OM603 rough idle until throttle blip
 

My 1991 350SD that had a broken cam after my shop replaced the HG is now running beautifully and strong. There are a few things to shake out but I’m happy with how it runs.

But I notice one odd thing, the cold start idle.

When the car has sat a long time, fully cold (which in this weather means an 80F heat soak), I start the engine… glow until the light goes out, crank, it fires and runs on the first cylinder, easily. But it shakes. Not as bad as when it was misfiring with one cylinder inoperable, but it does shake.

Blip the idle and it’s smooth as can be and doesn’t do it again until the next fully cold sit it goes through…

So something is off. I don’t let it idle, shaking like that for long enough to know if it smooths out. I blip the idle and it’s perfect. I wIll note that after the afterglow goes away, and before the engine is full hot, it will slightly show some white smoke. I do need to verify timing.

Any thoughts on what this might mean? A leaking injector perhaps?

I'd lean towards a single dead glow plug. I've had this behavior in the past and that's always been what it was. A healthy 603 will start with no glow plugs, although kinda lumpy all the way down to freezing. A blip of the throttle will usually smooth it out.

Wouldn’t a dead or bad GP cause the light to signal an issue??

I know they aren’t bad because they are brand new with less than 2 hours and the resistance is the same on all six. I literally just tested them because I had one out when messing with my prechamber due to diagnosing misfire.

They’re Beru. But maybe one has some other issue?

I also notice that sometimes idle reads really low. But the eds works fine. And it’s not related to this. The tach reads appropriate rpm at this cold start shake.

This is extremely common with older tractors, especially indirect diesels and I've never really questioned why, I usually start them with a bit of throttle to get the oil pressure up and then idle back anyways.

Something sticking in the IP or injectors under low flow conditions? Run some diesel purge through it, can't hurt?

Combustion chambers need to warm up a bit, carbon on the injector faces messing up spray pattern, oil ran down valve guides and needs to burn off?

Doesn't sound like the motor has that many miles on it though and after a serious top end repair I would assume those problems would have been attended to.

Thanks! That’s reassuring- the engine seems smooth and perfect otherwise after my HG/cam debacle…. The engine only has 116k.

Yes, cleaning the fuel system and getting on fresh diesel will be in the cards.

It does seem like maybe an injector loses its prime or something needs to be worked through…

It's also possible the engine just needs some runtime to get everything seated properly and give good compression/combustion when cold. You said the car's been off the road for a while, it may just need some running in to get the valves and everything worn in to give a nice smooth startup.

Any air getting in the fuel supply?