The problem I'm having is that when cold start the engine stall no matter what but when I unplug the MAF it start up with a little help applying on the throttle. When the motor warm up I plug the MAF back in and it hold idle at 1500 rpm. So what do you think is the problem?

 

Have you checked the resistance values of the water thermosensor?

 

I replace the coolant sensor behind the waterpump. Also does a rebuilt motor on an s5 TII hesitate on full throttle?

 

Hesitations, high and bouncing idles, etc. are all control system issues in general, and not the engine block itself. An engine is just a big air pump that holds coolant and oil. It just 'does what it is told" by the fuel/control system bolted onto the outside of it. So your problems will generally be related to wiring and grounding, fuel injectors, vacuum leaks, that sort of thing.

Rebuilds are a little down on power and starting ability, and idle a little rougher than normal since compression isn't 100% yet, but there isnt anyting you can do about that except let it come up naturally.

 

I know there is no vacuum leak since I went over 3 times taking the UIM out. Why does the motor start better with the MAF unplug and when the motor warms up plugging the MAF back and it hold idle? I think it's the injector since the guy sold me the car told me he had it garage for over 3 years. Also I fried the ground wire that goes from the tranny to the firewall cause I forgot to ground the starter.

 

The computer has a pre determined base cranking map that does not rely on the AFM flapper position. This is what the car initially starts on, but as soon as the rpms rise and the engine revs up, the computer has to immediately look at the afm flapper for fuel calculation.

Unplugging the afm makes the computer try to rely solely on the cranking map and no other correction for higher airflow/rpms. In general this would mean that it runs far leaner without the afm, and this is why most cars will not run at all without the AFM, they go lean and stall, especially when you try to open the throttle and rev it up. So if the car runs better without the afm than with, then you are running seriously rich, would be my guess.

You could have a screwed up or out of adjustment afm, or a fuel injector issue.

 

Thanks for your help I'm going to look into it.