I have a 1989 GMC Sierra 1500 Regular Cab with the original 5.7L V8 350. It has almost 227,000 miles. For the past four or five months it has started having idling issues. It will start normally and run fine, then the next time I start it, it will not start unless I press the accelerator. When I have to start it this way, it also will not idle at stops unless I continue to press the accelerator (while I have the brake depressed). This will last, on average, about a week and a half, then it will start as normal again.
About three weeks later, it does it again for another week or so. I use only 100% 87 octane gas, no ethanol. The fuel pump and fuel filter were replaced less than two years ago. I also put Heet fuel line antifreeze and water remover, gas treatments and fuel cleaners into the gas tank, but the problem persists.
You're getting your money's worth at 227k miles, but of course problems do begin to mount up.
I can't give an accurate diagnosis to the condition without being on-hand with my scanner, fuel pressure gauge, spark tester, and what have you. But a general rule-of-thumb when an engine won't start normally, but does start when the accelerator is depressed fully, indicates the engine is flooded with fuel. Opening the throttle supplies the engine with extra air to compensate for the overabundance of gas. Not always the case, but pretty often.
A couple of items you can check yourself will only require a quick peak under the air cleaner at the throttle body assembly. It's that thing sitting where the carburetor used to be with two big fuel injectors on top.
First, turn the ignition "on" to prime the system with fuel (apply pressure at the injectors) and then look to see if any fuel is dripping out of either injector. If so, replace it, it's no good.
Next, remove the vacuum hose leading to the fuel pressure regulator. Check for any visual signs of gas in the hose or the port at the regulator. Also give it a sniff-test to see if you smell gas. Any sign of fuel at the regulator tells us its internal diaphragm is cracked and leaking. And the attached vacuum hose is sucking excessive fuel into the combustion chamber. Actually, the technically-correct way is to check the regulator with a vacuum pump to be sure it's holding vacuum.
Again, these are just a couple of quick checks you can do yourself. But if everything looks good, have a technician evaluate the condition and test for other possible causes.