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The chart below shows the most common questions asked by owners of Detroit Diesel® engines 12V71 Natural when trying to troubleshoot why their engines underperform.  It categorizes areas of possible malfunction, likely cause and recommended action to bring your new, rebuilt or used  Detroit Diesel® engine back to proper operation.  The chart will give you confidence when speaking with your Detroit Diesel® engine mechanic or ordering parts.  As a quick reference guide only, the chart must not be thought of as a substitute for the use of your Detroit Diesel® engine 12V71 Natural Owner's operating or service manual.

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Engine will not start Lack of air intake or restricted air intake
Broken blower shaft  
Worn out blower shaft  
Defective fuel pump  
Defective starter  
Discharged battery  
High exhaust back pressure  
Black smoke coming out of engine Defective injector  
Contaminated fuel  
Broken or worn cylinder parts  
Engine overloaded  
Faulty intercooler or aftercooler (for turbo engines only)  
White smoke coming out of engine Broken piston ring  
Incorrectly installed ring set  
Defective injector  
Fuel quality or grade  
Long idle periods  
Broken or bent push rod  
Injector or governor improperly adjusted  
Low engine oil pressure Water in engine oil pan  
Fuel in engine oil pan  
Defective oil cooler  
Engine overloaded  
Crankcase overfilled  
Low engine RPM Improperly working injector  
Governor calibration  
Hi engine RPM Governor calibration  
Engine overheating Defective fresh water pump  
Defective raw water pump (marine engines only)  
Defective thermostat or thermostat seal  
Clogged radiator or heat exchanger (marine engine only)  
Grey smoke coming out of engine Long idle periods  
Defective intercooler or aftercooler (for turbo engine only)  
Restricted air intake  
Defective injector  
Contaminated fuel  
Blower or turbocharger seals leaking  
Geartrain makes noises Low oil level  
Engine driven accesories  
Damage main or rod bearing  

Detroit Diesel 12V71 Usefull Information 

We sell high-quality Detroit 12V71 parts that you can rely on. That is how we have grown from a small business to one of the leading providers of replacement parts for these tough engines. We have had the pleasure of serving many satisfied customers over the years, and believe that if you try us, you will become a customer for life. So whether you have this engine, or another Detroit Diesel engine, you have come to the right place. All of our products are guaranteed to work, and live up to their promise of being truly premium products. That means no more wasting money on used parts that are rusty and breaking apart, no more unreliable classified ads, and no more wasting time trying to search for a part number. Just straight forward information, clear product descriptions, and helpful pictures to make selecting the part you need fast and easy. We also offer a well-trained staff of Diesel experts who can help you with locating your engine type, placing an order, or anything else you might need. Try us today, and you will see Diesel Pro is the best way to buy parts online.

The Detroit 12V71 goes by many names, and has been used in many applications. To clear up any confusion, please locate the model number. The number should be similar to 7122-7201. The first number indicates the series, and the next two digits represent the number of cylinders. All 12V71 engines should start with 712x-xxxx. Of course, if you need any help, just give us a call and we will get the right part to you.

What we refer to as the Detroit 12V71 is also known as the GM 12V71 by some. These engines are the same and the parts listed here will work inside it.

12V71 Specs

This engine has the following properties:

Two cycle 12 cylinder engine

Bore: 4-1/4 (4.25) inches

Stroke: 5 inches

Compression Ratio: 17:1

Total Displacement: 851.2

7 Main Bearings

The maximum size of the block bore for a standard cylinder liner is 4.628 inches (bore dimensions of 4.625 to 4.6275)

The maximum size of the block bore for a .005 inch oversize cylinder liner is 4.633 inches (bore dimensions of 4.6315 to 4.6325)

The maximum size of the block bore for a .010 inch oversize cylinder liner is 4.638 inches (bore dimensions of 4.6365 to 4.6375)

The maximum size of the block bore for a .020 inch cylinder liner is 4.648 inches (bore dimensions of 4.6465 to 4.6475)

The maximum size of the block bore for a .030 inch cylinder liner is 4.658 inches (bore dimensions of 4.6565 to 4.6575)

Fuel Consumption, Horsepower and Torque

All horsepower and torque based on an ambient temperature of 85 degrees F, at an elevation of 500 feet. Horsepower is measured as brake horsepower or, for marine engines, shaft horsepower.

