Hondata

I have a:
- Honda Accord 1999 OBD2B 2.0L Vtec (F20B6), running perfectly fine with original ECU.
- Honda Prelude 4th gen 1992 2.0L (F20A4) with Hondata S300 v3.

So I want to boost the Accord and pluged in the Hondata S300 from the Prelude with the OBD2B to OBD1 conversion harness to the F20B6.
Values from MAP, TPS, temp etc are showing realistic numbers in SManager.
I'm also able to start the engine so everything seems in the basics to work fine.

I tried several calibrations and tried some tuning myself but still have the same problem with the different base maps.
The engine revv's to around 4000rpm
Engine speed falls down to 1000rpm and after that starts revving to 3000 a 4000 rpm, and again and again.

For example I use the calibration found here:
viewtopic.php?t=20463
Set "current injector size" to 310cc.
By changing the injector size (I use 310cc as "current injector size" for the Accord) the revving chages a bit to 1000-2000rpm

I also connected a Wide band lambda (only a display so not connected to Hondata yet) showing low values (8 - 10 AFR) while revving up and high values 30-60 AFR while decreasing rpm's, so obviously there fuel is cut while decreasing rpm's.

I also played around with the "Idle" tab but I don't see any differences in the revving problem.

See hereby a piece of my log file
Does someone has any idea where too search or how to solve my problem?

Thanks!

Try unplugging the IACV and starting the car. You'll get an IACV check engine light, but if the idle surge goes away, it's something in the IACV circuit.

Thanks for the suggestion.
Disconnecting the IAC valve works partially, now the engine is revving from ~950 to ~2300rpm when throttle is not pressed.
I will try to explain what I did last days so hopefully someone will have a good suggestion to fix my problem.

I installed a Wide band lambda sensor (Innovate LC2).
The digital output values match with the readings in Hondata so measured lambda values are correct (calibrated in open air).
The car/engine is original except the wide band lambda sensor and Hondata s300 ECU.

I used the basemap found here: viewtopic.php?t=20232
Made some changes to the settings (VTEC off, injector size: 310cc, disable IAC error) and tuned the fuel map based on the Lamba readings (so ignition map is unchanged).
The ECU is running always in Open Loop.
The IAC valve is disconnected.
The SManager file is attached in this message.

I can drive around, slow/high speed, partially and full throttle with this map, so working fine.
Attached a log file while driving on the highway and stopping at a parking.
You see at the end of the log file that the engine is revving from 950rpm to 2300rpm without touching the throttle pedal.
When the engine is cold, the engine is running stable around 2000rpm to warm up.
When the engine is warm we see this revving behaviour.
The engine is running perfectly fine with the original OBD2B ECU and IAC valve connected so it should be a Hondata tuning issue.

See following video for the behaviour: https://youtu.be/qdnaaHTt13Q

Looking forward for any suggestions what this could be.

Is your idle controller 2 or 3 wire?

It's the 3 wire from Denso
I attached 2 pictures to show the type of the valve.
As you can see, it's disconnected.

You will need to get a 2 wire idle controller and an adapter plate to properly mount it. The OBD1 ECU can only control a 2 wire idle controller.

I did a bit more research and I'm not sure if another IAC valve will solve my problem.
Because now it's not connected and the engine has enough air to revv up to 2200rpm with closed throttle.
So when I install the 2wire IAC valve, I assume that the ECU is only able to add additional air and therefore the engine could revv only more but this will not influence stationary rpms right?

Please explain your thoughts.

I've also added a log file showing engine warming up without driving.
So the engine runs stable 1200rpm at the beginning (really rich).
When engine heats up the rpm's increase slowly over a period of 4minutes till a point (2200rpm) and then starts fluctuating.
So it feels more that there is a setting/temperature/whatever relation that kicks in and creates this revving problem.

Hope you can give a more detailed motivation what you think is technically happening.

I figured something out but not completely understand it.

I started again with a this calibration without any changes: viewtopic.php?t=20232
I figured out that this calibration is working kind of acceptable to run the engine idle around 1500rpm.
The engine is running really lean and I've tuned the fuel table 2700rpm and up.
When I try to tune the fields too close to the idle fields to get a 14.7:1 AFR, it starts oscillating again.

The ECU is running in open loop.
The fields in this figure are used when the engine is oscillating around idle, as you can see the engine is running really lean. So my question is:
Could the 3 wire IAC valve stay partially open when no connector is connected and therefore the engine is running lean?
I assume Increasing fuel in the idle area will results in an oscillating engine because of the additional air which is present the engines spins faster till a certain point and drops again.
I've tested with the original ECU and IAC valve connected (and lambda disconnected) and the engine is running stable with AFR 13.2-13.7:1 so it should be possible for me to run with comparable AFR.
Therefore I'm thinking the IAC valve stays a bit open when not connected.

Please let me know if my reasoning is correct.

Oke, the problem is solved!
The 3 wire Idle Air Control Valve is providing too much air when the connector is disconnected.

For this test I made a gasket to be sure the IACV is doing nothing.
Now I was able to run the engine idle at 500rpm. Thanks for pointing me in the right direction!

Good sleuth work - glad you got it solved.