A tuner's nightmare. '88 Camaro, '02 Corvette ECU, and a 6.0 LQ9, with no sensors.

[SolarSupremacy]

TL;DR: The only airflow/fueling sensor this car has is MAP. No IAT. No MAF. No O2s. It won't idle properly (go figure). It's a straight piped 6.0 LQ9 with very low restriction intake and a cable actuated throttle and IAC (idle air control). I think the IAC tuning is both the problem and the solution, explained more below.

The car runs. I've been working on it over the last couple days trying to improve the drivability. Owner complained of frequent stalling. On idle (800 RPM), it was very rough and merely turning on the AC would stall it.

I fiddled with the idle PID settings and got it to maintain idle smoothly and more aggressively recover from RPM drops. I also dialed back the idle spark advance to 10, and 12 at 800 RPM and below, and roughly doubled the idle requested airflow. I also dropped the spark timing adjustment for idle overspeed and underspeed by about half so it wouldn't be reacting so fast. Currently, underspeed adds 12 degrees at -100 RPM relative to expected idle. With the current in-drive idle base spark set to 10-12 degrees, that would give it 22-24 degrees advance for idle recovery. Increased torque model of the AC. And lowered the engine's inertia model by 1/3. As of yesterday and this morning, that worked great. It would start up fine, idle fine, I could switch gears and screw with the AC and it would run great.

Now this afternoon in hot Florida, it usually stalls on start. It'll start, hit idle RPM for a split second, and then straight to stall. If it's idling on its own (with 0% throttle) after starting or while driving, it idles fine. PIDs seem to be good. However, in circumstances such as coming to an intersection and letting off the gas, it'll stall within a couple seconds. Sometimes it'll dip very low and recover. I have to keep my foot slightly on the gas to keep it from stalling and even after coming to a stop, with me needing to apply throttle, if I release the throttle, it'll stall. It can only maintain idle on its own after it's already had a moment to maintain itself.

I've tried boosting the throttle follower by 3x and I've tried enabling the throttle cracker but I couldn't tell a difference. And technically, when I first flashed the throttle cracker settings is when it started stalling all over the place but that was also several hours after the tests this morning and I flashed the previous version and it kept stalling anyway.

My hypotheses:

It was around 80F this morning and now it's 85F. The IAT is busted and giving crazy readings (which was largely why it had rough idle to begin with) so I flatted the entire IAT map to 120F. Maybe the temperature fluxuation is causing a problem.

The idle PID could be "learning" while the throttle is still pressed and when the throttle is fully released, the idle PID had incremented down too far and causing it to stall.

The IAC isn't opening enough when the engine is dropping RPMs (obviously) due to bad tuning. I don't think the IAC itself has an issue with how quickly it can open. I just cant figure out the proper settings to get it to idle properly.

I've attached the step 9 tune (the one used this morning where it ran fine, and then didn't run fine for the last couple hours).

I also attached two logs:
"Throttle Cracker Terrible Run" was a step 10 flash where I enabled the throttle cracker and started to experience the frequent stalling, removing it after.
"Still Stalling on the way home" was back to step 9.

Camaro Step 9.hpt Throttle Cracker Terrible Run.hpl Still Stalling on the way home.hpl

Any advice would be appreciated. This car is never going to run properly until it gets the sensors it needs... but it just needs to run "good enough" for now and that seems completely doable. I don't understand why it ran fine this morning and not now.

[TheMechanic]

Why would you want to mess with something that would have a blah outcome at best and a safety concern at its worst?
Tell him to park it until you can do quality (doesn't mean expensive) work. Fix the problems and charge him accordingly. Nothing in life is free and your time should never be unless you are looking at this as a training lesson.

[SolarSupremacy]

Solid advice. I already made that suggestion to the guy and his outlook on it is "it just needs to be good enough, do what you can."

I knew it would be a mess to begin with... but I am charging him for my time. And he just wants it to not stall.

Since yesterday, I took the car from barely starting and stalling every time you turned on the AC to starting and running reliably... until this afternoon where it's a stall fest again.

I want to learn. I want to get the experience. I want to figure out this problem.

And I'm fairly confident the guy is going to want me to get the missing sensors fixed/installed and properly tune it in the future. Meanwhile, I think fixing the stalling can be done with proper tuning of the requested airflow via IAC and ignition timing in those low-RPM/idle conditions.

[horsepowerguru427]

Your VE table doesnt look accurate and is way off from a factory LQ9. is the TB factory or aftermarket? If its a larger TB then you need to adjust the scalar or whatever on that PCM (id have to look) needs to be adjusted, otherwise the PCM will not make accurate adjustments because it thinks the TB is factory. If your set on not fixing the sensors then Id put a factory LQ9 VE table back into it and the stall saver settings as well, maybe even adding a a little to the stall saver to help it from dying out. I wont even take a whack at trying to adjust the tune with all problems that are evident on this vehicle, but ill see if i cant help a little. i understand where youre coming from but its not a good situation because i dont think the customer is ever going to be happy with it seeing as you WILL NOT be able to accurately tune anything without any sensors...might as well slap a carb on it at this point.

[SolarSupremacy]

I'll give the stall saver a shot. I hadn't actually tried it yet because my intuition figured it would just raise target RPM, but at this point it's worth a shot.

And yeah, when I got the car and pulled the tune the last guy had on it... it was even farther off and the guy disabled the knock sensors. I think it's borderline flooding the cylinders with gas and choking itself out which would explain why the five degree difference in ambient was enough to make it run rich enough to not run. I re-enabled those and raised the AFR from mystery to mystery+2%, but I'll up it a bit more. So far even with high throttle it hasn't done any retard.