With N60 injectors:

Torque ranges from 1130 LB FT at 1200 RPM to 990 LB FT at 2100 RPM.

Rated horsepower ranges from 260 at 1200 RPM to 396 at 2100 RPM.

With N65 injectors:

Torque ranges from 1161 LB FT at 1400 RPM to 1080 LB FT at 2100 RPM.

Rated horsepower: 310 at 1400 RPM to 430 at 2100 RPM.

Marine Engine, intercooled, with N70 Injectors:

Horsepower, power-propeller load: 225 at 1600 RPM to 510 at 2100 RPM.

Fuel consumption, power-propeller load: Ranges from 11 gallons per hour at 1500 RPM to 27 gallons per hour at 2100 RPM.

Marine Engine, intercooled with N75 injectors:

Horsepower, power-propeller load: 180 at 1600 RPM to 555 at 2300 RPM.

Fuel consumption, power-propeller load: Ranges from 9 gallons per hour at 1500 RPM to 31 gallons per hour at 2300 RPM.

Recommended Maintenance

The Detroit 12V71 Engine will last for a long time with proper maintenance. That is why it is important to only buy high quality replacement parts, to make sure the engine keeps running as it should. Here is a list of recommended maintenance, with instructions for some basic procedures.

• Change the oil filter element every 100 hours. To remove and install a new filter element, use the following steps as a guide.

Step One: The filter will be in a cylinder on the side of the engine, with two tubes leading into it, one for oil inlet, and one for the outlet. Locate and remove the drain plug at the filter base. Drain all of the oil from the filter.

Step Two: Take out the center stud, which will be a long bolt like object, and take off the shell surrounding the shell. Take out the filter element and discard.

Step Three: Clean all of the oil from the shell. At this point, the spring or retainer, or any similar parts, can be removed by removing the retainer ring first.

Step Four: Replace the gasket and put in the new filter element. Be sure to place the filter element on the center stud. When tightening the center stud, make sure not to damage the gaskets.

• Check Temperatures Every 1000 Hours

Step One: Check the engine oil temperatures. Do this with an engine that had been running to get an accurate measurement, but stop the engine when using the thermometer to get the temperature. For the 12V71 Detroit, the oil should be between 160 to 185 degrees Fahrenheit. If it is more than 60 degrees higher, then the oil cooler might be clogged or otherwise need to be replaced.

Step Two: Clean the cooling system with a radiator cleaner. After cleaning, rinse with fresh water, with a rust inhibitor. Connect all drain cocks and valves before starting engine again. Make sure to inspect all hoses connected to this system, and replace if worn.

• Replace Fuel Filter Every 300 Hours

Step One: Like the oil filter, the fuel strainer and filter will be located on the side of the engine, in a cylindrical housing. To replace the fuel filer, first make sure the engine is stopped. Then, place a drain pan under the unit and open the drain cock on the bottom. Drain all of the fuel from it.

Step Two: Unscrew the the cover nut, located on the top, and remove the shell and the filter element.

Step Three: Wash the shell with clean fuel oil. Dry thoroughly. Then place the new element over the shell stud. Close the drain cock on the bottom. Fill about 60-70% full with clean fuel. Place a new gasket in the shell.

Step Four: Thread the cover nut back onto the top. Remove the plug on the top of the unit and fill the rest of the way with fuel.

• Check Blower and Air Intake System Every 1000 Hours

For the Detroit Diesel 12V71, a supply of fresh air is needed to keep the engine running as it should. Check the blower screen, air cleaner cup and the breather every 1000 hours. This can be adjusted, and if the unit operates in dirty or dusty areas, this should be done more frequently. Check the blower and anywhere that air enters the system. Too much debris will rob the engine of its power, and can even cause more serious problems, such as overheating or failure.

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