Eagle7

I'm getting the engine to idle pretty well, but it has a periodic stumble in it. I observed that the MegaTune RPM gauge jumps quite a bit when it stumbles, so I suspected a noisy signal from the CAS. I moved a plug wire close to the CAS cable and the engine died. I rerouted the plug wires as far away from the CAS cable as I could and the stumbling frequency and amplitude diminished.

I need to dig into my harness and see if the CAS wires are in good shape, but in addition, do you have experience/success in suppressing CAS noise inside the MS? I'm speculating that some caps might work wonders. But I need to support revs up to 9K RPM.

I also will check out my plug wires. They are fresh, but they were chosen for spark performance, not suppression. Maybe the stock wires are better for this situation.

I'm confused by the result of this noise. Looking at an example from my log:
* RPM goes from 800 to 1700 in one 67 ms log sample, back to 900 the next sample.
* MAP stays constant at about 40
* VE stays constant at 46
* Injector pulse width stays constant at 2.8
* Injector duty cycle jumps from 3.6 to 7.6, so I'm getting more fuel.
* The AFR starts climbing from 12.2, eventually reaching 13.5 in about 500 ms, then falls back to 12.2.
My question is - why did it go leaner with more fuel?

Thanks,

pmrobert

A misfire/stumble will show as a lean transient in the WB signal. Uncombusted AF mix has lots of oxygen in it, just like a very lean mixture.

-Mike

jayroc

What kind of signal wire are you using for the CAS? Stock? Also, where are you grounding it to? Is it a stock FC, or did you rewire it from scratch?

muythaibxr

yeah, if you don't have the shield around the CAS wire properly grounded, you WILL have noise. Properly grounded means ground it only on one end, do not ground it on both ends or it will do nothing.... Also, if you are using a 2.2 board, and you are using the error* daughterboard, adjust the load potentiometers to have more resistance... especially the one you have the G signal connected to... I have found that Noise on that wire influences these tach spikes greatly.

Eagle7

Thanks for the AFR explanation.

V3 board. Stock S4 harness.

I traced the CAS cabling in the stock harness. The shield is unconnected at the CAS end in the <forgot its name> harness. It connects to the Front harness near the trailing ignitor, exposing about 6" of unshielded wires. The shield in the <forgot its name> harness connects to shield in the Front harness through a separate connector with about 10" of wire. The Front harness shielded cable enters the passenger compartment left of the steering column and runs over to the ECU on the passenger side. Near the ECU connector, the shield connects to a wire that runs back to the instrument cluster area, where it is jumpered to a number of ground wires. Eventually it ties to a chassis ground - I expect somewhere in the instrument cluster/fuse box area. I think I'm stuck with this wiring based on my racing class rules.

If the other threads are correct, I've got my G and Ne polarities wrong. I'm wired with 1Q connected to the VR circuit on the V3 board (24 teeth) and 1P connected to the LM1815 (2 teeth).

muythaibxr

the other threads are definitely correct... your wiring is probably backwards. I've put a note in the FAQ, and will post the proper diagram asap.

Eagle7

Fixed. MUCH better. Back to tuning.

Thanks,