Yes, but i've removed the exhaust system, and edge extrude it closed, remodelled the exhaust in polygons, and merged it again, and now it's fixed, but i'm just wondering why 2 merged objects (combined offcourse) don't merge together, can it be something with the normals? I dident checked that out, it's to late now, i allready solved it by remodelling it.

But anyways if somebody has a answer to this mistery i would love to hear it, so i can edit it when i encounter this problem again.

Thx