Hello, my friends S4 NA that we just swapped the motor on wont hold idle on its own for no more then 20 seconds at the longest. You can keep it running if you feather the gas over and over again and it will drive and rev all the way to redline but it will not idle. We have checked for vacuum leaks and have not been able to find any and have adjusted the tps. We tried to adjust the idle from the screw on the UIM and we jumperd the check connector but it made no difference turning it. We checked the voltage at the fuel pump and it was at 12v when running and held voltage when it will die. This is the video we got of it doing it, sometimes it will hold idle longer, when starting it cold it will rev up to 2500-2000 rpm and it will die when it tries to lower the idle under 1500rpm.

 

check and see if the throttle body gasket is installed in the correct orientation if you adjust the screw and it doesn't do anything that may be the case.

Also to rule out worst case ontario, check compression.

 

take soem plastic cling wrap and wrap your air filter and put it in the air box. then take a vacuum pump and apply vacuum to the hose of your choice, preferably one that goes straight into the manifold and watch the gauge. it will take a bit of work to build any vac, but you should get something and it shouldnt fade all too quickly.

if you cant make vac or it just dissipates quickly theres a leak somewhere.

do a quick compression test. it wouldnt really even start if it had that bad of compression frankly.

 

check the maf

 

A while back I ran into something similar, the thing just would not idle. I could lower the idle to the ~875 RPM point but when I turned the TPS adjust screw the moment the light came on it would idle like crap and stall. I finally tracked it to the purge valve, that's the round valve on the drivers side of the engine that feeds blowby gases back into the intake. If it goes bad, the large hose won't seal and you can't hear the vacuum leak. It's only supposed to open over ~2000 RPM.



I just stuck a vacuum cap over the intake nipple, the problem went away. Never did replace the valve, that might be a problem for you guys in emission control **** states.

 

replace your injector grommets.

 

Good catch, primary injector seals will do this too. Quick test: spray carb cleaner under the intake with the engine running. Any change at all in engine speed or running (up, down, runs rough, smooths out) indicates primary injector mounting grommets that aren't sealing. I learned the hard way the AutoZone etc type do NOT seal properly.