-02-2002, 10:15 AM

Here we go again. I've seen many threads in this forum on rough idle and the cure being adjustment of the damper bolt. Also many opinions on which cars have them and which don't. I have a 76, 240D with the Bosch M type pump (pnuematic governor), that has the classic symptoms of rough idle. I have adjusted the valves, installed new timing chain and verified valve timing using valve opening measurement technique, retimed injection pump with low pressure drip method, and installed rebuilt injectors. Car runs great except for the rough idle and there is no excess smoke problems. The only thing I see that resembles what I have read described as the rack damper bolt is a slotted stud with a 17mm (wrench size) nut on it on the end of the pump closest to the firewall. After trying to adjust it clockwise the only thing I could accomplish was to lower the top speed of the engine with a moderate adjustment, and if I went several turns the car would stall. Counterclock wise seemed to raise the engine top speed and started to create some smoke.
Now if anyone is still with me after this long description, can any one tell me where else to look for the damper bolt and how do I properly set the bolt that I did mess with? Any other suggestions if I don't have the rack damper bolt?

franklyspeaking · #2 10-02-2002, 12:04 PM

I have an 82 240D - it doesn't have the rack damper bolt that is on the 300D's. Not sure about the pre-123's.

Another cause of rough idle that you can check is cylinder compression. Compression should be > 200PSI (Preferrably 400PSI) and fairly even across the four cylinders. My guess is one may be a bit low (especially if it starts okay).

Good luck.

leathermang · #3 10-02-2002, 12:27 PM

Here are a couple of places you can see the titles of tech service bulletins whichi MB has put out.. and safety recalls...

http://www.nhtsa.dot.gov/cars/problems/tsb/central2.cfm

http://alldata.com/

If a title sounds interesting then you can order it from MB or ask people who have copies of the tsbs to share with you...

Beagle · #4 10-03-2002, 12:03 AM

Johnny

A few questions for you..

1)Have you had a compression test? If so what were the results? Poor compression in one or more cyls. will cause a rough idle as will contaminated or poor quality fuel.

2)Is the rough idle a regular rythm or is it hunting (i.e. uneven RPM)?

Slacken each injector pipe in turn while idling and note any missing or knocking cylinders.

Your problem could be simply your pump calibration out - very sensitive at idle as the fuel vol. is very small, just a couple of drops. Re-calibration would have no benefits other than smoothing out the idle and will cost you $$$$! But can you live with a rough idle??

There is no "damper" inside that pump. Some are fitted with a damper in the throttle linkage, a small shock absorber about 3 inches long.

There are two adjustment srews with locknuts on the rack lever at the back of the pump, one is idle adjustment and the other max delivery. I guess one of these is the screw you refer to?

(Check the spring on the linkage is pulling the rack lever firmly onto the idle adjust stop)

#5 10-03-2002, 09:32 AM

Hi Beagle,
Answers to your questions:
1. Compression test was done about 12000 miles ago,readings between 300psi and 325psi all cylinders. Fuel and fuel filters don't seem to be an issue.

2. The rough idle is along with a slight hunting.
When loosening the injector feed pipes one at a time each cylinder would drop out the same and recover when tightened again.

Could be pump calibration I guess. 280000 miles on the car and no pump service done in the last 200000 miles have owned it.

I have to look again but I don't recall seeing the linkage shock absorber you mentioned.

I also have to double check the two adjustment screws and the spring for the idle stop.

I guess I will also check the compression again since it's possible something happened in the last 6 months.

Thanks for your reply.

Beagle · #6 10-03-2002, 01:58 PM

Hi Johnny

Your compressions look good and even for that mileage.

Mean't to ask you what was done to your injectors. New nozzles? Were they stripped? What was the leak-off like?

15sec/10bar is good but more immportant they be more or less the same. One bad one will give exactly the same effect as a pump out of calibration.

New nozzles usually take some time to settle in, about 500ml or so.

#7 10-07-2002, 10:30 PM

I finally got a chance to get back to working on this problem.

Here's the answers to Beagle's questions about injectors. They were sent out to a highly recommended diesel shop and rebuilt. All I know is that they did replace the nozzles and set the opening pressures. No mention of the leak down rate when they were returned. I don't suspect any poblem with settle in time as they have been in the car for at least 3500 miles.

There is no damper in the linkage and springs are returning the linkage to the stop screws.

I am wondering if the diapham in the injector pump is a problem. This pump has a pnuematic governer so I suspect the vacuum at the throttle plate may come into play with idle control. I disconnected the line at the manifold and tried to pull a vacuum with a hand pump. The reading never got higher than 12" of Hg and leaked back to zero within 5 second after I stopped pumping. To me this looks like the diaphram is leaking. Is this a leaker or is this normal? Is this a possible cause for rough idle if this is leaking at the diaphram?

Beagle · #8 10-08-2002, 01:46 PM

Johnny

Your injectors are obviously not the problem since you have new nozzles, leak-off is only an issue with worn nozzle needles.

The classic symptoms of governor problems, whether pneumatic or mechanical, are firstly a lack of power and secondly a surging or erratic running. Idle may be affected but the former should be the most obvious. Your discovery that the venturi line will not hold a vacuum certainly indicates that the diaphragm is leaking. I assume that you have checked the line from the pump to the manifold for leaks.

I would suggest at this stage, that you take the car along to the diesel shop that rebuilt your injectors and let the boss there listen to your engine, he should recognise immediately if it is the governor at fault and can confirm for you that the diaphragm is leaking. They shouldn’t charge you anything for that and can give you a quote. (alternatively perhaps you could post us a few sound bytes?

)

The bad news I’m afraid is that the pump will have to be removed, re-calibrated and the gov. reset on a rig. You should save some $ if you R&R the pump yourself.

psfred · #9 10-08-2002, 08:44 PM

Leaking pneumatic governor diaphram will cause uneven idle, I think, and it should definitely hold vac. It is leather, and has worn and gotten holes in it.

I would not recommoned doing this yourself -- it is somewhat of a pain the sitting part to get the new one adjusted correctly (the thickness of the diaphram affects the governor rod length) -- you have to accurately measure the lenth of the pushrod as installed and add or remove shims to get it correct. You also run the danger of not getting it installed correctly and having the engine run away when started -- this has been know to happen on new cars, too, by the way! -- and it will destroy itself if you don't get the cap nuts loose on the injectors fast enough -- even two cylinders running can blow it up with an automatic. Manual you can usually kill with the clutch if you are fast. Take it to a shop and let them do it, the pump can remain in the car.

Peter