Greetings.

My wife’s ’95 turbo Miata (Begi 2) with MSPnP has developed a very loud intake drone or hum; it sounds like a resonant air noise.

I eliminated the turbo, vacuum leaks, and several other possibilities to determine the source was the Idle Air Control valve (IAC).

It appears this valve opens and stays open at idle, and the air rushing through the passage causes the droning sound. With the throttle body (TB) exposed, a great deal of air can be felt being pulled into the bypass circuit below the TB. If I move the throttle slightly the sound completely stops until the throttle moves back to its stop, at which point it resumes. Likewise, if I unplug the IAC from the harness by pulling the green plug, the sound also stops. This problem also happened simultaneously with an increase in idle speed to about 1,500 or 1,600 rpms; but that’s explainable by the IAC valve remaining open and the additional air the engine now sees at idle.

I don’t know why, but it appears the MSPnP is signaling this valve open. It’s a recent problem and nothing’s been changed to the tune, so I’m at a loss as to what happened and how to correct the problem. Any thoughts?

 

John151 over in Miata.net offered this explaination:

"While most valves operate at 100 hertz, some older style 2-wire valves will work better at frequencies as low as 39 hertz. There is an inverse relationship between frequency of operation and control precision because of the algorithm Used to control the PWM output. Lower frequencies have more steps between dc values. 100hz operation has 100 steps and therefore has 1% control steps. 50hz operation has 200 steps and has 0.5% duty cycle steps and that is why the duty cycle values put into MT need to be double the true DC% value.

Some valves may emit a buzzing sound which can be eliminated by increasing the frequency above 100 hertz."

The link is here:

http://forum.miata.net/vb/showthread...31#post4850431

I set my frequency to 115 hertz, and that pretty much solved that problem. Thanks for looking, everyone.

Jim

 

see if it had been described as a hummmm/buzz and not a drone, I could have told you that

 

I swear, it sounded like a flight of B-17's over Dresden...

 

well how am i supposed to know what that sounds like!? I'm not hilter.

 

Slaughterhouse 5, 12 O'clock High, stuff like that...

 

I dont know what that is, Regis.

 

Don't you worry yourself about it, Kelly. You just enjoy your lemonade and thank Brain from me when you see him next.