98 silverado 5.7 hard starting when warmed up starts fine cold what could cause this?

Think about how the vehicle is acting. Starts fine when cold, hard to start when warm. In these computer operated systems, the computer runs everything. Starts fine when cold: the computer has the engine in cold start mode putting more fuel in till it warms up. Temperature sending unit in the manifold sends a message to the computer telling it to stop the cold start mode and it stops dumping excess fuel into the cylinders and it runs fine. IF, mind you, IF the temperature sending unit is faulty, it's not telling the computer to stop the cold start mode and is flooding the engine with fuel cause it's sill in cold start mode. This is my take away on your situation and the temp sending unit is something to check to be sure it's functional. Check it out. I hope this WAS the answer to your problem my friend. It's typical. Luck to'ya Bro.

How to Start a Car in Freezing Cold Winter Weather

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wikiHowFreezing cold weather can take a toll on car batteries. ... It's much more difficult to start a cold car with a mostly-empty tank, so do yourself a favor and gas up ...

Why is it so hard to start a car in the winter? - HowStuffWorks

auto.howstuffworks.com/car-driving-safety/.../start-car-in-winter.htm

To start a car in winter, your engine has to work extra hard. Learn the top 3 reasons your car is hard to start in cold winter weather.

Debunking 5 Cold-Weather Car Myths - Popular Mechanics

it could be the starter, they can be affected by heat from being near exhaust components, or it could be the starter solenoid... also bad ignition timing could cause that

Fuel Pressure Regulator !

Maybe battery or starter is going or positive cable to the starter is going bad.

You may have a leaky fuel pressure regulator located on the LH side of the engine on the side of the fuel rail. The internal diaphram can leak fuel which then gets into the vacuum line attached to the regulator. From there the leaking fuel travels through the line and gets into the intake manifold causing the motor to be slightly flooded when you restart it. The regulator is round, about 1-1/2" diameter, and about that long and has a small 1/8" vacuum line attached to it. After the engine has been off for a few minutes if you remove the vacuum line from the regulator and find even a trace of fuel in the line then you've found your problem. Typical symptoms for a leaky regulator are: Starts fine cold and will restart fine warm if the engine has only been off for a minute or so. Restarts hard warm if the vehicle has sat for several minutes or up to an hour. May produce a bit of black smoke during the warm/hard start.

A warm engine takes more fuel pressure to start then a cold engine. A clogged fuel filter can cause this or a weak fuel pump can cause this. There are also strainers on the fuel pump in the tank that can become clogged. If the vehicle has been ran out of gas, I would suspect clogged strainers.

Try replacing the fuel filter also Has it had a tune up? they need to have the plugs replaced at 100,000 if the fuel filter does not work then the most probable cause is the fuel pump is losing pressure when hot. The pump warms up and the check valve fails to close all the way allowing a now fuel or fuel starvation.

hi a bad water pump will not let the engine run normal unless it is frozen up. it sounds like your coolant temp sensor has gon bad.that will cause rough cold starts and work fine after warmed up

My 98 Jimmy runs on synthetic oil and starts well after sitting in cold weather. If the weather is the problem, try checking that. If it is warm... sorry no ideas.