Hey guys,

First let me clarify that Ive searched through many old threads regarding idle issues dating back to the early 2000's, mid 20 teens, and everything about idle issues lead to no resolute solution or it showed adjustments for an S5 Turbo FC or it had to do with high idle situations (over 1000 rpm)

My car is an 87 FC NA

My issue is a bouncing idle at 600rpm to 900rpm.It is a consistent bounce between that range. ALSO if i blip the throttle, it drops to below 500 momentarily and comes back up to bouncing.

This is a new rebuild with maybe 5 miles on it. I dont have an air pump. I do have a dual belt pulley dor the alternator. I dont have an ACV. and I dont have the sub zero stuff. I do have the BAC still that i verified is working (clicks when 12v is applied).


The car starts up everytime. It accelerates smoothly. But at idle bounces up and down.


I would love some pointers to dig into. I assume TPS adjustment? I have Banzais two lamp tester Ill give a try tomorrow.

 

For me whenever I had a bouncing idle it was the TPS.
When adjusting the TPS make sure the car is fully warmed up, or that the thermowax is not keeping the throttle plate slightly open; or your zero will be incorrect.

The drop to 500 might also be the TPS. But also check your dashpot, it is like a damper for your accelerator pedal when you let off and prevents the throttle plates from closing too fast. It might help smooth it out.

 

Youre right. It was the TPS that needed to be adjusted.

However, using Banzai's lamp tester got me in the ballpark but I still needed to use the volt method to ensure the ecu was seeing 1 volt.

The idle is steady now but when i blip the throttle, it dips to 600 then comes back up to 750-800 and kinda bounces around there and smooths out after 15 or so seconds. Guess i should take what i can?

 

i would follow the rest of the idle setup stuff, if its a stock ecu.
there is a connector by the battery, green 2 pin, you jump that, and then you can set the idle speed using the screw on top of the throttle body, and then the mixture on the shock tower.
since you don't have the air pump ACV anymore, its probably going to be closer to the R. (on a stock car, with everything right, it should be in the center)

 

Yep stock ECU.

So should i still set the idle speed if my idle does eventually smooth out (kinda) between 700-800? I got it to bounce inbetween there by adjusting the TPS, as opposed to 600-900 in my original post.

Idle speed is used to adjust where the idle falls? Is that correct?

Edit - reread your post @j9fd3s ill adjust the idle speed screw and play with the shock tower mixture.

 

the ECU is trying to hit its target idle speed, and the closer the actual idle speed is to that the happier everyone is, if that makes sense

i'll put it another way, the ECU starts its idle strategy with a default, and then adjusts from there. the closer you have the thing to the default the better

 

I removed similar parts and it is great to know that adjusting the mixture screw on the shock tower is necessary.

Thanks!

 

And for posterity, when you mention the "idle strategy"... you're talking about whatever the ecu is doing to try and get it to maintain 750rpm (the default i assume) ?

 

yep! its kind of backwards. the ECU sees the throttle is closed (TPS), and you're in neutral (Neutral Switch) or in gear with you foot on the clutch (clutch switch), decides this is idle, and then tries to hit its target rpm

 

@j9fd3s could you explain where the thermowax pellet comes into play in that chain of operations?


I got my car to idle just fine and it runs great. Idle will drop to 700 and hover around 700-850 perfectly after driving around for 30 minutes.

But, what's happening now is that the hot start immediately after i shut off the car, the idle will be bouncing up and down from 400 - 900rpm and evenutally settle back down to 700-800 (after about 15-25 seconds) and be perfect from that point on.... and this only happens after a hot start.

 

thermal wax pushes the throttle open when its cold, its supposed to click off around 65c coolant temp, but it is adjustable. you can adjust both the rpm it goes to and when it turns off. and then sometimes the hoses are full of air or rusty junk too

 

Hmm. I have an NA S4.

It looks like the thermal wax is off the screw when cold and extends down and touches the screw when the car is fully warm - is that incorrect?


edit - just going to leave this link here https://www.rx7club.com/2nd-generati...stions-506407/

it kinda helps figure out the purpose of the thermowax.