The SD? If so you may want to adjust, or replace the Rack Damper bolt. It resides on the back of the IP, just in front of the oil filter canister. Do this with engine at operating temp while running. 14mm lock nut and a 12mm adjusting bolt. Loosen lock nut, back out screw till you have a smooth idle. I had the same problem on mine (SD), replaced bolt with a gold colored one, worked fine for a few months, had to pull bolt and re-seat "o" ring. Problem solved. The spring in those bolts wear out over time. This is far less trouble than dealing with the IP. The best way to determine if the injectors are working properly, is to have them tested, preferably at a Bocsh authorized shop, or just about any place that works on diesel trucks. Hope this helps.

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83 SD

84 CD