How To Troubleshoot Why a 6.2 Diesel Engine Will Not Start

by William Boyce
itstillruns article image
Auto Engine image by Andrew Breeden from

GM installed the 6.2 L diesel engine in light trucks and Sport Utility Vehicles between 1981 and 1993. The 6.2 L diesel is naturally aspirated, unlike the more recent 6.5 L turbo-charged diesel. The fuel system on this engine includes the fuel tank, mechanical fuel pump, fuel filters, fuel line heater, injectors and fuel lines. The electrical system is less critical to this engine than on later 6.5 L diesels because the injection system is purely mechanical. A troubleshoot of this engine should walk through the essential components one at a time.

Fuel System

Step 1

Check your fuel. Make sure the tank is not empty and that you are running the right type of fuel. If your tank is full of summer diesel and it is now winter and cold, your fuel may be clouding up and clogging the filters and injector nozzles. Make sure you haven't contaminated the fuel with gasoline. Gasoline will not damage the engine but is not capable of igniting under pressure as diesel is. Pump out your fuel tank and fill with clean diesel.

Step 2

Examine the fuel lines between the tank and injectors to see if there are any crimps or damage. Check the connections for leaks and loose fitting as well as any obvious fuel leaks. Replace damaged fuel lines and connectors using a wrench to loosen and tighten connections. Bleed air from the system after replacing lines by removing the fuel line inlet at the primary fuel filter and cranking the engine until diesel flows from the line. Re-attach the line to the filter.

Step 3

Open the primary and secondary fuel filters and check for paraffin deposits. Clean the deposits and replace the element-type filter. Check for water in the primary filter and open the drain on the bottom and petcock on the top if water is present.

Step 4

Put your head close to the fuel tank and have an assistant turn the key on without cranking the engine. You should hear the fuel pump in the tank come on. Tap the top of the fuel tank lightly with a piece of wood or rubber mallet and see if the pump unsticks. This will sometimes be enough to get the truck running until you drive to a garage.

Electrical System

Step 1

Use a battery tester to test both batteries. Replace defective batteries. Diesel engines have very high compression ratios and require a large amount of electrical energy to crank.

Step 2

Check the glow plugs. Most 6.2 L diesels came with one relay but early models had two. Test the glow plugs by unplugging the electrical connection from a glow plug. Hook up an ohmmeter to the glow plug terminal and ground the other end. The resistance should read between 0.8 and 2 ohms, depending on the glow plug type. If it reads infinitely large the plug is failing or has failed and must be replaced. Unscrew the failed plugs with a wrench and replace them.

Step 3

Turn the ignition key to RUN without cranking and listen for the clicking noise as the glow plug relay system cycles on and off. The glow plug relay system requires extensive diagnostic experience and tools and may require a professional diesel mechanic to troubleshoot.

More Articles

article divider