MX5 mk2 1.8 idle drop

[Joeigglypuff]

Reposted from MX5 forum:

Some may have seen me post before about this but I haven't been getting anywhere. Been driving the car with this problem for 6 months or so now and it's no massive issue, mechanic says the car looks just as solid still - but it's annoying. I have a problem where when slowing and revs falling (i.e. not in gear basically) the rpm drops to around 400/500rpm before boosting up again to where it should be around 800 (give or take 50 either side). Almost like it's about to stall, although it never does.

I've had it in the garage and checked all the hoses but doesn't seem to be an air leak anywhere. Also replaced knock, cam, crank and MAF sensors.

Final thing I've done is followed this guide to set the base idle.

It seemed to be a little low, so I upped it - but the idle kept dropping down to around 500rpm for a second then back up to where I set it at about 800rpm for 7 or 8 seconds, then back again for a second and so on. No idea what could be causing this but it seems the idle drops even when trying to set it's base setting. Any further ideas of what this could be?

[cheshire_cat]

Mine does the same occasionally. Fixed by cleaning the throttle butterfly. Just pop the big pipe coming from the air box off and get a rag in the throttle. My thinking is by clearing the gunk, a little more air can pass to keep the idle up.

[Joeigglypuff]

cheshire_cat said:

Mine does the same occasionally. Fixed by cleaning the throttle butterfly. Just pop the big pipe coming from the air box off and get a rag in the throttle. My thinking is by clearing the gunk, a little more air can pass to keep the idle up.

Have tried this but hasn't worked

[cheshire_cat]

Tried screwing in the Allen key grub screw to slightly open the throttle? Revs should never drop below a certain amount that way (at the sacrifice of slightly higher idle).