So I've been having a strange issue that seems to be the memcal by process of elimination, though I don't understand how the memcal explains it.

I'll be driving down the road and if I get to say <8% throttle (I can glance at Tunerpro when it happens), all of a sudden a I'll get intermittent losses of power and then recovery...almost like a bucking/jerking situation.

AFR on the WB pegs at 10.0:1 (minimum reportable value) during the event.

However if I give it any throttle the problem goes away and the car runs absolutely perfect. It only ever occurs at very low or 0% throttle. I've never had it happen above say 10% throttle. That's the strange part is why only at low throttle... why would the EPROM malfunction "care" what throttle position I'm at?

A few times I'd get CEL codes thrown for "ECM Data Out of Range".

The whole thing started happening about a month or so ago.

Other than that, Tunerpro data shows nothing unusual happening, since I was able to do logs that capture the event. No anomalous sensor readings or anything. TPS voltage looks perfect, MAP voltage, everything looks perfect.

I thought it might be the IAC since I recently changed it (coincidentally approximately around the time this started happening). So I put in a different one, but the problem remained. Hence my recent post in DIY PROM regarding IAC Steps being commanded or actual.

So I started looking at the ECM side of things, which is when I started doing that process of elimination on the various pieces of hardware, and it wasn't until I swapped out the PROM that the problem hasn't returned.

But again, I don't understand what about the memcal would affect things only at low throttle.

 

https://www.thirdgen.org/forums/diy-...d-density.html

I wonder if it’s a cylinder select issue, v6 use resistors v8 is grounded to get the firmware of the ecm set. If you want to test I’ll look and can modify a memcal to try . Just pay the ride on a modified v6 one.

 

Thanks!

So per our earlier PM's, I tried the other memory adapter with the suspect memcal and the problem came back. Which made me think it really was this memcal.

But then I got to thinking a little more...

One thing I've noticed for a while was that when I let off the throttle and end up at very low TPS voltage, the WB AFR on the passenger side drops to ~11:1 for a bit... It never affected how the car runs or feels so I ignored it. But I always attributed it to some weird side effect type interplay between the DFCO and how I have my "quasi-closed loop" setup (can search on that if interested).

Any rate, fast forward to a little over a month ago when I had to replace the IAC, which is when I started noticing this new issue where it bottoms out the AFR gauge at 10:1.

The IAC has a lot of steps while cruising. I actually have my throttle follower set to about 1.80 as I like the feel of it via seat of the pants. So I started wondering if the new IAC is somehow more responsive, shutting completely (or near completely) and cutting off the airflow to the extent that I get this ridiculously-rich condition (almost like the choke on a carb). Plus if the airflow cuts out, it would also help explain the sudden loss of power. Not that the engine wants to shut off... but the same feeling if you suddenly went from 10% throttle to 0% by abruptly letting off the gas pedal)

Again since the IAC steps are commanded and not actual... a sudden loss of airflow with no change to fuel delivery would in fact cause a rich spike.

The more recent new IAC did the same thing, so it could also point that my old-old IAC was losing its responsiveness at low step counts or something. BTW, the IAC throttle follower makes a big difference throttle feel... which is actually what made me replace the old one in the first place... I was getting a lot of variability in throttle feel w/o any tuning changes, so I suspected the IAC and sure enough that's what it was.

So as a test, I leaned out the VE tables by 10 points from 20-25 kPa and the problem seemed to go away with the suspect memcal for the limited amount of time I was driving it. It's only dipping down to about 12:1 now.

I'm not sure why the IAC would respond differently between one memcal vs another. But the other memcal didn't exhibit the issue. But the way it recovers immediately with just a little throttle suggests the IAC is part of the equation somehow via the throttle follower.

Any rate, I'm going to drive it around a bit more over the next week or so and see if the problem comes back. If not, then I guess it was a tuning issue with the new IAC.

 

That must have been the problem... I just got back from a decently long drive and gave it every opportunity to repeat by hanging for long periods of time in that low TPS zone. Never once repeated... AFR's would touch 11.5:1, but I don't feel anything strange, so I"m not going to sweat it.