I've got an ML350 in (241k miles) that I've been helping the owner get in good nick. The customer indicated that he thinks that the truck is "shuddering" at idle sometimes. When I was watching actual values the intake manifold pressures were occasionally out of spec. Could this be an indication of plugged cats?

Is there an easy way to check for plugged cats? I am not getting any fault codes. I do plan on changing the engine/transmission mounts.

Other than the shuddering it looks like it probably uses a little oil and it runs/drives out good.

tia

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Jim