View Full Version : 2.7 OM612 idle problem

10-15-2017, 05:58 PM
Hi, hopefully I can pick someone's brain here!

I have a very frustrating idle problem with my 2004 sprinter OM612 - it surges up and down by about 1-200rpm, almost down to the point I'm worried about it cutting out. It gets worse when warmed up. I have tried a new filter, injector cleaner and changing the seals in the pressure regulator. I can't see any leaks or air bubbles.

A Mercedes mechanic looked at it - there are no fault codes. The injectors are operating at the same output and apparently it is over-fuelling very slightly. He suggested replacing all of the injectors which is mega expensive and it seems unlikely that all 5 would be faulty at once? Mileage is only 80,000.

Any thoughts what I could try next? I really don't want to start parts changing!


10-15-2017, 06:32 PM
You can do a leak-off test to see if there is one injector that is worse than the others. Rebuilt injectors are around $100 now, I think.
My OM612 didn't have a steady idle, but that was when it was cold. Is it possible you have an electrical issue, and the surging is the alternator kicking on and off? I don't know much about that.

10-16-2017, 01:22 AM
What is the fuel pressure doing? The ECU needs a good fuel pressure signal. The injectors needs a well controlled fuel pressure as well.

10-16-2017, 02:09 AM
A dirty or malfunctioning MAF can also do this unsteady idle, as can a dirty EGR valve, a lose connection of the fuel line to the HP pump too

10-18-2017, 09:06 AM
Hi thanks for the replies...

Another mechanic has now looked at it. Again no fault codes but he told me that the injector values vary quite a lot - he has suggested changing the worst two injectors.

I have no idea what the fuel pressure is or whether it has been checked (I will ask) - if this was bad would it throw a code? Again with the MAF and EGR (I tried cleaning it).

10-18-2017, 09:13 AM
The fuel.pressure regulator can fail but from what I saw, it causes an irregular idle when cold and it clears up when warm.

10-18-2017, 09:33 AM
Hmmm. Mine is actually worse when warmed up?

By the way, I think it's a bit harder to start when hot than it should be. I takes 3 or 4 turns - my dad has a W211 E320 CDI and that starts almost instantaneously.

10-18-2017, 12:00 PM
W211 E320 is a v6, right? If yes, then it's a different injection system. All common rail diesels of this age which I've heard have the same starting "speed"

10-20-2017, 01:27 PM
Well, the mechanic changed the worst injector with a genuine part. The surging is much reduced but still there, I'm comfortable driving it now.

How hard is it to replace the other bad injector? There's no black death or leaks. Is it just a case of remove & refit with new washer & bolt?

10-20-2017, 05:00 PM
In a perfect world, it's almost as easy as changing a sparkplug.
In theory, you simple disconnect the fuel lines and use a socket wrench to remove the hold-down bolt and pull out the injector, clean the injector well and especially washer seat, put in new washer and injector and tighten the hold down bolt to spec (60 INCH pounds plus two 1/4 turns).

In reality, the hold down bolt has a good chance of binding and breaking off in the head, and not near the top, but 3 inches down where the threads engage. Then you are in a world of hurt trying to remove it. In worst case scenarios you need to remove the head and take it to a machine shop to get it out. On the numerous threads here, people have come up with solutions that work. Personally, I had to drill it out and install a 6x1mm helicoil. And it's not just removal that is dangerous, mine broke off when installing it because I didn't clean the threads before putting the new bolt in.

Because this simple procedure can cause big problems, read through all the threads and advices before attempting.

Ok, here's why you don't just use a grade-8 replacement hold down bolt so it won't break: the hold down bolt is long and thin because it's SUPPOSED to flex a bit, acting like a spring to accommodate the slightly changing variables as the head and injector get hot and cold. That way it keeps tension rather than stretching and failing; the difference between a bungee cord and a chain holding a tarp over a pickup bed on the highway.

10-20-2017, 10:11 PM
Given the recent rebuild, the injector should be easy. Read the black death thread for the process. You will need to clean the bolt hole and injector well. A new bolt and seal are mandatory.

10-21-2017, 11:21 PM
Just to be sure we are discussing the proper engine: Is this a 2004? If so, shouldn't you have an OM647? I thought the last year for the OM612 was 2003.

Aqua Puttana
10-22-2017, 04:18 AM
Just to be sure we are discussing the proper engine: Is this a 2004? If so, shouldn't you have an OM647? I thought the last year for the OM612 was 2003.
Only for NAS aka NAFTA land.

The rest of the world can have some different options for their Sprinters. The North American import models are of a specific MB selection. Eg.- 4 cylinder EURO T1N's. The NCV3 (906) model was introduced EURO 2006, not 2007 as for N. America.

:2cents: vic