Puzzling problem with Mercedes OM651

[JohnC]

I have a Mercedes GLK250 with the OM651 engine. I'm getting a code for excessive EGR flow and it sets at idle or during decel. We've tried a number of things, and the only fault left is a slight leak around a shaft in the EGR cooler bypass valve when smoke testing the intake. (Red arrow in photo.) I'm wondering if the manifold is under vacuum under those conditions and it could be drawing in air where we see the smoke. (It's a very small amount of smoke.)

Is there something we could spray on the area of the leak to see if it's being drawn into the engine? I was thinking propane? It would be easy of it were a gas engine...

[a5150nut]

Maybe a short shot of starting fluid. Who's going to ride the fender to check under deceleration?

[DmaxMaverick]

Propane or butane. Starting fluid/ether can pool then ignite explosively, especially on a hot engine, and may also trigger the knock sensor, if it has one. If you're seeing smoke there, no telling how much excess air/gas it's sucking in operation. I believe the intake is throttled, and should have strong vacuum at idle and decel, although the intake vacuum should be less restricted during decel. In any case, the incidence of the most leaked-in air would be at idle or deceleration.

In all likelihood, if the cooler valve is leaking at the shaft, you can't repair it (looking at the pic). You can pack it with silicone grease or PTFE paste, which may slow it enough to get under the sensor range. A shaft leak may be minimal or insignificant, and an internal leak is a much more common point of failure. Probably not an inexpensive repair, if it can't be "cleaned" or bandaged. I doubt you'd get a code for excessive EGR flow if fresh air is leaking in. More likely a MAF sensor error. EGR gas is exhaust gas, containing much higher levels of CO/CO2, which hinders/cools combustion.

[JohnC]

I need EGR almost as much as I need that stupid throttle plate! The EGR cooler and the plastic intake manifold were all clogged up with carbon.

The valve doesn't appear to be repairable. It's all swaged together. However, it's easily fixed by applying $400 directly to the problem.

Who's going to ride the fender to check under deceleration?

I think we'll try it at idle first.

[DmaxMaverick]

It was hashed out decades ago: EGR is a ecological and economical net loss. From the adaptation and control engineering, materials, component manufacture process, combustion efficiency (fuel economy) loss, to repair, disposal/recycle, it never returned a positive. We will find the same for DPF and DEF, in the end. "Rolling Coal" is a thing of the past, save a few ignorants, probably compensating for something. Almost no one is opposed to clean(er) air, but simply throwing money at a problem for short-term feelgoods isn't a solution for anything. If it's any consolation, California thanks you for your contribution to this boondoggle.

[JohnC]

If it's any consolation, California thanks you for your contribution to this boondoggle.

Had I known that would happen I probably would have bought something different.

[DmaxMaverick]

Had I known that would happen I probably would have bought something different.

You mean like something 1960's or earlier? The problem is the mandates and the nature of the concept. We still don't have the technology to do it right at an acceptable expense, but the political mandates require us to use what we have, despite the efficacy. Every vehicle model with smog equipment will have failures. Some more than others, some less than others, but no model is immune. A $400 repair cost isn't surprising, at all. If you want to keep a vehicle a long time, replace expensive parts with long/lifetime warranties, and don't lose the receipts.