I need to pick your brains.

I have a 98 Dodge ram 5.9, presently it won't start unless you put your foot to the floor and then it will start and revs and runs smooth as long as you keep the rpms up but as soon as you let off the gas it dies and won't start again unless you put your foot on the floor. Any ideas as to what's going on here? Could a bad throttle position sensor cause this? Ideas please, thanks.
Ken

Some GM systems would turn off the injectors completely if the gas pedal was floored during starting. This was in case the engine got flooded somehow.

R.

Strange, because I didn't think you need any throttle with EFI and electric fuel pump for startup. Do you hear fuel pump when you turn key to on position (not start)?

You could try the easy stuff first like using spray cleaner on the inside of the throttle body.

Does it try to start wuthout throttle?
Did this issue start over night or slowly?
Throttle position sensor?

Check your battery. I had a 99 Dakota that did the same thing, turns out it the battery had some dead cells and was killing the PCM's learned idle information.
I didn't believe it because the truck started OK, but they tested the battery and it was toast. New battery solved the problem.

Idle air control motor, or whatever dodge called it, gotta make sure it's working. It's what handles the idle speeds for different loads etc.

It's flooding. Cranking the engine while the throttle is wide open is a fuel clearing procedure. All the EFI cars I know of are like this.

Now you need to find out why it's flooding. I would look at the wiring for the coolant temp sensor behind the thermostat housing. If the insulation is cracked and broken you have probably found your problem. The wires short together and the PCM sees -40*F. It thinks it is in a deep freeze and feeds the engine a lot of fuel.

After you repair that you will need to do a cap discharge. Remove the positive battery cable from the battery, touch the positive cable end to the ground terminal on the battery. Hold it for 30 seconds and then reconnect the cable. You will now need to clean the throttle body or it will not idle properly. The first fire after this will probably result in a start and stall. That's ok, it's programmed into the PCM to keep from dry firing a new engine at fast idle on the assembly line. Second start should be good and it will relearn from there.

You may also want to replace the thermostat as I have seen them stick partially open and cause a rich condition because the engine takes a lot longer to reach operating temperature.

Ding ding ding, we have a winner, the IAC valve was stuck in the closed position. I cleaned the old one and try to exercise it but it was very very stiff. Put it back in but still would not idle at lower rpm, so I remembered a friend of mine talking about the IAC and I knew that it served as sort of a controlled vacuum leak, thats why i looked there, and sure enough, it fired right off and idled very well. Interestingly it never showed any check engine lights or codes, until after I put the new valve in period I guess I will get it cleared and keep an eye on things. Thanks for all your input.

An update to this problem i was having. I thought the new IAC valve had fixed the problem, but it started acting up again a few days later. As someone mentioned above, it was the battery AND the IAC. Apparently the IAC has been bad for quite a while cuz now with the new batt it starts and idles like new again. Lol. Its actually pleasant to drive again, so smooth at all times. Thanks for the input guys.

You also need to make sure the throttle blade is not real dirty and has any carbon build up on it as that will also do it. And thats because the IAC (Idle Air Control) motor is programmed in the PCM to go to a certain amount of steps at a certain temp and it also needs the extra air that gets around the throttle blade or it will stall and may not start just as you said. The PCM is not smart enough to know that it needs to open more because the throttle blades are dirty and not letting enough air by them. Its made so the IAC just fine tunes the idle speed but when at its setting it must have the air around the throttle blades to have the right idle. The IAC will adjust some but not fast enough to start the car or keep it from a stall when you let off the gas. Its like say the IAC motor is set for 35 steps when it sees the eng at 65 degrees. So now when you turn the key on the PCM sees the temp of 65 and sets the IAC at 35 steps but if that extra air is not getting around the throttle blades the eng will not start or will barely start and stall. Now if you put your foot 1/4 to 1/2 way down the eng will start since you open the throttle and if you bring the rpms down slowly and take say 60 seconds to do it you will slowly see the IAC steps go from the 35 steps it is set at by the PCM for that eng temp up slowly to about 60 or 70 steps and then you will see the eng idle like it should. But if you drive down the road at part throttle while you drive the PCM will adjust the AIC back to 35 steps where it is programmed at that eng temp and the eng may die when you let off the gas and come to a stop. Step on the gas and the eng will start and do the same thing.
Now if you pull the throttle body and I use a tooth brush to clean all the carbon off the throttle blades you should see when you put it back together it will start and idle fine at the 35 steps the PCM has the IAC set at. Remember the IAC steps are higher on a colder eng as at 20 degrees it may be at 70 steps but if the throttle body is to dirty and blocking the air made to go around the blades it will still die. And on a completely warm eng at idle with all acc's off the IAC may only be set at 20 steps. Course it can change if you modify the minimum throttle blade setting which you should never have too. But definetly make sure the throttle body blades are clean. And as was said if the battery goes to low the PCM can forget where the IAC is set and needs to relearn that also. Good luck , Ron

Also as was said on fuel injected engines when you put the throttle to the floor while the eng cranks below a certain rpm its a flood clear mode and turns off the injectors. But it dont sound like your eng was flooding since you said it runs fine after you start it. If it was flooding it would be loading up and chugging and blowing black smoke when you got it started. Ron

Another simple diagnostic check, the IAC should only bump rpm when needed, like cold start or AC kicks on. When the engine is fully warm, and AC off, you should be able to plug the IAC port with your thumb and the engine shouldn't die. If it does, then the iac is doing too much work and the throttle blade needs tweaked