Any ideas? I have a power FC on it but I was just wondering what controls the idle during hot starts. If I drive the car and start it within 5 minutes the idle is perfect 820 rpms. If I wait anything longer than that and start the car, the idle bounces up and down from 1500 to 700 rpms and bounces for approx 1 to 2 minutes.

Alex

 

Give it a large rev (say 4000rpm) during the hot restart, and see if it clears up after that.

Mine did do this to a lesser degree, and turned out to be the air intake temp sensor under the UIM. It heatsoaks badly when the car sits hot.

When you come back and start it up again, the sensor thinks the air intake temp is scorching, and so it sends this info to the ECU, which richens the idle mixture up too much, causing the lumpy idle.

A big rev on my car would clear the excess fuel out, as well as pull in a gulp of fresh cooler air, and things would settle down.
These cars do heat soak very badly when just parked hot and left alone for 10 minutes or so.

 

Ill try that and let you know thank you so much.

 

Get a fast reacting IAT sensor. That will help too. Onve the factory metal encapsulated sensor becomes heat sosked, it takes time for it to cool off.

 

Raise the idle fuel cut until it doesn't do it anymore.

thewird

 

I'm having the same issue, but not sure if it's the same source.
The coolant is going hotter after stopping the engine because of the turbos which still very hot, when it reach or go over the fan2 setting the engine is going up and down when I restart it. After a couple of seconds the fresh coolant coming from the radiator is cooling the engine down and the idle stop bouncing.

Did you check the engine temp on the PFC Commander during bouncing idle ?

 

If its a LHD RX-7, did you cut the 4 wires?

thewird

 

It's a RHD, and I don't know anything about the 4 wires ^^"

 

RHD's don't have to worry about the 4 wires.

I suggest the same thing then, raise the fuel cut idle.

thewird

 

The 3? I've tried to move them without success.

But I don't want to go over Peruvianrx7 who is the first to post the problem.

 

Try 1500 RPM. Basically get it higher then the highest bounce point. Once you get it stable you will either A, have fixed the problem or B, make it easier to find the problem.

thewird

 

I'll try this

Thank you

 

I believe in my case like thewird said. it has something to do with the settings in the power fc and my engine being the original engine with 90K miles on it and below 100 psi in compression. thewird how do i go about adjusting fuel cut or idle fuel delivery? Alex

 

i tried adding more fuel through the PIM volt but no change.

 

Not to doubt you Thewird, but I still believe it needs less fuel. The hot restart idle-hunting is due to the hot air and coolant heat soaking in the engine bay. If you leave a twin turbo 3rd Gen parked after a run for 15 minutes, they do heat soak pretty badly. Peak engine bay heat soak temps can occur 10-15 minutes after hot shut down, and then they will very slowly begin to drop off.

Hot air has less available oxygen and is less combustible than cool air. The large masses of hot air sitting in the intercooler, UIM, throttlebody elbow and turbochargers just sits there and absorbs heat out of the surrounding component materials (thick, hot aluminum castings) and next time you start her up, the poor engine is immediately sucking in all this very hot air that has been just sitting in the intake tract, baking.

It then runs rich momentarily because everything is working against the engine management in this scenario- less oxygen available from the hot intake air, and the PFC is still injecting start-up AFR quantities of fuel, which is now too rich for the situation, because the available air is too hot and not very oxygen-dense.

You would notice it clears up and runs better after you nurse it out of the parking space and then drive it down the street at 50mph for a minute or so? This is the car taking in cooler, denser air with more combustible oxygen, and the mixture is no longer rich and it clears its throat, so to speak.

Look at a cold December night's run for the opposite situation. Its a frosty 6C outside. Your water temps are low, intake temps are low. You wail on it hard, teaching a Honda a lesson. It overboosts by Xpsi and blows an apex seal because it leans out momentarily due to the cold air, and lean-detonates. This is the opposite of what happens on a hot, 35C day, with the car parked and heat soaked. Cold air = leaner AFRs, hot air = richer AFRs, in most all situations

Idle mixtures at temps over 95C can be leaned back in the pfc. There is a setting in the pfc through the Datalogit interface to adjust AFR vs water temp/air intake temp. Try that in small steps until you see the hot starts clean up. If you're unsure it's running rich during a hot start, have a buddy hot start the car while you stand at the rear bumper and see/smell the exhaust richness til it clears up and idles clean after a minute or two (gets through the hot engine bay air and begins to pull in cooler, denser air from outside the engine bay).

 

Hunting comes from a lean-rich hunt. You can't just randomly add fuel, you have to add it to the right spot, that is where proper tuning comes in. If it was too rich, it would just not want to idle instead of hunting.

You go to your idle settings to adjust the fuel cut. It would be the 3 settings with the "F/C" label and is higher then the other 3 values which are the target idle values.

thewird

 

This right here.

I would look at the cells left and right, above and below and see if they have a smooth transition all around it.

it sounds like the car wants to jump between two cells, one being a lot lower than the other. so the powerFC cann't interpolate well between a say low cell and a super high cell, so it jumps back and forth hunting.

 

Thank you thewird will try it now

 

i tried it but nothing... same issue