Hard to Start when Warm...1999 GMC, 5.3 engine



Truck starts fast, when cold, starts hard when warm. Already replaced the fuel pump with new oem pump. Also replaced spark plugs. Same problem. Truck has 90,000 miles…


You need to determine if the truck is getting too much, or too little, fuel on a warm start. For a possible flooding condition, press the gas pedal to the floor, and HOLD it there, during cranking. As it starts, let up on the gas pedal.For possible over-lean condition, spray Starter Fluid into the intake tube, and try to start the engine. If it starts, follow the charts, at alldata.com, for instance, for that symptom.


I… wouldn’t do that on a modern fuel injected car.

Why did you replace the fuel pump? Did you have a fuel pressure problem? Just from your brief description, I’d be inclined to say that you have a weak coil or failing ignition module.


Mrjosh could be correct to suspect a weak spark. You can get an adjustible spark gap tool ($6) to test the strength of the spark. A weak spark could cause a weak start; but , it’s usually during a COLD start. The design IS in modern cars to unflood an engine, today, using the same technique as yesterday of holding the gas pedal to the floor while cranking.


About a year ago, my local garage, checked the fuel pressure…said was low pressure…about 40 psi.
Also the fuel gauge would never read accurate. Also in the Cold season, my check engine light would
come on. That tech. said, “You need a new fuel pump”. I drove it for another year, real little milage
added on. Anyway, decided, and had the fuel pump replaced, new fuel filter, & new NGK spark plugs too.
Engine still hard to start when warm, but in the Morning…Starts right up. It has not been that cold
here in Denver, no check engine light either. What’s Next?