Coolant flows first through the oil cooler AND THEN through the EGR cooler. Sand will plug up the oil cooler (small passages) on the coolant side and then stop coolant flow to the EGR cooler. Both coolers will fail this way, but it is the sand in the oil cooler that is the originating failure mechanism.
Now I guess it is possible that if the EGR cooler fails first, that you could get some exhaust gasses into the coolant and this could cause "two-phase" flow. This type of flow will cause extra pressure drop and consequently restrict the coolant flow somewhat. If the coolant flow is restricted, it will impact the oil cooler also. The issue I have with this theory is that it is much more common for the coolant to be at higher pressure and therefore the coolant will flow through a leaking EGR cooler, mix with the exhaust gas and flow into the intake.
Edit (to clarify I guess):
The second paragraph was just "theorizing" how it could be possible for a failed EGR cooler to cause problems with the oil cooler. The first paragraph (oil cooler failing first) has been proven to happen quite frequently.
Last edited by bismic; 04-08-2009 at 06:04 PM.