To summarize, My 2015 302.7 D has been down for the count for about 2 months. Myself, two independent mechanics, and CAT have not been able to diagnose or fix my problem. Machine has 1100 hours. Symptom: Machine Starts good, runs smooth through all rpm, and no smoke. During travel or when holding a function against a relief or heavy digging the engine bogs down to a stall. Summary of diag: clean fuel tank, fresh fuel, replace fuel lines, clean pre-filter, replace fuel filter, test lift pump pressure and flow, test lift pump and return check valve, new air filter, verify tank breather is not plugged, clean and inspect Banjo bolts, bench pop test injectors, Disassemble and inspect main hydraulic pump, new hydraulic fluid and filter, disassemble and clean all relief valves, remove head and inspect engine condition, vacuum test head, set valve lash. This problem appeared at 1100 hours, after cooling off the machine it was ok for the rest of the day. The following day it returned on startup and has since never been resolved. My intuition told me it "felt" like the hydraulic system wasn't properly unloading. There is not black smoke when loading down so it led me to go through the fuel system first. I had one independent mechanic condemn the main hydraulic pump. he stated that the plungers responsible for compressing the spring on the pump regulator were worn and not allowing the pump to destroke. Although this would make perfect sense, I had two reasons for disagreeing. 1. his method of diagnosing this was to put compressed air behind the plunger, when it allowed air to bypass and not compress the plunger, he claimed it was worn out. 2. I visually inspected the plungers and bores and did not see any signs of wear or scoring. My main disagreement is that air pressure will leak past much easier than hydraulic fluid, the spring it needed to compress was very large heavy and after talking with a few equipment mechanics they agreed it was not a solid way to diagnose this. I then took it to CAT and they essentially walked through the same diag I did at home believing that it seemed like a fuel problem but did not find anything wrong with the fuel system. They then came back with a diagnosis of low compression on all 3 holes at 300 psi. Since I dont have a compression tester that will go up to 500 psi or adapt to my threads I took their word for it and pulled the head, The engine looked brand new inside, Factory crosshatching was excellent and the head sealed as if it was new on a vacuum test. Hardly any carbon buildup led me to believe rings were not stuck and the cylinders all held an acetone/ATF mixture over 24 hours. Given my smooth starting, running, and no smoke I have determined the engine is very healthy. After re-assembling everything I put hydraulic gauges on it. Even though cat said everything was in spec I wanted to verify. Couple oddities prompt me to want to perform another test. Some of the hydraulic functions did not align with their test port according to SIS. Assuming this could be due to the control selector for changing from cat/bobcat controls? There was no information in SIS regarding what test port the auxiliary lines use. I found it odd that when cycling the thumb in either direction it showed full operating pressure on P1 of the primary and P3 of the secondary pump. These were tested one at a time not simultaneously. I did have a theory that a hydraulic circuit could be deadheading so I found this interesting and plan to investigate further. Largest breakthrough I have found is that after backing out the main pump regulator screw approximately 4 rotations the machine is almost back to normal and still hitting full relief pressure (maybe the first mechanic was right after all regarding the pump not properly destroking) however; by backing out the regulator and allowing it to destroke sooner it proves that the pump it still capable of destroking so I'm not entirely sold. If something in my write up flagged your attention please comment but my main question it this: Based on backing out the pump screw restoring operation what can you tell me based on that information? Ive read that sometimes when hydraulic pumps wear the screw has to be backed off over time. Interestingly I found a 500 and 1000 hour oil analysis through CAT from the previous owner that showed elevated wear levels in the hydraulic fluid that CAT claimed were normal during break in but advised to monitor and they did significantly improve on the 1000 hour test. The hydraulic fluid was clean and the pump looked very healthy inside. My understand is the only way to properly set that regulator screw requires a flow tester to run the conversion to match hydraulic HP to engine HP. Even then if I were to confirm that it would be hard to know if the pump is the culprit or the engine is down on power (assuming the original screw position was the correct one). I am very lost at this point and I'm tempted to throw a new pump in but I'm already upside-down in diag costs for this machine. Any help on this is hugely appreciated.
Isn't this the same machine you posted about before.? Not quite sure why it merits a new thread for the same problem. HeavyEquipmentForums
Are you the forum police now? Do you plan on helping or just here to be obnoxious?
Multiple threads on the same subject just confuses everybody.
You either saw the original post that was over a month ago and know the story or didn’t see it and now are up to speed. Nothing confusing about it. No one bothered to read my last post anyway given all the suggestions were telling me to do things I had already tried. Post went dead so I make a new one. Big deal
Here is my Humble opinion, The Nige asked you a question and instead of answering the question you chose to be argumentative. The Nige, who is very knowledgeable along with many others on this board, are here to help others. If this board does not suit you, you did not pay anything to join. I am VERY pleased to be on this board, have received a tremendous amount of information, and, I hope I can be helpful to others. This is my opinion and your mileage may vary.
He was being obnoxious and cluttering the post with his crap. You say he is very knowledgeable yet what value does that comment bring? Everyone likes to hide behind a keyboard and bitch yet no one has commented anything to help
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?