HARD TO START HOT

I have a 1982 half-ton 4x4 Chevy with a 6.2L diesel. It had been sitting for about two years under cover, so I put a new battery on it (we put a high-torque starter on it to make it start on one battery—been that way for years), and it fired right up. However, after being driven for a while and then shut off (even for just a minute), it won’t start.

A mechanic said it had low fuel pressure, so he installed an inline electric fuel pump, which seemed to work at first, but after the engine reaches full operating temperature, I discovered it is still hard to start—even after cranking over a long time. We then installed a new fuel injection pump because the original began leaking. We also installed new fuel filters, which seemed to help, but the problem remains the same as before when the conditions are repeated. If the truck is allowed to sit all night or all day it will easily start. I want to keep it original, so your help would be greatly appreciated.

Michael Tuggle Via Email

Hi Michael,

Usual causes of “Hard to start hot:”

1. WORN FUEL INJECTION PUMP AND/OR INJECTORS. This means the pump has trouble producing injector pop pressure at cranking speed and/or the injector spray patterns may not be efficiently atomizing fuel.

2. SLOW CRANKING SPEED BECAUSE OF WEAK BATTERIES OR A WEAK STARTER. A normal engine, starter, and batteries should spin the engine to at least 180 rpm when the engine is warm. Most healthy systems produce cranking speeds in excess of 200 rpm.

3. INCORRECT FUEL INJECTION TIMING. Should be 3-1/2 degrees before top dead center at 700 rpm idle speed on a warm engine. Injection timing naturally retards over many miles of use because of timing chain wear. A less than optimal timing advance effects how easily the atomized fuel combusts during a cylinder’s compression cycle.

4. OLD, STALE, OTHERWISE BAD/CONTAMINATED FUEL. Normally, diesel fuel can sit in a fuel tank for two years without deteriorating. However, moisture can accumulate in the fuel tank in a wet/moist environment, which can greatly affect the fuel injection system and start-ability. Drain a small amount of fuel from one of the fuel filters. If you see evidence of water (it’ll sink to the bottom), a lot of particle contamination, or if you can smell gasoline, now would be a good time to drain the fuel tank and flush the entire fuel supply system.

5. POOR CYLINDER COMPRESSION. This scenario helps explain why a cold engine sometimes starts easier than a hot one. An engine has more cylinder compression when cold (higher oil viscosity) and the fuel injection system can more easily produce injector pop pressures (higher diesel fuel viscosity).

The fuel lift pump should produce a fuel pressure somewhere between in the range of 4-7 psi with an idling engine. If your new electric fuel lift pump is generating more than 7 psi, that could explain why the fuel leak developed in the fuel injection pump. The “throttle

shaft” in these fuel injection pumps are sealed in the housing using just two small neoprene/viton O-rings. The O-rings weren’t designed to contain fuel pressures of more than 7 psi.

These engines shouldn’t need the assistance of the glow system when the engine temperature is above about 80 degrees F because the heat generated during the compression stroke of each cylinder is enough to combust the atomized diesel fuel. If manually activating the glow system allows the engine to start better when hot, I'd have the cylinder compression checked. If cylinder compression pressures are OK, then I’d have the injectors checked for proper spray patterns. Poorly atomized fuel sometimes needs help to combust at cranking speeds.

A normal healthy 6.2L diesel should produce cylinder pressures of about 350 psi at cranking speed (after about 12 puffs). GM considered compression bad when pressures fell below 250. The lowestreading cylinder should be no lower than 80 percent of the highest-reading cylinder. Many thousands of miles of light-duty use can produce excessive carbon fouling of the piston rings, which can result in poor cylinder compression pressures. Unless there’s been gross neglect in engine service or damage to the air filtration system, the pistons and rings in these engines rarely wear out even after many hundreds of thousands of miles.

Somewhere here is the answer to your problem. Good luck. Jim