Massive dip in RPM before returning to idle.
 

My GSL-SE has started doing this only a few weeks ago. Last thing I did was reseal the oil pan. Before that it seemed fine. Didn't touch anything else...

Basically what happens is when I'm coming to a stop and I push the clutch in, RPM drops dangerously low before returning to normal idle speed. Even stalled a few times.

It only happens in the first few minutes of driving, then it seems to stabilize, possibly once it's fully warmed up. And it only happens when clutch is held in. If I pop it in neutral and let go of the clutch, it just returns to idle without a dip.

Vac leak blah blah, everything is rebuilt and in mint condition, and it was alright last month. Feels like some sensor is giving false reading until warmed up and ECU can't keep up.

Check the dash pot. 85 FSM, Sec. 4B, p. 40. If problem persists, see the Trouble Shooting guide. I'd start on p. 18.

+1 classic dashpot symptom; slamming the barn door, catching the horse's tail in it.

I'll look it over. I remember I adjusted it a while ago, but maybe with a bit of use it's getting worn in or something...

might be time to go thru and set the TPS and idle speed too. it might also be a sticky BAC valve too, but i think we covered that?

Yeh, it might be my TPS is a bit off when cold.

It's not a big deal, though. It just kind of confused me out the first time it stalled, I was like whoa wtf, but then I was like oh.

Looked over the dash pot and such, everything seems ok.

Further playing around reveals that this dip only happens IN GEAR with clutch in. I know there are sensors on my tranny, and I think one of them is the "in gear" sensor or some such. Could I have wired it wrong or something?

Anyone know what the "Neutral" switch is used for exactly? FSM is not clear on this. Just says it sends signal to ECU about the in-gear condition, doesn't say what the ECU does with said information.

Edit: Never mind, found the answer. Gonna try disconnecting the neutral switch.

the other thing to look at might be the clutch switch, the FD's stall when the clutch switch is bad. the FB/fc doesn't seem to be as effected, but its very easy to check.

the spring in there is the same as a normal pen...

double check that the check valve is functional on the brake booster vacuum line
and do the suck test on that line to see if it will seal to your tongue
( from manifold end to the booster )
-- if not,, you have a vac leak in the booster,, or in the hose

I'll check the clutch switch, but simply pushing the clutch in doesn't cause the dip, only being in gear does. When I'm not in gear, everything is perfect.

Ok, further developments and further confusion.

Disconnected the neutral switch and re-adjusted TPS one more time just to be sure. Now it dips almost all the time. Sometimes it dips so low that my instrument panel lights up for a moment, and sometimes it barely dips at all. It actually dips just revving at a standstill - clutch in, gear out. The RPM just drops low on the way down, engine stumbles a couple of times and then recovers to a perfectly smooth idle.

I played around with the dash pot. Didn't make any difference. Took it off completely, even. Not much changed. Then I set it to engage earlier - that made the engine rev up a bit when I let go of the gas, but it would still dip coming down.

Then I tested the clutch switch. That was a PITA just getting to it, and I took it out to test it properly, since I couldn't probe it and press the clutch at the same time with any amount of consistency. Seems like it's ok. I don't know how much resistance I should be seeing on it when it's open, but it does appear to function somewhat.

Further driving around I could not get the car to consistently dip on the way down. I tried clutch in, clutch out, gear in, gear out, coming down from 2K, 3K, 4K, 5K, popping it in and out of gear, pushing the clutch in slowly and then quickly. Sometimes it dips a lot, and sometimes it dips barely at all. Couple of times it didn't dip at all and in fact didn't drop below 1200 RPM.

Only thing that's consistent is the dip that happens when I just pure-rev it, no gear, and not touching the clutch.

I donno. Something is causing a delay at idle recovery.

This sounds odd and may be way off base, check the engine and tranny mounts. If broken, they may be allowing the engine to shift, catching/pinching a wire or cable. It's a long shot in the dark.

Mounts are stiff. Not much movement.

I'm trying to think what else could cause the RPM to shoot down right past the idle. Feels almost like the engine is either drowning or starving for fuel as it goes past the 900 RPM mark, and ECU can't catch it fast enough.

My car is doin this did you fix and how???

I know what it is i think. Just learned this at tech college. The diaphram in your brakr booster probably is leaking or split and will cause a massive amount of air to be pulled through from the manifold under braking and causes it to almost die from air starvation on one rotor.

ok its worth checking. i since changed TPS and plugs. it ran sweet for 2 min then it fell on its face. it started stumbling and puffs of black smoke. whatever it is its in both rotors because all four plugs were black(looked nice few days earlier). here is link to whats been goin on
https://www.rx7club.com/general-rotary-tech-support-11/2nd-gen-rpm-drop-while-idle-1000836/
any ideas would be great