Our ML350 has been experiencing intermittent power limitation. After some period of driving, usually less than 30 minutes, it'd start shifting at or below 1500rpm. This results in no boost, and thus no power. Turning off the car and starting it back up gave some period of time where it behaved normally, after which it would again limit RPMs and thus limit power.

I'd been troubleshooting using my BlueDriver OBD2. Not ideal, but it did give me a view into codes. I was seeing an intermittent pending code, a P0405 indicating an EGR valve issue (EGR Sensor "A" Circuit Low). As a pending code it didn't set the CEL.

Making things more complex, our ML350 had recently shredded the tensioner pulley, which snapped the drive belt. That logged a ton of "voltage low" codes that have taken their sweet time to finally clear. I know low voltage can cause all sorts of fun issues, I confirmed the alternator is solid (14.8v), battery is solid (2 years old, measures 12.6v and tests over 800CCA vs. the 850 rating). The "voltage low" codes stopped logging after I was able to get in some driving time to help with diagnosis (below).

I did some testing yesterday to see if I could confirm the problem. I suspected the EGR, but needed to link the code to the issue. After about 15 minutes driving it started shifting at 1500rpm or so. I stopped immediately, found the P0405 code. I turned off the truck, cleared the code, restarted, drove off normally. A few minutes later, same issue - except the CEL set this time. New codes, P0404 (EGR "A" Control Circuit Range/Performance), P042E (EGR "A" Control Stuck Open), P042F (EGR "A" Control Stuck Closed). Cleared these, it immediately set the P0404 and P042F, CEL lit.

Seems certain there's a problem with the EGR Valve. I plan to pull it apart once I get the turbo inlet seal, PCV seal, and turbo outlet seal later this week. I plan to clean and test my EGR, reinstalling if functional and replacing if it fails. That in mind, I'm not sure how to bench test the EGR on this car beyond basic verification the valve moves smoothly. I don't have an MB tool and my BlueDriver doesn't have the ability to trigger. Anyone have information on how to test this EGR?

The EGR for the W166 OM642 seems to be unobtainium, at least online and my local auto parts shops. I've not checked my dealer yet, I will tomorrow. If mine ends up failed, I'm also interested in where else I may be able to find one.

Thanks!

 

You can remove EGR from the Intake and with the connector still in place, ignition On, ignition Off and so on. You can start the engine too, with the EGR removed ( if you do that in open air), for testing.

But...you should place the order, as the availability is very limited.