Hi guys, I've had my RX-7 for a few months now and been loving it, however I've come across a rather tricky problem.

It's had occasional idling issues but as of late they've been getting worse, to the point now where it's the norm rather than the exception. What it does is, the RPMs drop sharply, almost (and occasionally succeeding at) dying before jumping back up to normal 750 RPMs, repeating the process continually. I've done enough diagnostic work to figure out that my coasting valve is causing the drop, as the RPM drop occurs when I watch it close, which I know cuts off the rear rotor. It will continually slide open and closed with the RPMs. I've pulled the large hose from the CV that connects to the air cleaner and when I plug it, the RPMs settle out and it purrs like a kitten. I've also checked by unplugging the yellow rat's nest plug, and this leads to it being closed all the time, certainly not desirable.

This makes me think the control switch for the CV is the one doing the pulsing, and so my question is what could cause this? The car is 99% unmodified, the previous owner had it since 1983 and never changed a thing from stock. The engine was rebuilt ~50,000 miles ago, and just recently had new plugs and air filter. I've changed some of the vacuum lines, but it got dark before I could finish last night and I'm working today. I don't think it'd be the vacuum lines though as it seems to draw considerable suction.

 

As some additional information, the problem sometimes cures itself after the car has been driven for 20-30 minutes, but in the meantime, without the hose being plugged it idles horribly. This makes me think something warms up enough to a point where it functions as it should, but I just don't know enough about this car yet to know what that is.