On my 83 300SD, I recently started having a problem that concerns me. It has started to run rough at highway speeds especially when its warm and humid.

Starts and purrs when the engine is cold. Good power and pretty smooth running with very little grey smoke.

After it warms up and over the course of a day, it really starts to run rough. More like a flat thud miss and not a knocking or nailing. I can cruise at 60 mph but anything above that causes concern. I have pulled over and revved the engine up to rpm but can not identify anything amiss. Fuel filter is golden and full.

The fuel filters have been changed more than regularly. This is a daily driver with many stops and in Dallas-Ft. Worth traffic. Oil and air filters get changed every other month. I need to adjust the valves but am thinking that I would notice more problems when cold if they were the problem. No black/white smoke of any kind and no coolant loss.

Since this mostly occurs above 60 mph, I have had to scratch my head on this one. Am I headed in the right direction? Are there other tests?

I have read on this forum about injector heat shields. What exactly do these look like? Since I bought this car used I have found a few other mechanic "short cuts" that have caused me problems and I suspect they may be missing.

I appreciate your help. BTW this forum is the best thing to come along for DIYers.