81 300SD 180 miles:

Upon Cold Start-up my exhaust is black [obiviously], for about 1-2 seconds. Then it becomes pretty clean and starts to become invisible [at idle] once it warms up. Apparently , although I can't see it im my side mirror, my exhaust becomes noticable agian at about 2500 rpm when crusing on the freeway. My shadetree mechanic friend claims that might be an indication of a faulty injector pump $$$. My engine sounds and runs pretty smooth.

Should the exhaust be slightly visible at 2500-3000rpm warm or should it be practically invisible.

Thanks in advance for your time and expertise.