A persistent misfire in a 2020 Cadillac CT6-V has already survived multiple repair attempts and more than $10,000 in work, making the decision to return to basic diagnostic principles especially important. Rather than immediately replacing another component, Dave and Kyle begin by confirming the complaint, scanning for codes, examining misfire counts and fuel trims, and considering mechanical compression. That methodical opening gives the repair story credibility even while both technicians openly speculate about what they expect to find.
The scan data provides useful direction without producing an instant answer. Misfire history appears across several cylinders, while long-term fuel trims are substantially positive, particularly on one bank. Kyle checks sensor readings, fuel-pressure data and possible intake leaks, including monitoring fuel trims while introducing carb cleaner around potential leak areas. The explanations of why he creates a smaller custom data list for faster updates and why the engine should reach operating temperature before judging fuel trims make the diagnostic process accessible without oversimplifying it.
Dave suspects carbon deposits on the intake valves, while Kyle initially considers a sensor problem, and their willingness to disagree is one of the more engaging elements. Neither possibility is treated as established simply because someone suggested it. Once the available sensor data appears reasonable and an obvious vacuum leak is not found, the carbon-deposit theory receives more attention because of the direct-injection design and mileage, but it remains a hypothesis that still needs physical confirmation.
The Cadillac's unusual 4.2-liter twin-turbo V8 makes that confirmation considerably harder. The discussion of the intake and exhaust arrangement, tight packaging and apparent requirement to remove the engine illustrates how vehicle design can turn a relatively straightforward inspection into a major operation. Dave's insistence on checking compression before authorizing that work is therefore sensible within the diagnostic process shown: if the engine has a fundamental mechanical problem, committing to extensive disassembly based primarily on the carbon theory could send the repair down another expensive path.
The conversation with the owner adds important context, including his account of previous spark-plug, injector, seal, wiring and other work that failed to eliminate the complaint. Dave uses the situation to argue that shops should confirm a diagnosis and verify that a repair actually corrected the original problem rather than simply replacing parts. Some of this becomes repetitive and occasionally shifts from diagnosing this particular Cadillac into broader commentary about repair-shop practices, but the underlying emphasis on confirmation before and after a major repair is consistent throughout.
Removing the plugs finally produces the episode's most important concrete result: compression readings are reported at roughly 145 to 150 psi across all eight cylinders, with the cylinders described as being within 5% of one another. Kyle also discovers a damaged coil connector apparently being held with silicone, which introduces another potentially relevant issue that deserves further investigation. The strong compression eliminates one major concern, but the central diagnosis remains unresolved; the engine still has to be removed before the suspected intake-valve deposits can be inspected, leaving the actual cause and successful repair for the continuation.
Pros
- The diagnostic process begins with fundamentals rather than immediately replacing additional parts.
- Scanner data, misfire history, fuel trims, leak checks and compression readings give viewers specific evidence to follow.
- Dave and Kyle openly consider competing explanations instead of presenting an early guess as a confirmed diagnosis.
- The unusual engine packaging provides a useful explanation for why inspecting the suspected intake deposits is unusually labor-intensive.
- The compression test delivers a meaningful result before the shop commits to engine removal.
- Discovery of the damaged, silicone-secured connector shows the value of careful physical inspection alongside electronic diagnosis.
Cons
- The suspected carbon buildup remains unconfirmed by the end, so the episode stops before establishing the actual cause of the misfire.
- Several explanations about compression testing and the importance of confirming repairs are repeated at greater length than necessary.
- Broader criticism of previous repair shops sometimes distracts from the otherwise disciplined focus on diagnosing the Cadillac.
- The damaged coil connector is potentially significant but cannot yet be meaningfully connected to the multi-cylinder misfire.
The combination of a rare performance engine, extensive previous repairs and a stubborn misfire creates a strong diagnostic mystery, and the methodical testing makes the investigation more substantial than a simple parts-replacement story. Strong compression narrows the possibilities without prematurely solving the case, making this an informative first half of a repair whose ultimate diagnosis still needs to be demonstrated.
