1999 Merc Mystique barely runs
 

I'm working on my friends 1999 Mercury Mystique. When I picked it up off the side of the road it would start, but only for a second. I had to keep my foot on the gas at all times just to keep it running and get it back to my place. I ran it up and down the road, came back home and it would idle @ about 500RPM very rough for a brief time. When you're going down the road it goes just fine up until a certain amount of throttle given, about half way open it won't go over 60mph but when eased off it goes just like normal however fast you want it to. In Park or Neutral it is about the same way, give it 1/4 to just under half throttle and it revs fine any bit more than half throttle and it's got a hesitation and is slower to climb in RPM. I used a diagnostic scanner and it said that it's running too lean. It didn't come up with any trouble codes other than saying "too lean". I first thought one of the sensors might be junk, but if that were the case I believe they would have showed up as a trouble code. I checked the fuel filter and it seems alright, I can easily blow through it and it was fresh clean gas that came out of it. My best guess is (due to the 205,000+miles on it and what it's doing) the fuel pump or maybe a screwed up regulator? - Oh, I also checked the lines. None are pinched or anything and there's no leaks anywhere. Any help would be appreciated, she needs this car running ASAP. Thanks

oxygen sensors sometimes won't show up in codes as bad. this could be the problem. it could be running with reduced fuel pressure due to a clogged fuel regulator, weak fuel pump, etc. check the pressure on the fuel rails while someone has it running and that will tell the tail. if the pressure stays up, then it's the injectors instead. it could also be worn plugs. I would definitely clean out the throttle body. the carbon build up in there can cause all kinds of weird idle issues. it could also be the idle air control motor. if it's stuck in a closed state, it would cause the engine to stumble when you let off the throttle.

I disconnected the line at the fuel filter again, and did the old thumb-plug test to see relatively how much pressure it was producing (my father broke my fuel pressure tester) it seems to have a decent flow, but I barely have to put my thumb on it to stop it from squirting. I'm pretty sure the pump is the problem because it's supposed to run 28+psi and the 14psi pump from my truck puts up more of a fight with the thumb-plug test. Sounds like a dumb test but hey, it works if the pump is really weak. The female friend didn't listen to me and wait to get the pump so I guess I'm putting it in today, and I'm going to dump in some fuel system cleaner as well. Thanks for the suggestions I'll look into them if this doesn't work and let you know what it was.

P.S. this problem was out of the blue according to her. I think she's probably been running it low on fuel quite a bit since she got it, seems to not concern her when it's sitting a needle-width over empty. So, it'd make sense that the pump would be crappy.

yeah... if you can stop the flow of the pump that easily... it's the pump. That should fix it for the most part... the fuel system cleaner is a good idea after changing the pump. it will definitely help get rid of the low pressure build up (if any) in the system. after that, you shouldn't have any trouble accelerating after that.