Engine turns over but will not start only when engine is warm

Here's some FYI on your situation. When you turn the switch on, all sensors are activated. One sensor is the cold start switch/temperature sending unit. When working properly, the sensor will send a pulse to the computer telling it to put the engine into cold start mode. Once this is done, a bit more fuel is added to the normal amount to the cylinders and the throttle is advanced slightly making the engine idle a bit faster for the extra fuel coming in. The engine is running to warm-up and the sensor then tells the computer to set the engine back to normal run mode. Once the engine is warm, the sensor is not being used for cold start mode. It will start right up and run smoothly. Now, since your truck is hard to start when cold, first thing you should check is the temperature sending unit wire connection first. if that connection is good, replace the temperature sending unit/sensor, and you'll be good to go. That's a simple repair that must be done when it gets hard to start when cold.

The temperature sending unit tells the computer that the engine is cold when first started. After engine is warmed up, if the computer things engine is still now warm, it's still in cold start mode and dumping too much fuel into the engine and flooding it making it hard t start or no start at all. I'd have the temperature sending unit in the engine replaced to see if it's cured. It's separate from the gauge or light and they would show warmed up, but the computer is not getting the message.

the coolant temperature sensor is faulty
when cold the sensor tells the computer its cold adding more fuel into the engine, when warm the engine needs less fuel so the temperature sensor tells the computer its warmed up and to use less fuel. when this snesor goes bad it always reads as if the engine was cold so the computer adds more fuel than needed making the engine not start. Trust me on this ive had this problem happen to me before

Its probably a problem with the water temperature sensor. Also do not rev a cold engine. The engine has a "cold start sequence" which is similar to a choke. When the engine gives a cold signal, the fuel mix is made richer and the timing is changed until the engine warms.

Let the engine warm at the preset factory settings. Sometimes the O2 sensors will cancel the cold start sequence before the water is warm. Racing the engine will warm the O2 sensors too quickly.

Most likely is the temperature sending unit/sensor. Nobody ever thinks about how it works. When engine is started, and cold, the temp sensor tells the computer to put the engine into cold start mode, putting more fuel into the engine and to a bit higher idle. Once it warms up to operating temp, said sensor tells the computer to turn off cold start mode and it stops dumping excess fuel in. If the sensor puts it in cold start mode, malfunctions and doesn't take it off cold start mode, it's still dumping excess fuel in and flooding the engine. Sometimes a re-start will reset the computer, but not always. Makes sense now, doesn't it. Find the temp sensor and replace it like you did all the others.

yes chevys 3.4 and 3.5 have common lifter noise, have you checked the oil level. I would a better oil ,and I put lucas oil treat in my customers cars after oil changes

A common problem with the 90's engines was the ECTS (electronic coolant temperature sensor) going bad. Seems when attempting to start the car, the computer in the car will only allow enough gas it thinks is needed to start. When a car is cold, the computer gives more gas initially, until the engine warms up, then gives less gas. When starting a warm engine, less gas is needed.

If the sensor goes bad, it always tells the computer that the engine is hot. The computer give less gas to a cold engine, and sometimes it will start, sometimes not. Its a pretty cheap part, and a do it yourself one, just research it a bit on line...good luck

Ok, time for car school. The vehicle is operated by the computer, every electrical concept that is. When the engine is cold, it starts fine because the computer has placed it in "cold start mode", putting more fuel into the engine than normally would until Once the engine is warmed up, a temperature sending unit or temp sensor tells the computer to change to normal run mode. IF said temp sensor is faulty and is not telling the computer to make said change, it stays in cold start mode, still dumping excess fuel in the engine that is now not required, cause it's warmed up. Like keeping the choke closed on older vehicles. Now I think you can answer your own question, when this no start action is happening after warm up and it always starts fine when cold, what's the first thing you check ? Right, the unsuspected temp sensor. Change that and no more start problem. Have a wonderful day. Glad I could be of service.

you might want to check your return line to the tank

You might want to check the ECT (engine coolant temperature sensor) It could be defective and telling the PCM (car's computer) that the engine coolant is cold all the time.