I posted this question over on the HPTuners forum, the thread is linked here, however I'm not getting much of an answer over there so I figured I would ask you all. I've been working on tuning my car for E85 now that I have it setup for it, but I am seeing enleanment at small (under 20%) TPS values. On the 91 tune I had previously, I would see this enleanment as well, however it would only lean out to about 1.05 lambda, where as now I am seeing up to around 1.15 lambda. Seems to me the car is leaning fueling at those TPS values by a set AFR adder, which is why with a lower stoich value the lambda reading is higher now. I have messed with all the tables I can find that may cause this but nothing, and when I posted the question on the HP Tuners forum someone suggested the PCM may be doing something called a cat test. I've tried finding information on what this test is but I haven't found anything, have any of you heard of such a thing? The commenter made it sound like this test happens when the rear O2 sensor doesn't exist and the circuit is open, however I still have my rear O2 and after logging the O2 Voltage B1S2 PID shows a voltage reading, implying the sensor still works. Any ideas?

 

I don't know anything about HPT, so I can't be of much help (I use DHP / TT).

I am curious though - what are your narrow band O2 / fuel trims doing when that happens? Im just thinking the narrow band has to be practically pegged if your wide band is at 1.15, so is the computer just ignoring that completely and isn't touching fuel trims at all?

 

The reply in my HPTuners thread suggested that DHP should be able to disable this "cat test" that they think is causing it, have you ever seen anything in DHP like that?

I should have mentioned, this is all during OL, in CL the car stays at 10.9 AFR which is the the value I have in for stoich. My B1S1 O2 sensor jumps down and reads low (around 30mV) during this, as you would expect since at that time the wideband is reading 1.20 lambda (13.0 AFR). Here is a section of the log during OL. (Orange EQ is the wideband reading, blue and purple are commanded).


As you can see in OL my commanded AFR is all over the place, its not until I pass about 20% TPS that it drops down and stays at 10.9 like it is supposed to. This has been a thing for as long as I have been tuning this car, except now it is far more pronounced, probably because some hidden AFR adder is being applied and adding 0.7 AFR results in a much larger deviation from stoich when your stoich-ratio is only 10.9 instead of 14.7. Looking back at an old log from when I was running the normal 91 tune in low TPS areas it would increase to about 15.4 AFR commanded. It just never really bothered me and I barely noticed since with 91 that is only 1.05 lambda, and honestly I kinda assumed at the time that it was just the car leaning a bit for better fuel economy.

What I've been doing lately as a workaround is being my own fuel trim and made a table not of the difference in wideband reading from the commanded but just from stoich. That way I can tune the MAF curve so in OL so it drives in those areas at stoich despite the computer wanting to run at 1.15 lambda, but that's a pretty messy and not ideal workaround :P

 

I'm not an HP Tuners expert either, but the only cat related item I know of in these PCMs is cat over temp (COT) in the fuel>temp control page. I don't run a cat, so I have that disabled by setting the enable temp to 283*

Ever since I got HP Tuners, my narrowband sensors have been disconnected, and I've tuned the car in OL with my wideband by adjusting the MAF table. I tried messing with the VE table as well, but that didn't turn out well. I also have the AFR modifiers in the fuel>open loop page zeroed out.

Honestly, I'm tired of the stock ecu. From weird fueling issues, to the way it controls timing, the complete lack of idle air control tuning. I'm going to be so glad whenever I get my Holley efi swap done.

 

Honestly I'm not overly familiar with the cat test. The pcm I run on my monte is from a 98 GTP, so its not new enough to have that issue.

I do recall hearing something to disable that in DHP (or maybe it was in the separate tiny tuner program which had more stuff mapped out), but Ive not done any kind of tuning on others cars in probably 5+ years at this point, so I really dont recall.

Outside of a less than ideal AFR, is it causing any drivabiltiy issues? E85 seems a lot more tolerant to running leaner, so I'm curious if you're feeling any issues as a result, especially just given its at lower engine speeds.

I am curious what major issues you're having with idle air control? I've got a very choppy cam and huge injectors and mine idles fine.

