2000 Jrsc m45 rough idle/howling
 

I know this question has been asked before, but of all the threads and websites I've read, I cannot seem to find a straight answer. Background: Car driven with sc for 60k, no problems. Pulled engine, rebuilt (had 150k miles), put engine back in. After starting the car, the sc has developed a horrible erratic idle (drops to near stall, then shoots up, repeat) and a howl from the intake. No hoses are kinked/crimped/bent that I can find. Any thoughts as to why this would have developed would be great. After running for several minutes (5-10), things seem to level off, but the howling comes back if you touch the throttle. Again, thank you for any help in advance.

idle valve? vacuum leak? both?

We went back over every hose and resealed everything thinking it was simply a vacuum leak. I've heard these bypass valves can cause a fuss if they go bad. Is there any way to check it to see if it is worn out?

If your talking about the one in the idle tube you can blow into it and see if it holds in one direction....

I might replace the actuator bypass unit.

I've got a new valve and actuator on the way. Haven't changed them in around 30k miles, so I suppose it's about time either way. Moss Motors seems to think the hoses connecting to the dummy throttle body are swapped around, but I won't be able to get back to the car until Monday to find out. Hopefully we will get this solved then.

Update: after re-installing every vacuum line (some with new hose if the least bit questionable), and playing with the idle screw to no end, the idle is still terrible, and the intake is still howling. With the air filter on, the car will nearly stall out, but if you cover the intake with your hand (essentially cutting off most of the air), the car begins to idle correctly, and stops howling. I wish I had a better adjective for this sound.

Get a can of carburetor cleaner and start spraying random ---- under the hood while the engine is running. Listen for the idle sound to change pitch or the engine to change RPM. If either of these things happen, you found your vacuum leak. Obviously, don't spray your air filter or INTO your MAF. Don't light yourself on fire in the process and report back.

Check that your throttle body is closing all the way, and doing so consistently. Make sure that SC install isn't binding the cable or something silly like that. IIRC, its pretty tight around the back end of the blower?

Also, what are you using for engine management?

Have you checked the coupling? Maybe it sounds different from no load to slightly loaded?

After replacing all of the vacuum hoses, I sprayed the carb cleaner as precaution. Although the already unstable idle made it hard to tell whether it was the carb cleaner, or just the idle problem, the cleaner did not make any noticeable changes. The bypass actuator is steadily pushing down and back up with the throttle. When it's about to choke down, the actuator pushes down, the back up when the rpms shoot up, repeat. The roughness of the idle can be compared to a flooded engine. The engine management is the old magic black box that alerts the IAT under boost. Although it's not the best, it worked fine before so I don't see why it would cause problems with a simple engine swap. Nothing was changed with the setup of the car, that is why this is so confusing to me.

Alters*

Fixed my idling issue. The MAF was bad, thankfully I had an extra left over from my last Miata. The howling from the intake has mostly* went away. At idle, it is completely gone. Under part throttle (cruising), or under load from the AC, the howling becomes constant. What could cause this? I have replaced the IAC, bypass valve, bypass actuator, ALL vacuum lines (mainting perfect vacuum psi now). Thank you for all of your previous thoughts and ideas, they really helped me figure this out.