Chevy Suburban will start cold, but only once unless it sits for 4 to 6 hours, seems to run fine, full power smooth Idle but exhaust pipe black black, "Starts once", Then either right away or running an hour will not start again, No check engine light dash, errors and diagnostic codes clear,

has 4 new O2 sensors, and air cleaner filter, new air flow sensor, 7.4 liter engine 98 suburban 2500.

Answer: Very strange that your Suburban would not have a check engine light or codes stored with the problems your truck is having.

I think the fact that the exhaust is black is going to be the key to this. Black smoke or soot in the exhaust is a sign of the engine running extremely rich- way too much fuel. Normally this is caused by an engine misfire, but you say when it starts that it runs fine. I would first replace the spark plugs since i am sure they are as black as the exhaust is. This would BE the problem, but a result of the way it is running. The problem with it not starting sounds like a leaking fuel pressure regulator. When these fail fuel gets dumped into the engine. On a cold engine that is not too much of a problem because a cold engine needs to be rich in order to start and run, but not on a hot engine.

Locate the regulator- see pic below. With the engine off for a few minutes, remove t5he vacuum line and look for raw fuel in the hose or even just the smell of gas. If you find either, replace it, replace the spark plugs, start the truck and rev it up several times real good to "Blow it out". This should take care of your problems.

Another way is to install a fuel pressure gauge, start then shut off engine and wat5ch for the pressure in the system to drop- this indicates system leak down. If the regulator is not the culprit, then you could have a few fuel injector sticking open causing this problem with your Suburban.