94 miata with an M45 blower on powercards (yes i know, but i've put a few thousand miles on it with no real issues)

I've had a low hot idle for a bit (can drop to 600rpm), and decided its time to play again. The issue never stalled the car, just felt rough at lights and stumbly. Car runs and has run fine besides just a little low rpm at lights. Decided to jump the ten/grd to raise base idle to 800rpm. Now when I blip the throttle, it will go back to the 800, then about 2 seconds later increase to 1500, 1750, and close to 2000, hang there for 5-10 seconds, then work its way progressively back. I checked the IACV, and ohm value is good and it operated with power to the prongs. cleaned the IACV. Removed the throttle body as well, which was pretty clean but I cleaned gently with throttle body cleaner anyway. cleaned the maf. checked pcv. unplugged the battery. took it for a long drive to try to "relearn." Still the same issue. I've unplugged the IACV, and am still having the same issue. It seems to occur when i blip the throttle. If i slowly raise it and let off, it doesn't happen. It is very consistent in its raise and drop, which seems to be an ECU controlled thing.

I'm not quite sure where to go from here, seeing as all I did to initiate the problem was to turn the set screw. Does the TPS need to be readjusted, or is it bad? Or is there a way to adjust the idle jump? seemed before i touched it, it was only raising idle to 1200 or so, then dropping back to 600-700, which seemed on track for how the ecu should manage idle off throttle or under load (a/c, etc).

Update: Just unplugged the TPS, and the issue went away. Still doesn't tell me if a new tps will fix the issue, but at least some forward movement. I'll bench test it this week and try readjusting. i also have a spare i found that i can throw on to try. not sure that will be the fix, so any input is appreciated.

Thanks in advance.

 

Put the idle screw back where it was?

 

then i'll have a low idle/stumble. one problem or another i guess. any idea what it may be?

 

https://www.miataturbo.net/superchar...54/#post750230

This is a common issue with blowers on the exhaust manifold side that moved the Throttle Body pre-blower. Look up "idle dip" on other forums. No one really found a quick fix due to the large throttled volume and the constantly spinning blower. It's not so much a piggyback thing, but how the blower's set up. The two confirmed fixes are hardware - either a slightly complicated "Dual throttle body" DIY upgrade, or move the TB back to the IM and put a BOV. Either way, putting the TB back on the IM will cleanup the response. The idle dip is worse in cold weather, so you'd just adjust once the weather turns.

Cheeseball workaround is to put the idle and throttle stop back to stock. Then adjust the IAC valve slightly. Adjust it so the idle "floats" down to idle RPM - won't be crisp anymore. But that gives the system a little time to get its bearings, adjust fuel trim and not hunt for the last 3 seconds. Not optimal, but better that blipping the throttle at every stop.

A full ECU can fix this because you're switching to MAP based fueling. But you're basically tuning out a workaround based on the notes above.

 

Thank you, thats good input. I had done this swap a long time ago, kindof forgot that could be an issue, and since i hadn't really had a problem before never considered it now. I'll plug away with that theory and maybe consider the BOV route.