TBH on your other two points, that just comes with experience / learning how things interact from playing with it long enough. IMO it seems worse than it is on the surface for 2 reasons:
1.) Theres no good tuning guide for 3800s (especially heavily modded setups). Both fuel and spark have several modifiers that can make it confusing to understand why the computer does what it does for a specific output. For whatever reason, the 3800 tuning community seems a lot more tight lipped on how to get advanced mods dialed in. I bet people would gladly pay a hundred or two for a legitimate tuning guide to avoid having to endlessly wade through forums for info / read through the existing (and very basic) guides that exist.
2.) Some modifiers aren't mapped out at all (more so on HPT), so you've got to work around them in other tables that you do have access to. Someone good at programming could certainly map the rest of the tables out in DHP, but plenty of people already get confused with the stuff that is open already so it could have mixed results / make it more confusing for people just getting started. Tiny tuner helped to address that to an extent by helping open up more stuff that people had been begging for for years.

 

It doesn't seem to have any drivability issues, I'm just not very comfortable cruising around town at 1.15 lambda haha. Its also making it difficult to tune my MAF curve when the commanded fueling is jumping around so much. The replies I got on the HPTuners forum did say HPTuners can't disable this cat test but DHP can. Looks like I need to find a shop locally that has DHP that can disable it for me

Edit: I called up a few local shops as well as ZZP, they all seem to think its a cat test running, and apparently it runs when the rear O2 sensor is bad/missing. I have a rear O2, and I think its still working as I don't have a code and logging B1S2 I see a voltage, so idk. ZZP told me there is a code I can disable to stop the test from running but the lady wasn't able to tell me what the code is . Anyone know what the code would be? B1S2 is the rear O2 on these cars, right? If I'm seeing it go between 0.3-0.5V coinciding with the front O2 sensor bouncing between high and low, doesn't that mean the sensor is working? If so why is this test running still?

 

The reason I'm talking about it, is because I feel like the IAC is far too aggressive, and idles too high. I have my idle set to 800 rpm, but depending on what the ecu feels like doing, it'll idle at 850-900. The only idle control HP Tuners has is the base idle rpm table. As far as I'm aware, without purchasing the "User Defined Parameters" and knowing how to map that all out if it's even possible, there's no way to control the IAC. Technically you can control it with the "Vehicle Controls and Special Functions" tab in the scanner. But that's not what I'm after, cause that's manually setting the IAC to a certain position. I feel like the Holley ECU would provide a more consistent, much easier to control idle. I believe the IAC is also what causes the engine to stumble after revving, just like in this video:

 

I only ask because you're honestly the first person Ive heard on this platform that complained about lack of adjustability with stock pcm idle controls.

I do wonder though, are you sure something isn't wrong with your setup? Those numbers don't sound too crazy, but you should be able to get it much lower if thats what you're after. The stock table is like 675 warmed up in gear and you should be able to get relatively close to that unless you've got a big cam thats causing it to think its going to die. Shouldn't really be any reason for it to run at 900 unless you're artificially forcing it to to smooth out cam chop.

Sorry for the tangent OP

 

Based on the info of disabling a certain rear O2 code should stop the test, what I'd try is disabling all codes that mention the rear O2 (bank 1, sensor 2), and see if that fixes the issue. Then you can either leave them off, and turn them back on at your next inspection date. Or you could re enable each code one by one and see if the issue pops up again.

Apologies for the tangent as well.

I'm still on the stock heads, cam, and injectors at the moment. And I can get it to idle much lower by forcing it to with hptuners. But that's only while the engine is running. If I turn the engine off, disconnect hptuners, and restart the engine, it would go back to whatever is set in the base idle table. I think my issue is I'm comparing the stock ecu to a Holley HP, where you can set the IAC to do anything you want. My main complaint with it is how it holds rpm after you get off low throttle. With a Holley HP, you could tune it to act like a car w/o an iac, where it falls right back to idle once you get off throttle.

 

I just had my front O2 sensor start failing, so I'm just going to replace both sensors and see if that clears it up. If the test is still running after that I'll try disabling codes until it finally stops. I don't have any DTC codes currently for the rear O2 sensor though so idk why the computer thinks it needs to run the test in the first place.