Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

RX7 PowerFC weird throttle response / bouncing

General Tuning Discussion

Forum Posts



Tech Articles

Discuss all things tuning in this section. News, products, problems and results. 

= Resolved threads


Setting up RX7 S4 NA (stripped, no electronics, only FPR and MAP on vacuum) with PowerFC, keeping a low stable throttle to fine tune idle/timing but RPM/s bounce for some reason and MAP reading also bounces. In my understanding constant throttle position = constant MAP because intake directly wired. But MAP varies, what may cause it? MAP sensor seems to be valid when testing it with vacuum.

Attached Files

I've not had much to do with the PFC on a rotary but I'll offer what advice I can. Firstly is your TPS correctly adjusted? It's important that the ECU is seeing the correct VTA1 and VTA2 voltages at closed throttle. If the ECU is seeing a closed throttle but your throttle is actually open slightly then it may be bringing in over run fuel cut when the rpm rises. This would oscillate on and off giving the effect you're seeing. There is also an idle learn procedure for the PFC which you need to go through. This is covered in the PFC documentation which a google search should find easily enough.

TPS is very close to factory spec and has no spikes on whole range:

VTA1 (3G) (Full Range): Closed 0.76V [spec 0.1-0.7V]; WOT 4.5V [spec 4.2-4.6V];

VTA2 (3F) (Narrow Range): Closed 1.03V [spec 0.75-1.25V]; WOT 4.7V [spec 4.8-5.0v];

O2 Feedback and IG-Control disabled, target idle RPM's set to zero, so PFC should use user provided IG maps and ignore internal idle logic.

Attached Files

MAP will show the engine load / vacuum, which is not exactly a mirror image of Throttle position. MAP will generally increase when the throttle increases, but MAP can also change even when throttle is constant at idle. MAP at idle generally depends on how much load the engine is experiencing. For instance if the cooling fans turn on, or the AC turns on, or if the engine runs through a fuel map cell that is so lean or so rich it loses power... any of those things will make the MAP signal will show less vacuum (higher PIM value on a PowerFC). This isn't RX7-specific or PFC-specific, it's just part of how engines work. Most of the HP Academy videos about idle tuning or fuel map tuning should be useful for dialing in the idle so the RPM doesn't bounce.

You might simply need to adjust your fuel map cells to give the engine the right amount of fuel so it can idle smoothly. Try to show Injector Pulse / milliseconds, Idle Air Control Valve position, and Lambda / AFR from wideband sensor on the datalog. Remember there is sometimes a slight delay between the engine's combustion and the wideband sensor's reading... in other words the O2 reading might be telling you what happened in the engine 0.1 - 0.8 seconds earlier. PowerFC was very popular with US-based RX7 owners about 10-15 years ago, you might find helpful info by searching online for 'Chuck Westbrook PFC Tuning Notes'.