Symptoms
Hard cold starts that get worse as temperatures drop, or black smoke and a stall right after starting. Flash Off-Road's 1996 turbo diesel wouldn't start below 50°F when its plugs failed.
The fix, step by step
- With the ignition on, the voltmeter should dip as the plugs draw current.
- Measure about 50–55 amps per bank and 13–14 amps per plug while they cycle (6.5 non-turbo procedure; military 24-volt trucks are different).
- Replace failed plugs; Flash Off-Road recommends the self-limiting AC 60G.
- Check the controller and alternator voltage too, because both can burn out plugs.
- They aren't a routine service item.
Tools & part numbers
- Multimeter with a DC amp clamp or in-line ammeter
- deep sockets
- anti-seize
Where to buy the parts
Diesel glow plugs, 1998 H1 6.5 TD (catalog)RockAuto ↗
Hunting a used or discontinued part? Search eBay Motors for AM General Hummer H1 parts →
Rated for a competent home mechanic with a decent tool kit. Confirm torque figures, fluids and procedures against the factory service manual for your VIN, support the truck on stands rather than a jack, and work at your own risk (disclaimer). Some parts links are affiliate links, which pay us a small commission at no cost to you.