The 1998 GMC Sierra is a pickup truck that was sold in 1500, 2500 and 3500 models. There are significant differences among the three models, including different engines, transmissions, suspensions and bed size. No matter what model you are driving, troubleshooting the non-starting issue is the same. Because your Sierra is turning over, you can rule out a dead battery or a siezed engine as the cause of your problem and shift your attention to some other areas. In order for your Sierra's engine to run, it will need fuel, compression and ignition of the fuel, so those are the areas to focus on.
Confirm that your Sierra has not run out of fuel. A faulty fuel gauge could be giving you a false reading. If you cannot remember how many miles you've driven since your last fill-up, add a few gallons of fuel to the tank and attempt to restart the Sierra. If the tank and fuel lines had run dry, it will take some extra cranking of the engine to get it started and work any air out of the fuel lines and fuel filter. Do not crank the engine for more than 30 seconds at a time and space out the starting attempts to prevent the starter from overheating.
Check to see if your Sierra has any fuel pressure. To do this, attach a fuel pressure gauge in the fuel line. Crank the engine while someone else checks the reading on the gauge, which should be between 55 and 62 PSI. If there is nothing registering on the pressure gauge, the fuel pump has failed, which will need to be replaced. If the pressure reading is low, it is a sign that either the fuel pump is weak or the fuel filter needs to be replaced. GMC suggests changing the fuel filter ever 30,000 miles, so if you have not changed the fuel filter within that mileage range, change the filter and test the pressure again. A low pressure reading after changing the fuel filter indicates the fuel pump needs to be replaced.
Check the spark plugs to see if they are worn and need to be replaced. Depending on your specific Sierra model, you either have a six or eight cylinder vehicle. Use the spark plug wrench to remove the spark plugs and check the tips. If they are coated with black carbon deposits or excess fuel, the spark plugs cannot provide proper spark to ignite the fuel and should be replaced. GMC suggests replacing the spark plugs on the Sierra every 100,000 miles.
Check to see if the engine is getting proper compression. To do this, remove the spark plug on the first cylinder and insert a compression gauge. Have someone crank the engine while you take a reading from the gauge. The cylinder should have a compression in the range of 125-160 PSI. Perform the same test on each cylinder and record the readings. The lowest pressure reading should be within 10 percent of the highest one if your engine is working properly. If your readings fall lower than the 125-160 PSI range, or if there is more than a 10 percent drop off between the low and high reading, the most likely cause is a timing belt that needs to be replaced.
