I just put a used motor into my car. I started it up and it will not hold an idle. It will stay on for about four seconds and then die. I've double checked all the vacuum lines, gaskets, AFM, BAC, TPS, wiring harness, changed plugs, and now I'm stumped. I can't figure it out and its frustrating the hell out of me. I don't know whats left.

 

my rx7 had an idling problem. Wuldnt stay alive but i could keep it alive by revving it. Turns out it was a switch on my microtech that was screwing up due to my thermos. Aparently my thermos was playing arnd with the voltage or current or something like that. Thermos was replaced with a stock engine fan and now its all good.

 

Go back to the basics. First, check the engine compression. If compression is at an acceptible level on all faces of both rotors, check fuel delivery, spark and correct timing. Test for fuel delivery and spark according to the shop manual. Do a basic stab of the crank angle sensor to set a basic timing position.

If all that checks out, the problem could be un-metered air entering the engine or a major vacuum leak. I know you checked for that, but do it again. Look for a leak, especially somewhere between the AFM and the throttle body. Like unconnected air hose, loose clamp, crack in intake tube, etc. Make sure the AFM connector is plugged in and that the wiring back to the ECU is intact.

 

Well I got the car "running". It will drive, but still wont idle. I can't find an intake leak anywhere. Are there any sensors that if unplugged would cause the car to not idle and yet still drive? I've replaced pretty much every thing I can think of, even the dynamic chamber and upper intake manifold.

 

Check the gaskets on the BAC valve, ACV, EGR (nm if it's an S5 motor), or any block-off plates you might have. If any of those are originals, they're gonna seal like crap.

 

Yes, As said before, check especially if you have a ACV Block off plate

that vacuum leak cost me a week! (and alot of back pain!)

Other than that, i'd fuel and spark seems fine if you can drive it (no idle right..)
not sure if it is tubro, but sometimes a "looks like" connected intercooler can be the culprit too... (it gave me a Blitz BOV sound when i was at boost.... but idled fine)

 

Maybe I should leave a little more information here.

S5 NA
No block off plates
All emissions equipment on the car except for the split air tube going to the cat(test pipe)
All vac lines replaced when new motor(used motor) was put in.
BAC has RTV.

I haven't touched the ACV nor the lower intake manifold. The motor came with one on there so I decided not to mess with any of it to reduce headache since I didn't have a new gasket for it.