Injector Question: Can an injector miss only at idle?
 

I think I may have a bad injector, I have not cracked the lines yet to check but strangely as I coast up to a stop, my engine will shake (rack damper fine, new motor mounts) Problem is somewhat intermitant -engine is super smooth at speed with plenty of power, no perceived missing under a load -any thoughts? can our injectors miss at idle but clear up under a load? This is for a '82 300SD OM617 Thanks!

short answer is yes.

these injectors have new bosio nozzles that were set up correctly by a Bosch shop -I guess I am surprised that I already after an issue after only 10,000 miles- may pop in a set of Monarks next and go from there -

then in that case the short answer is probably no.

unless your fuel filters have blown.

Replacing the valves in my fuel pump made a vast improvement in my idle quality, as well as off-the-line performance.

Swap injectors around see if the problem follows them.

Swaping injectors is okay. If the problem is still there first a check of the valve clearances then a compression check of that cylinder is indicated if the valves for that cylinder were not tight. Remember I said a quick check. No reason to adjust them if the need is not there.

Try to keep in mind that many owners of long standing did not know or ignored the concept of checking the valve clearances for endless miles. Mercedes wanted it done every 15k. This attitude that a diesel engine requires no service is held by many yet is false.

What makes you think the rack damper is OK? Either way dirt can always foul an injector...

Sorry as thought you had already identified a missing cylinder. Rough idle can be other things as well of course. Air in the fuel is also a prime suspect.

rack damper replaced and adjusted correctly many mile ago - last valve adjustment about 18,000 miles ago - time to adjust anyway!

Valve adjustment first.

How about a small Air Leak?

Is your idle speed within the amount on the Emission Sticker?

Maybe the Rack Damper Screw is now broke in and needs re-adjustment.
Briefly remove your Oil Fill Cap; if this improves things your Crank Case Pressure is effecting your Vacuum Shutoff Valve.

Briefly Pinch off your Cigar/Fuel Return Hose; this should raise the Fuel Supply Pressure to the Fuel Injection Pump. If this improves things it is likely your Return/Overflow Valve is not holding the pressure well; a long thread started by Cervan on that.

Or, it could also could be a Fuel Supply Pump issue.

Really pleasant to notice many suggestions by many people are coming into more common useage. Vindications of some thought processes by many over time.

I have checked connections for air, not an issue -this problem seems to develop as engine reaches full operating temp so it mystifies me that an injector would only miss at operating temp idle versus cold idle . . . I may wire in an electronic pump anyway but either way, I will check valve clearances this weekend along with trying to isolate a potential faulty injector. It also occurred to me that perhaps I have a injector nozzle (bosio) with a factory defect. My trained MB indy explained to me that defective/malfunctioning injectors are fairly common. If dirt, I wonder if diesel purge would clear? Yesterday, I filled car with B100 -biodiesel is also a very effective solvent and I was curious to see if it would impact injector after running a tank through. I enjoy the challenge of learning more about these cars as they are for the most part, pretty straight forward and easy to work on. Thank you everyone for your assistance and thoughts

A cylinder that is down on compression will miss at idle and hit at higher rpm where sealing is better. Can be rings or valves.

Well you may have presented a clue it could be valve clearance related. When the engine is cold the valve stem is at ambient temperature. After engine heats the valve stem increases length by expansion. That is why you need so much valve lash clearance to start with cold.

So if the valve is just barely sealing with not enough clearance cold. The valve will be open a little after the warm up.

Unfortunatly your doing the valve clearances 18k ago reduces that possibility. Yet it still remains a posssibility. At least the valve clearances were done in recent history compared to many examples on your car.

Back off your injector lines one at a time with the engine at operating temperature to see if you can find the cylinder. That gives you more incentive to pull the valve cover and check. You do want to establish it is an individual cylinder as it could well be something else.

Or read the milli volts at the glow plug harness with the engine started but still cold. Then when the engine is fully warmed up read them again. If it is a cylinder miss you should be able to pick it off with the comparison between the two very easily.

With your revised description I doubt it is an injector but one never knows.