My 86' NA FC has ran great, it's only at 93,000 kms, always a constant steady 750 rpm idle, the only fault being the aux ports don't work as they should, because the exhaust is straight piped and doesn't have the back pressure. This past weekend, I did an oil change, cone filter mod, removed the air pump, and capped off the big vacuum line going from the air pump to the ACV. I did notice one of the solenoid valves (blue one closest to the front of the engine) had the nipples for the vacuum lines snapped off, not sure how long it was like that, but I capped the lines for now till I get one from the wrecker. The car ran just fine since I did all of this, smooth idle, good power, sounds great.

Then today while leaving work I gave the car a hard blast down the highway, and once I got back into town I was going down a hill, and noticed the car sounded differently on decel, like it was missing, or something. Sounded like gurgling almost, and was very touchy on the throttle. Got to a red light, and it was idling at 1500 - 2000 rpm, and surging. I shut the car off, started it again, it revved to 3k as normal, and then when the cold start finished, right back to 1500-2000 and surging. I tried disconnecting the TPS, and it idles at 2.5k constant, but sounds like it misses the odd time.

I have been meaning to replace the fuel filter, not sure if that could be the cause, but this was very sudden to me, I didn't mess with any adjustments, car ran great on the way home, and was idling fine when I left the parking lot at work.

Anything I should check? I'm almost thinking the filter and MAF shaking around might have damaged a wire (the filter is only wired in place for now, building a cold air box next weekend), my solenoid valve (but why all of a sudden) or a vacuum leak. But I'm trying to think of things that fail suddenly, maybe my TPS, O2 sensor, or coolant temp sensor failed, any advice, or tips on where to start hunting.

I'm posting up a vid after my phone charges a bit

 

Could be a vacuum leak. But w/the engine trying to idle, press upwards on the throttle linkage and see if it budges or not. If you can press upwards on it w/the car fully warmed then the idle speed ought to drop. If the idle drops then a throttle cable is too tight or the plates are sticking open due to some internal reason or the Fast Idle Cam is not working properly.

 

Issue appears to be resolved. Seems the throttle plate was jammed, I was moving the Fast Idle Cam, heard a click, and it went back to normal idle. I found this out after doing a poor mans compression test, sounded good, checked for a flat spot in the TPS, didn't find one, checked for a vacuum leak with propane, no rpm increase.

Here is the video just in case anyone else has this problem:

86 Mazda RX-7 Idle Problem - YouTube