17549 load cross check implausible could be caused by anything that will effect the load data (calculated value). One of the main inputs to this calculated value is the MAP sensor. A high MAP signal will also cause it to run rich and so could cause the other code which is for fuel trim rich. I think you have taken the codes too literally without looking at what will cause them to set. I would be looking for a mechanical issue which is effecting manifold vac.
On that engine a worn timing chain can cause this.