Black smoke and power loss under load on a Cadillac CT4 diesel usually mean too much fuel or not enough air.
EGR, DPF, turbo, and sensor faults are the first places a good tech will look.
Why your CT4 diesel is smoking and feeling flat
When a diesel loses power under load and rolls black smoke, it is almost always an air-to-fuel problem. Black smoke is unburned fuel, so the engine is either getting too much diesel or, more commonly, not enough clean air. On a Cadillac CT4 diesel, the usual suspects are a clogged EGR valve or EGR cooler, a partially blocked DPF, a sticking turbo VNT actuator, a leaking charge pipe or intercooler, a dirty MAF or MAP sensor, a restricted air filter, or injectors that are over-fuelling. EGR and DPF clogging often travel together because carbon builds up in both places, especially with short trips and stop-and-go driving.
