My son's 99' 4.0, has a hard start prob. that has me pulling my hair out. He couldn't get it to keep running, did a quick search on here, symptoms seemed like the tps, changed it. It started ran rough for a little bit, figure prob. fouled up a bit, then seemed to run fine. couple weeks later, it's doing it again, but it will stay running, just have to keep on the throttle, til it warmed up, dumped some injector cleaner in the tank. Again seemed to run fine. Yesterday couldn't get it to stay running when he went to work, when I get home, it starts right up. Really hard to diagnose when it doesn't do it all the time and doesn't throw a code. Could a injector be leaking fuel into the cylinder? Any ideas apprec. Thanks.
Charlie

 

Take it to autozone or any of the box auto part stores and have the check for codes to see if it has any stored codes in the system.
Also check all the grounds and clean them all. That would include the battery and the ones that come off the engine. At least that's where I would start.

 

I already checked for codes, none. Thats why it's driving me nuts. It's just intermittent. Thanks, I'll check the grounds.

 

The fuel injector can be leaking in to the cyl. like you suggested. You can check by pulling the plugs and see if the plug are damp or if it smells of excess gas. Also check the fuel pressure at the fuel rail with a mechanical gauge. Also make sure you have a good strong spark.