So i just finished installing a complete T04b turbo into my S4 RX7 Turbo II. It's running great at the moment. The only problem i've got is that it idles a bit lean when it's running at normal operating temperature. It deceleartes ok, stops at 900rpm and runs with AFR 12.5 for about 5 seconds. Then it leans out to AFR 13.9-14.5. It's jumping between 13.9 and 14.5 like it's trying to regulate something.

I first thought it could be the tps but it's set to 1019Ω when hot. I also tried to set the AFR by turning the adjustable idle mixture resistor but i had to set it to full rich to get those AFRs above. By the way the resistor is ok. I can adjust it from 0,45kΩ till 5kΩ.
I also searched the whole engine for vacuum leaks. Everything ok there.
Next thing i tried was to adjust the BAC and recognized something strange. I can set the idle speed to 750rpm as long as the jumper wire is in the initial set coupler. If i remove it the BAC will set the idle back to 900 imediately. I don't think that should happen.

By the way i removed all the emissions stuff and the whole rats nest but kept the BAC and that other valve on top of the intake manifold in place.


What can i do to get the idle more rich and the idle speed down to 750?

 

Well first off all what size injectors are you running ? Upgraded fuel pump ? Upgraded fpr ?

Aftermarket ecu ? piggyback ? boost controller ? ( i hope you have a fuel cut defender at least )

 

A turbo BAC should be adjustable. A leak within the BAC could cause it to idle higher than normal. What does it idle at if you disconnect the plug to the BAC? With little to no load (brake pedal depressed) it should idle at close to 500 rpm unplugged. Perhaps you could lower the idle below 750 w/the coupler jumpered then remove the jumper and see how the idle adjusts.

 

If i disconnect the BAC i can make it idle where i want by adjusting the screw. So i guess i will need to get it out of the car and check it for leaks but first i'm going to try what you mentioned satch. Might work.

Nothing like that yet but that can't be the problem because at idle there's no boost at all. By the way i would never ever use a FCD. That would work for a few psi more but never for what i'm going to do with my car. A Greddy emanage is allready ordered.

 

goodluck. how much boost is a fcd good for? i figure stock turbos should do 10psi with one no problem

 

But for 10psi you would at least need an afc or something similar but i guess with only those two things the ignition timing wouldn't really be good under full boost.


Back to the problem. I solved one part of it. the tps connector was a bit corroded. I cleaned it and set it up correctly. Now the car is idling with AFR 13.0-13.1. I guess that's more or less ok but it's running at 1000rpm now and if i try to get it down to 750rpm with the BAC it goes down but get's leaner. I'm totally clueless why it's getting leaner when i close the bypass air passage.

 

Is the timing spot on? If you press upward on the thottle linkage does the idle stay the same or drop some?

 

The timing is ok. I checked it some minutes ago. I was allready thinking of checking the throttle body. The throttle sits exactly at that small stop screw so it can't move. I tried to move it but it seems to be ok. There's only one strange thing. That stop screw is turned completely in so that the nut sits exactly at the same level of the screw head.
Could be that the previous owner tried to adjust something there because the car had vacuum leaks nearly everywhere.
Should i try to adjust the throttle somehow maybe?

 

The throttle stop screw on the front is for the secondary plates and not the primary. The secondary lever should be level. Have you checked the throttle cable or the cruise control cable?

Look at pic #6 and #15. In pic #6 it is properly set or does yours look like #15?


https://www.rx7club.com/2nd-generati...ughts-1003065/

 

I checked the cables. They aren't pulling the throttle open when the car is idling. My car is a Turbo II so the throttle body is a bit different but the screw in pic #15 looks like the one i meant. Mine is on the back of the tb and screwed all in.

 

I was driving the car a few miles today. No problems but a lean idle. The idle is rock steady at 1000rpm but still very lean (at AFR 14 now) but as i said before the car start's idling normal and leans out 5 seconds after it started idling. Something must start or stop working after those 5 seconds. What could it be?

 

Huh,I didn't think that 13.0 - 14.0 was very lean, since mine idles at 15 - 16.6 afr when warmed up. Cold, it's down to 12-13.5 afr.

 

Do you think i should let it idle like it is? I don't really care if it idles at 850 or 1000rpm but i don't want to damage the car because of running lean.

 

Mine idles at 11afr(without adjusting the rtek), 13 without help would be great!

 

Just to finish this: I solved my idle problem. It was a combination of dirt inside the bac-valve and the big block off plate leaking some air because of a piece of an old seal which i missed when i cleaned the surface. The car is idling at AFR 12-13 now depending on whether the electric fan is on or not.

Thank you for all your help and suggestions.