Hi all,

Have followed the forum for a while and has been very helpful!

I have an om615 that’s giving me a few issues and wonder if some of you could help?

Main symptoms are: tough to start from cold, black smoke on blipping the throttle and under load, lack of power. (Eventually loses power under load and black smoke).

What I’ve done/checked…

On this engine no vac is needed for brakes etc so vac pump is blanked from manifold and dual ports left open on pump.

Removed, cleaned injectors, new nozzles and pop tested. Spray pattern good.

Adjusted valve clearances.

Timed ip to 24 btdc at roughly 1 drip per second.

Checked vac governor holds vacuum.

Turned in full stop screw until showing only a few threads with no change to smoke output.

Ran without air filter.

When the throttle is held open stationary it doesn’t smoke, only under load. Very low torque output too.

What am I missing?

Thanks in advance.

Is the hard plastic line from the back of the IP to the intake plenum cracked? Does the diaphragm in the IP hold vacuum? Are all your linkages set to factory spec?

Phil Forrest

__________________
1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson

Thanks for reply.

No cracks in tube, holds air well.

Diaphragm in good condition and will hold vac nicely.

Linkages seem irrelevant really as it’s just the throttle butterfly that’s moving when driving. I’ve tried with direct hand control of the throttle and black smoke appears.

Could the control spring have been stretched in the pump? (By previous owner) it’s 4”/100mm long at present. Can’t seem to find the spec on them but does feel like a lot for that Venturi to pull on..

Also does the vac pump have any bearing at all on the timing advance system on the end of the ip?

Thanks

It'll run fine without the rod that goes over the top of the engine, and also without the rod that goes down to the back of the injection pump.

It's possible. Plus, there are shims in there, and it is possible that someone fiddled with them.

No. The timing device is driven directly by the timing chain. It just so happens that the cam that drives the vacuum pump is part of the timing device.


Not sure if you ran into this in your searches:
Tuning of the Diesel Engine - OM615 OM616 (Pneumatic Governor)

I'm kinda thrown off by your report of seeing black smoke and low power. When I've experienced over-fueling in my car the smoke was grey, and smelt of un-burnt fuel. More fuel = more power, so the low power you're experiencing doesn't seem to fit an over-fueling scenario. My car had no shortage of power when the governor was leaking and wouldn't hold vacuum.

How sure are you of the engine's overall condition? I wonder if a compression test would shed some light on this.

__________________
- Greg -
1973 220D, The Prodigal Benz
1974 240D

This would have an impact on power output, for sure.


Is this engine installed in something other than a M-B vehicle? Just curious.

__________________
- Greg -
1973 220D, The Prodigal Benz
1974 240D

If you have a lot of blowby and the valve cover breather that vents to the intake manifold is really pushing out oil, that would cause some black smoke.
Careful with this going forward though. Since you're burning black, there is oil getting into the combustion chamber/s somehow and you could end up with a runaway. Always be ready to cap your intake and pull the clean air side of the intake off of the throttle body so you can starve the engine of oxygen just in case it decides to run away.

Phil Forrest

__________________
1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson

Not with your hand!

I'm wondering, like you Phil, if this is a worn out engine.

__________________
- Greg -
1973 220D, The Prodigal Benz
1974 240D

The breather issue could be partially solved by capping the connection at the intake manifold and letting the breather drain into a can filled with steel wool, just to keep things clean. If the black smoke disappears, you have a winner and will need to suss out why there is that much oil vapor pushing out the top end. If you just have stuck rings, they can be freed up. If it's really worn out, you could put some heavier oil or engine restore in there to see if you can bump up the compression a bit.

Does the oil cap blast off forcefully when the motor is running or just dance around?

Phil Forrest

__________________
1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson

Yes only turned in full stop screw to see if it had been wound out too much..

Yes the engine is fitted to a 1962 unimog 🙂

From what I’ve read, the black smoke is part burnt diesel. Blue being oil smoke?

I’ve isolated the crank breather into a catch tank and plugged the manifold for now to rule that out but doesn’t seem to affect smoke output.

It does blow oil vapour though.

I’m running 20w50 currently, oil pressure is very strong. Much higher than I’m used to vs petrol V8 engines etc.

Oil cap doesn’t lift up, it blows by it but can’t lift the weight of the alloy cap.

I’m planning to test compression this weekend as I haven’t done this yet.

Can compression be checked via glow plugs? Getting the crush washers for the injectors takes a while 🙂

Yes, with the right adapter.

I tend to agree with black smoke being related to partially burnt fuel. The lack of power is what's stumping me.

Inadequate pressure/volume from the lift pump? Overflow valve not holding adequate pressure in the IP's fuel galley?

__________________
- Greg -
1973 220D, The Prodigal Benz
1974 240D

Agree, a few things bother me, injection timing and the affect that has on combustion of the fuel. I keep questioning myself on it. I did set to 24 degrees…

The governor and the strength of the spring it has to overwhelm. Without the spring in place should the rod just sit loose and be moveable in either direction with little to no resistance? Assume probably correct.

Compression will tell me about condition of the engine but also perhaps double double check valve adjustment first if it is down.

Injectors popped at roughly 1600 psi after shimming and chattered nicely with a good pattern so I’m happy enough with them.

Only other thing I have considered is, fuel is a mixture of brand new and some that’s possible to be over a year old but with a bottle of system cleaner in it.

How stretched is the timing chain?
What is your altitude?
Man, I want a unimog.

Phil Forrest

__________________
1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson

There's a fair amount of flexibility in injection timing. If you are anywhere near the ballpark it should run OK. I've pushed timing way far advanced, like 1/4 inch of IP movement towards the engine, and it still ran and drove OK. It was a PITA to start but it ran. I did that when I was trying to figure out why the engine wasn't running well, before I discovered the leaky governor.

I've got a loose IP for parts. I'll pull the governor spring and measure its length and report back. I know from experience removing and installing the rear governor housing that the spring has to be compressed a good amount during install. So there's a fair amount of pressure on the diaphragm/fuel rack at rest.

Diesel fuel ain't like modern gasoline. It'll last a long time so long as it stays dry.

__________________
- Greg -
1973 220D, The Prodigal Benz
1974 240D