Troubleshooting a 12v92 Detroit Series Marine Main Machine

The 12v92 Detroit series is a two-stroke cycle, V12 diesel engine that was introduced in 1974 as a rebored version of the Series 71 engine. It has a displacement of 18.1 liters and a bore and stroke of 4.84 x 5 inches. It is used for various marine applications, such as yachts, fishing boats, and ferries.

One of the common problems that can occur with this engine is a loss of power and performance when the transmission engages. This can be indicated by a drop in the engine speed (rpm) from the full rpm at idle to a low rpm at cruise. For example, a user reported that his 12v92 engine reaches full rpm of 2100 at idle without any problem, but when he goes on a cruise, the rpm stays at 800 when the transmission engages.

This problem can have several possible causes, such as:

  • A throttle problem. The throttle regulates the amount of air that enters the engine, which affects the fuel injection and the combustion process. A faulty or misadjusted throttle can result in insufficient air supply or excessive air restriction, leading to poor engine performance and fuel efficiency. A throttle problem can be diagnosed by checking the throttle linkage, the throttle body, and the throttle position sensor for any damage, wear, or malfunction. A throttle problem can also be ruled out by disconnecting the throttle cable and manually operating the throttle lever on the injection pump. If the engine speed does not change, then the throttle is not the cause of the problem.
  • A mechanical problem. A mechanical problem can refer to any issue with the engine components, such as the pistons, the cylinders, the valves, the injectors, the turbocharger, or the exhaust system. A mechanical problem can cause a loss of compression, a loss of boost pressure, a loss of fuel delivery, or a loss of exhaust flow, resulting in reduced power and performance. A mechanical problem can be diagnosed by performing a compression test, a boost pressure test, a fuel pressure test, or an exhaust backpressure test, depending on the suspected component. A mechanical problem can also be indicated by abnormal noises, vibrations, smoke, or leaks from the engine.
  • A transmission problem. A transmission problem can refer to any issue with the gearbox, the clutch, the torque converter, or the propeller shaft. A transmission problem can cause a loss of torque transfer, a loss of speed synchronization, a loss of gear engagement, or a loss of propeller thrust, resulting in reduced power and performance. A transmission problem can be diagnosed by checking the transmission fluid level, condition, and pressure, the clutch adjustment and operation, the torque converter lockup and operation, and the propeller shaft alignment and condition.

To troubleshoot this problem, the user should follow these steps:

  1. Check the throttle for any problem. Disconnect the throttle cable and manually operate the throttle lever on the injection pump. If the engine speed changes, then the throttle is working properly. If the engine speed does not change, then the throttle may have a problem. Check the throttle linkage, the throttle body, and the throttle position sensor for any damage, wear, or malfunction. Replace or repair any faulty parts as needed.
  2. Check the mechanical components for any problem. Perform a compression test, a boost pressure test, a fuel pressure test, or an exhaust backpressure test, depending on the suspected component. Compare the test results with the specifications in the service manual. If the test results are within the specifications, then the mechanical components are working properly. If the test results are below the specifications, then the mechanical components may have a problem. Check the pistons, the cylinders, the valves, the injectors, the turbocharger, or the exhaust system for any damage, wear, or malfunction. Replace or repair any faulty parts as needed.
  3. Check the transmission for any problem. Check the transmission fluid level, condition, and pressure. If the fluid level is low, add more fluid as needed. If the fluid condition is dirty, burnt, or contaminated, change the fluid and filter as needed. If the fluid pressure is low, check the pump, the valve body, and the solenoids for any damage, wear, or malfunction. Replace or repair any faulty parts as needed. Check the clutch adjustment and operation. If the clutch is slipping, worn, or damaged, adjust or replace it as needed. Check the torque converter lockup and operation. If the torque converter is not locking up, slipping, or damaged, replace it as needed. Check the propeller shaft alignment and condition. If the propeller shaft is misaligned, bent, or damaged, align or replace it as needed.

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