B230FT Microsquirt Misfire

Been working this project for a while. Base car is a 1994 940 sedan. This is the work that has been done:

3" custom down pipe
Innovate Wideband O2 sensor
Microsquirt from DIY Autotune
4 LS2 Coils "near plug"
Red fuel injectors
Walbro in-tank fuel pump (bypassed factory high-pressure pump)
Gutted factory airbox
Electronic Boost Controller
GM MAP Sensor
GM IAT Sensor
No Throttle Position Sensor / No MAF Sensor
Factory CLT Sensor, calibrated

This is my first FI project and I was stoked when I was able to get it to fire and sort of run. I'm still a bit overwhelmed with the tuning process. I had it to the point where I could start it and drive it around the block and on to the trailer. And that is about as far as I was able to get it tuned.

This morning, I took it to a dyno shop here in LA. We got it all loaded up and everything was looking great. The tuner was able to get the AFR nice and stable and the torque curve was looking fat and flat.

That is until we hit about 4,200 RPM.

With or without a load, around 4,200 RPM the tach signal goes crazy. The lost sync count starts to climb and the motor misfires badly. We checked the plug gap to make sure we weren't blowing out the spark. We change the grounding of the shield wire for the VR sensor but that made little difference. The tuner felt we are dealing with some kind of noise that is interfering with (harmonizing with?) the tach signal and preventing the microsquirt from doing its thing.

The good news is I can replicate the problem simply by free-reving the motor. I'm going to start by trying to isolate the wiring to the VR sensor from the rest of the harness I built. I am also going to verify that the VR sensor is properly secured in position. I have spare coils so I may also swap out coils to see if one is bad and making an excessive amount of noise. And I may make a new coil harness. The first one I made includes noise filtering capacitors as instructed on the MSEXTRA site. I have ordered a new VR sensor as well.

I welcome any other suggestions. I am hoping someone has had a similar experience and can guide me to the promised land.

SK

1) check that the "Rev Limiter" settings are OK
2) turn off all "Ignition"->"Noise Filtering" settings -- it can sometimes act as a rev limiter
3) post a .msq, a normal log, and a high speed "composite logger" tooth log of the issue

edit: this may be relevant - http://forums.turbobricks.com/showthread.php?t=306770
(I think there's also an internal solder-blob jumper on the Microsquirt v3 boards that can be used to reduce the peak VR voltage)

Thanks for the input. I double checked the Rev Limiter and Noise Filter settings. Rev Limiter is set at 6,700. All Noise Filtering is off.

I have to read through the post you linked. It looks to be a similar issue.

bobxyz said:

1) check that the "Rev Limiter" settings are OK
2) turn off all "Ignition"->"Noise Filtering" settings -- it can sometimes act as a rev limiter
3) post a .msq, a normal log, and a high speed "composite logger" tooth log of the issue

edit: this may be relevant - http://forums.turbobricks.com/showthread.php?t=306770
(I think there's also an internal solder-blob jumper on the Microsquirt v3 boards that can be used to reduce the peak VR voltage)

Click to expand...

UPDATE: This morning, I opened up the pig-tail between the factory CPS plug and the Microsquirt. I left the sensor side alone so that I can plug-n-play a replacement sensor in the future. I spliced in a 10k ohm, 1/2 watt resister into each of the black and red leads (ie two resistors). Put it all back together and voila, no more top-end cutout. Starting was difficult, however. I had a lost sync count of 12 before it started.

I'm now off to pick-your-part to find another factory plug that I am going to wire in two 4.7k resistors in the same fashion to see if the lower resistance will give me the low speed voltage necessary while at the same time clipping the top end. I also have a new CPS arriving next week. Perhaps a new sensor will be more 'sensitive' at cranking RPMs?

I will post results as soon as I have them.

EDIT: see this page on DIYAutoTune's site. Scroll down to the "Bosch 60-2" section. https://www.diyautotune.com/support/tech/other/megasquirt-vr/

EDIT2: Further idea: if I cannot find a happy medium that allows me to control top end voltage without clipping the bottom too much, how about installing a DPST relay connected to the starter switch to bypass the resistors during cranking?