My "new" 3.0 OM603 is back in operation and everything seems good with one exception- once the engine is warmed up, the idle hunts. From a cold start, it idles smooth and steady for about 3 minutes. At this point it starts to cycle predictably between appro. 750-800 rpm, non-stop. We static timed the IP, and I have a request in to rent the A-B lights from a member here. Vacuum lines are all connected accurately according to the diagram. Any thoughts?

Thanks!

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Chief

1991 350SD

My 603 would exhibit similar behavior on occasion. About the time I would begin to get concerned, the behavior ceases.

I, too, would like to know what governs such behavior..........other than the obvious...........worn components in the IP.

I just had this IP overhauled by the Bosch shop in my area, and all calibrated well!

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Chief

1991 350SD

There is another variable other than the governor flyweights. The variable is much larger than the governor would typically see and the governor struggles to compensate for it.

Hard to narrow down from that!

The most annoying effect of this is that the car rocks up and down at red lights, almost like it's breathing!

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Chief

1991 350SD

I'll propose one possibility:

The engine will tolerate quite a bit of air in the fuel lines............but, not an unlimited amount. If air is getting in via the clamps for any reason, the issue should be addressed.

You can tell if you have air if you replace the very darkened factory hose with the banjo that runs from the fuel pump up to the filter. A new hose in this location will immediately tell you if air is present in the fuel.

Frustrating type of thing. I know nothing about the injection pump on the 603. Other than I thought there was an electrical idle control. What happens when this is disconnected?

A wire going to the injection pump I believe. One could monitor the voltage there and substitute a fixed one if required. That was if that voltage was fluxuating rapidly when warm but not cold.

I also wonder if the rack damper bolt needs adjusted if they have one. Mark your starting position so you can get back to where it was if of no help. Just a couple of things that perhaps the average guy can check out.

You also have more experience than the average guy. I doubt there is anyway to detect this type of fault with an injection pump on a test stand. I might try disabling one injector at a time to see if the problem stops. I doubt it will but once again for the time spent.

Basically just some easy quick managable shots in the dark. That probably will show nothing. Brians thoughs on air ingestion has a better possible chance. With air I would expect more of a constant rough idle though. Check your overflow for fuel expulsion as well. If for some reason the pump was being partially starved of fuel it might be erratic I suppose.

Also I do not know if it could create this type of problem. Still would want to make sure the egr valve is not acting funny. It normally does not activate when first starting the engine. I think there is a thermal control. This is a really an outside chance type of thing.

The workshop has not adjusted the damperspring correct.

What about a faulty coolant temp sensor? The EDS module changes idle speed based on coolant temp...

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

I'm still running down the cause of my idle fluctuation. I replaced all the fuel lines, using oem including the molded injector return and the thermostat to lift pump line, and the o-ring in the fuel thermostat which was loose, and leaking. I'm confidant that no air is getting into the system. When starting the car from cold, the engine runs and holds steady at about 650 rpm. I let it sit this way without driving and i could just detect the idle start to hunt when the temp guage hit 60. It became more pronounced and stayed that way.

While driving, every once in awhile it will idle steady, but usually hunts. Under load, that all goes away.

Compu_85 mentioned the temp sensor tie-in. Is this something that might show up in a code via the OBD?

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Chief

1991 350SD

I tried this again today. Started from cold, idled steady for approx. 5 minutes. At this point it started to hunt- the temp guage was reading 60. So my previous question is still the same- likely temp sensor tie-in, and would this throw a code on my OBD?

Thanks!

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Chief

1991 350SD

OK, silly question. If you disconnect the temperature sensor, so the engine thinks the temperature is back below 60, does the hunting stop?

A good (and in retrospect obvious) suggestion. Will do tomorrow.

__________________
Chief

1991 350SD

So my car has started doing this as well. Does the 603 have a rack damper like the older motors?

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

There is a full test diag in the FSM for the ELR.

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