Engine breaks up under load ~3000rpm

[Joeblob7]

Engine: 383 TPI with 7 pin HEI (running off of coil in cap)

Running MS2extra, using Tunerstudio

Engine starts, idles, and runs great until about 3000rpm under load. We are able to achieve RPMs above 3000 while in park.

While driving at a slow throttle increase the engine breaks up/sputters and becomes erratic when approaching 3000 RPM

We've also noticed that the dash tach (reading directly from the coil) will drop out when this happens. The RPM signal within Tunerstudio doesn't drop out.

I've tried attaching the datalog, but the file was to big, so we'll run a short one tomorrow of the problem and post.

thanks in advance

[Joeblob7]

Update:

Did some research and am leaning towards a noise issues. We installed the TPI plenum extension cover this morning (aluminum plate that goes over the distributor) and that seemed to knock down some of the random RPM spikes we were seeing. This didn't completely fix our issue, but seemed to be headed in the right direction.

I took a data log of the issue and have attached it.

Info on Data Log: got the engine warmed up and drove around the block. Everything ran nice and smooth. We then took it out on a road where we could open it up. That's when I started logging. Made a slow pull up to 4000RPM (I"m guessing the plenum extension cover let us go a bit higher in the rpm range) once we reached 4000Rs the break up started happening again. RPM signal fluttered and the tach on the dashboard would drop out intermittently. (not the tach in Tunerstudio...that stayed put)

We are in the process of shielding and rerouting some wires near the distributor and plug wires to see how that helps.

[jsmcortina]

The datalog shows horrible noise on the RPM trace. Try enabling noise filtering.

I think that perhaps TunerStudio is hiding the problem from you by smoothing the needle on the gauge - I think there's a way to turn that off, so it shows you the actual value, but I can't seem to find it at the moment.

James

[Joeblob7]

We were able to knock down the noise that we were getting randomly in the RPM signal during idle and low loads by shielding the wires that pass by the distributor.

Prior, the RPM signal would have small spikes here and there while idling under no load... once we shielded the wires those disappeared and the RPM signal seemed to be nice and steady until we reached that high load point where the signal would go nuts, engine backfiring, sputtering, and tach signal dropping out.

Of course I left my Hard Drive with the latest (shielded wires) data log at home, so I'll post that tonight.

We are wondering if we have a bad coil? Seems to only happen at high loads... almost like the coil gets hot and randomly shorts somewhere during this event, which corresponds to losing tach signal at the same time (tach signal comes straight from coil). I noticed the AFR's spike lean, then go rich, which tells me that the fuel portion is still working correctly, but ignition is cutting out and causing a rich condition.

It's just weird that the engine starts great, idles great, drives great at low to med loads, but once you slowly lean into it, it just hits this point where the ignition becomes unstable and won't let you get past the RPM at which it occurs.

We have not turned Accel Enrich on yet, so I know that's out of the way.

I do have noise filtering on, but am unsure on the settings. I basically just turned it on and used the stock values. Didn't notice too much of a difference in the RPM signal when turned on or off. I'm sure I just need to "crank up" the filtering.

[n20junkie]

Do you have a LM2904 chip in the VR circuit U7?

I had this exact problem and it was that chip causing it.

[cksiver]

Have you checked your fuel pressure through this range? Does the AFR go lean at the higher RPM?

[LilSki]

Are you triggering the megasquirt off the ignition module pin?

[prof315]

I recently had a similar problem with a BB Chevy triggering off an MSD Pro-Billet distributor. Noise filtering cleaned it right up.