'87 S4 RX-7 with stock N/A 13B rotary. Earlier this afternoon, for no apparent reason, the idle has begun to oscillate between 1600-1900rpm. The car bucks when decelerating with the clutch out and will not go below 1300rpm when I try to coast in 4th or 5th gear. Any ideas? Adjusting the idle speed control screw at the top of the intake manifold has no effect.
Dont know much about the FC and rotaries but my guess would be the TPS. Try adjusting it. Also cleaning the TB and all the ports and linkages might help
Vacuum leak. First place I'd look is the intake hoses for the oil injectors under the intake. The vacuum hoses going to those will crack with heat and age. Do NOT block them off, or the engine will starve for oil!
Hey Jensen, when you say to replace the vacuum hoses underneath the intake. do you mean on the driver's side or passenger's side? So far I've replaced all of the vacuum hoses on the passenger's side and on back of the engine (between the firewall and the throttle body), to no effect.
These are under the top half of the intake, meaning you have to take the intake apart. You can leave the bottom half of the intake in place, just remove the bolts which hold the top half on and lift it off. Sounds easier than it is. 
This pic shows the position of all the vacuum lines that I'm talking about.
There are two oil injectors at the center which screw into the center 'iron' and then there's one on each end. They all connect to one of those metal vacuum manifolds the Japanese are so fond of and they then splice into the intake behind the big intake tube between the air flow meter and the throttle body. There's five or six small hoses that connect all that crap together, when I pulled the intake off my S5 motor the hoses literally crumbled.
The injectors need those lines so that on the intake stroke the engine's vacuum pulls oil into the intake ports. Plug them, and the oil supply drops dramatically leading to ugly noises and smoking wallets.
After searching around the rx7club, I found this thread:
VIDEO - Still crappy idle, no response
The poster's Turbo II video has the same thing going on as my FC. There's a metric butt-ton of info in the thread and the website that summarized it (http://www.globalvicinity.com) is down, so it's hard to follow. The best I can parse from it is that the owner had to test the TPS, clean the BAC, and adjust the fast idle cam screw, which is conveniently located on the back of the UIM (thank FSM I have skinny fingers). I've been meaning to remove the UIM anyways so I can clean the ground, so I'll get to the vacuum hoses while I'm at.
Looks like I'mma hafta burn my unused vacation days to give my car a little TLC...
Vacuum leak or misadjusted throttle plates. The ECU has a rev limiter of around 1800-2000 rpm when it sees that the throttle is at idle. Pull the TPS and see if the idle flattens out, then go from there.
Check for vacuum leaks with an UNLIT propane torch; the idle should smooth out when you pass over the leak. There are a bazillion possible vacuum leaks on the TII. If you don't find any, try pulling the throttle body and checking the throttle plates per the FSM.
Misadjusted throttle stops will cause an erratic idle, but generally only after someone's twinked with it trying to 'fix' something else. It doesn't generally manifest as an 'all of a sudden it started idling funny' kind of thing. Propane works well for chasing vacuum leaks, so does brake cleaner or carburetor cleaner. Keep a fire extinguisher handy!
RexSeven,
This may help...sorry if you know all this already.
Removing the UIM and Throttle body is a pain in that there is a lot of prep work to do. De-pressurize the fuel system by running the engine and then disconnecting the fuel pump connection under the trunk carpet. Shut off the inginition when the engine dies and then remove the gas cap. Of course to do this you have to remove the driver's side rear speaker and the trim around the hatch to pull back the carpeting. Label every vaccum, coolant and electrical connection. Remove the Fan. Remove the belts. Remove the Fan Shroud to drill out the top Air Pump bolt if you break it like I did. Drain the coolant. Remove the coolant hoses to the BAC valve and water themo valve. Remove all the vauum and electrical connections from the UIM / TB. Pull chunks of skin off your thumbs trying to pull the fuel lines off the secondary fuel rail. Then give up and vertically slice the end of the hoses so you can remove them without superhuman strength. It is worth the $20 for new fuel injection hose...trust me. Remove the throttle cable. Remove the air valve under the dynamic chamber so that you can get to the two bolts that support the dynamic chamber on the driver's side. Remember to spray these with PB Blaster first...many times...same thing with the bolt in the support bracket on the font of the dynamic chamber. Spray the UIM bolt and nuts with PB Blaster MANY times over multiple days before attempting to remove them. One of the nuts holding my UIM was not "free" and the stud un-threaded...no big deal I guess because it did not break.
This is basically where I am at now. I have new vaccuum lines and the "vacu-spider" from Mazdatrix ready to install along with the new fuel injection hoses and fuel pulsation damper. I am debating whether or not to replace the OMP lines since I have the UIM off and I do not want to have to do that again. However my main issue now is my throttle body as I noted in my other post.
I wish you the best of luck with your UIM removal. Hopefully we can get both of our cars sorted out and running right soon.
Ray
E36 M3, I've got my work cut out for me...
Thanks racer_ace, I knew it would be complex, but knowing what I have to do ahead of time will make it a little easier for me to not screw up.
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