Hi Ironline: I've been ripped on a bit for using AI to supplement diagnostics. The reality is that if you ask the right questions to an AI search engine that search will pull up everything relevant online from sites like this, users manuals, and mechanics' forums. I dropped your text into Gemini and this is what it generated. If anything here helps then great. And, BTW, I did this once before and the AI search said for another forum member the most likely problem for his error code was a mouse chewing through a wire or bad connection (my Kubota mechanic had said the same thing). The forum member eventually found a bad connection. Hope something in this search helps your diagnostic journey. Best, A Gemini Search Response You are dealing with an incredibly frustrating but fascinating diagnostic puzzle. Your systematic approach is excellent, and your skepticism regarding the "low compression" diagnosis is completely justified. A diesel engine with low compression across all cylinders does not start easily, run smoothly, or produce zero smoke under load—low compression causes hard starts and heavy white/blue smoke from unburnt fuel. Here is the direct analysis of your breakthrough: Backing out the regulator screw 4 turns and restoring operation proves your hydraulic pump is functionally capable of destroking, but it strongly points to a failure in the hydromechanical control logic that signals the pump when to destroke. You have shifted the pump's torque regulation curve down. This prevents the pump from demanding more horsepower than the engine can currently provide, stopping the stall. Here is what this breakthrough reveals, along with the most likely culprits behind your symptoms. 1. The Regulator Breakthrough: What it actually means Your pump uses a torque-limiting (or horsepower-regulating) control . It balances internal system pressure against a spring-loaded spool to destroke the pump as pressure rises, ensuring the hydraulic horsepower (Pressure × Flow) never exceeds the engine's available mechanical horsepower. By backing the screw out 4 turns, you lessened the spring tension. This allows the pump to destroke at a lower pressure threshold or a faster rate than factory spec. Why it works now: The pump is no longer overloading the system. The Catch: Since you are still hitting full relief pressure, the pump can max out its pressure, but it is doing so at a drastically reduced flow rate (destroked state). This leaves two distinct possibilities: either the pump's internal regulation mechanism is getting an incorrect pilot/signal pressure , or the engine is quietly down on torque without showing the classic signs. 2. Primary Suspects Based on Your Data Suspect #1: The Negative Flow Control (NFC) or Pilot Signal Circuit Many CAT mini excavators utilize a Negative Flow Control (NFC) or pilot-derived regulating system. When all control valves are in neutral, a high pilot pressure is sent back to the pump regulator to force it to minimum displacement (destroke). When you stroke a function, this signal pressure drops, allowing the pump to upstroke and deliver oil. The Theory: If a spool in your main control valve is slightly sticking, or if the control selector (CAT/Bobcat pattern changer) has an internal bypass leak, a hydraulic circuit could be "deadheading" or sending a false signal to the regulator. The Thumb/Auxiliary Clue: You noted that cycling the thumb showed full operating pressure on both P1 and P3 independently. If a relief valve or resolver shuttle valve in the auxiliary circuit is leaking or stuck, it could be forcing the pump's regulator to remain at or near full displacement (upstroked) when it shouldn't be, instantly stalling the engine when a heavy load is applied. Suspect #2: The Regulator Spool / Servo Piston (The First Mechanic's Idea) The first mechanic may have been directionally correct, even if his air-pressure test method was flawed. Inside the regulator, there is a small control spool that directs internal system oil to the large servo piston (the one that physically changes the swashplate angle). If the regulator spool is sticky, chemically varnished, or has a broken internal centering spring, it will lag. When you demand heavy power, the pump upstrokes but fails to destroke fast enough to match the engine's torque curve, dragging the engine down to a dead stall before the mechanical forces can balance out. By backing out the screw, you gave the weakened or binding mechanism an easier mechanical advantage to overcome the lag. Suspect #3: The Engine is Down on Torque (The "No Smoke" Clue) Your observation that there is no black smoke when the engine bogs down is highly significant. Normally, when a diesel engine is heavily overloaded by hydraulics, the governor demands maximum fuel. This excess fuel results in a heavy lug and distinct black smoke. The Theory: If the engine bogs to a complete stall with zero smoke, it means the engine is not getting enough fuel under load, or the governor is failing to react to the drop in RPM. Because you checked the entire low-pressure fuel system, the issue could be the governor linkage, the fuel injection pump internal rack, or an electronic engine speed sensor/limiter (if equipped) failing to tell the injection pump to give it full rack travel when RPMs drop. The engine isn't weak due to compression; it's weak because it's being starved of high-load fuel delivery, making it easily defeated by a factory-set hydraulic pump. 3. Recommended Next Steps (To Avoid Buying a Pump) Before dropping thousands on a new pump, perform these targeted checks: Test / Check What to Look For Purpose Isolate the Auxiliary/Thumb Circuit Mechanically cap off the auxiliary/thumb hydraulic lines right at the control valve. Rules out a hidden, deadheading auxiliary circuit or leaking resolver valve causing the dual P1/P3 pressure spikes. Inspect Pattern Changer Valve Cycle the CAT/Bobcat selector back and forth firmly, or temporarily bypass it. Internal leakage in these switch valves is famous for cross-communicating pilot signals and confusing pump regulators. Monitor Injection Pump Rack / Governor Visually watch the fuel shutoff/governor linkage while someone deliberately lugs the machine down. Ensure the linkage is pulling forcefully toward the "full fuel" stop when the engine begins to bog down. To help narrow this down without costing you more diagnostic fees, tell me: When the engine stalls, does it die instantly (like turning off a key) or does it slowly groan down over 2-3 seconds ? Does your 302.7 D have an Eco-mode switch or an auto-idle feature , and do those features work correctly?