I'll update this thread with how it goes and try to give any useful information I gather for future readers and tuners who... want their car to run without any sensors... because... they exist?

If you guys have any other general advice to throw at the problem, I'm all ears. I read around which is where I found increasing requested airflow and decreasing idle timing advance is a good way to help with rough idle on these engines, but I don't think this car does well with "proper" tuning. Doesn't matter how well you fine tune anything because this car has no idea how much air it's intaking... it seems like the TPS works but its probably not accurate and even when on throttle I think the idle PIDs are trying to compensate for *my foot*. It's just... This car just needs to be aggressive and overshoot target RPM. There's no other way.

[SiriusC1024]

What about cranking fueling? Pretty rich.

[SolarSupremacy]

I think the approach for that would be proper VE tables but this engine is so far from stock I'm not even confident a stock LQ9 VE table would be better than the Corvette VE table (I assume) in this thing right now. Adjusting the cranking fueling directly would be a bandaid on a bandaid on an unknown wound. I believe it has plenty of power when cranking. It's catching itself after crank that it was having a problem with yesterday... would just drop back down and stall if I didn't encourage it.

Edit: I really do appreciate you guys taking the time to seriously consider and help out. Most likely every single table and calibration on this car is wrong and you're completely in the right to say "it's not worth it." I think this is a valuable experience though and I'm going to fix this car if it's possible.

[reubone]

Flat line your IAC position where it holds idle stability. In your case it looks to be 310. Way too much I agree but lets dial in idle. Make that change, write it and start it. Should come down and idle with low timing once warm. Now using vcm controls lower and raise the fuel stoich and see if idle rises or falls. Your looking for it to rise. Where is stops rising or surpasses maximum idle control, mark that percent change from 14.7 and apply this to your idle rpms in the ve table. Smooth and lower IAC to 250. See if it starts and check for idle. If it stumbles to life but still runs, repeat stoich mod for best idle fueling. Once idle fuel is good, try starting with the new fuel with the IAC at 250 or what ever you eventually had to set it at. If it starts and idles really good, check the rpm range and idle spark. You want enough IAC for clean start up and enough Idle spark overspeed control to reach the rpm range your after. Then move onto drivability fueling using vcm real time tuning. Your going to be shooing blind so apply logical changes to entire table from idle fuel and move on. It's not going to be perfect until you can drive on a table long enough to build color on your spark plugs. Even then it will change with temperature swings. Probably best to have o2 sensors or a wideband eventually but this will get you on the road.

[SolarSupremacy]

@reubone, I'll give that a shot a bit later (after a try some other settings and the car's warmed up). That's an excellent strategy. I need to hang out on the forums more often so I can learn shit like that. The desired IAC Area Max is set to 84mm^2, however the ECU seems to believe that thing can crack open to 120mm^2. Leave it at 84 or up it to 120? That information may also depend on what I'm saying to horsepowerguru.

@horsepowerguru427, Sorry for overlooking your question. I believe it's a 4" TB. OD of the intake pipe is just over 4" and the TB has a similar OD. That's my best guess without pulling it apart. Larger TB could also mean larger IAC valve (I'm not experience enough to know if that's a correlation) which would also make the IAC calibration and tuning out of wack. The ECU was pulled from a 5.6L '02 Corvette IIRC and was then thrown in the car and handed to a tuner who disabled the knock sensors so at this point the calibrations and tuning could literally be anything.

I'm going to start playing around with it and I'll keep checking back in here.

[reubone]

Screenshot (102).png
This table here for IAC control. Start with this set to 310 which is the position it was in for 800rpmish idle in your log.
Screenshot (103).png

[SolarSupremacy]

Good observation and shows why I need to add more values to the charts.

The parameter "Desired IAC Area Max" actually seems to be the minimum, not the maximum. The IAC never goes below that value. Might need to tell Bill to have them update it.

[SolarSupremacy]

Another note for future readers:

In the Engine, Idle, Airflow settings, under Adaptive Idle Airflow is the short term idle trim and long term idle trim. These have maximum learned values and on this particular vehicle with everything being out of wack, it may be necessary to crank up the maximum learned values for drivability.

All of the values on this vehicle were around 50 lb/h and I've increased them to 100 lb/h.

However, I don't think this car uses desired airflow for timing at all. I think it only controls the IAC which is usually wide open anyway if these readings are to be trusted.

[SiriusC1024]

Tuning that engine is possible. Marine grade solutions rely solely on the MAP sensor.

[reubone]

Idle air adaptive. Those tables can be left alone. The stock values will be fine. The desired IAC max can be left stock as well. When I say "310 is where you engine idled on the data log" That simply meant that after all the adapts were applied, It maxed out the travel of the IAC valve. Setting the Idle Air Control table to 310 all the way across will allow you to start tuning the idle fuel without worrying about the engine dying. You start having stumble issues when the IAC drops below 310. Which is what it will do unless the minimum on the table is set to a higher number than what causes it to stumble. In your case via the data, 310 is your starting number.

[kevin87turbot]

Your primary issue is that the engine simply needs more air. You'll find that most stock GM applications have an IAC position in the 40-70 count range at a warm idle. Yours is maxed at 310. You can't fix this issue in the tune. I typically would start with a 2002 LS1 Camaro tune, since it was a native DBC tune, but I've seen the Vette calibration work as well, since the System Options table has be changed to DBC.

Step 1 to fix this issue is to mechanically open the throttle body blade. In an application like yours, it might take a few tries to get the IAC in the proper operating range.
Start by turning the minimum air screw by 1 full turn. That will open the TB blade enough so that it isn't maxxing the IAC position.
Start the engine and take note of the IAC position once your engine is up to temp. If it's still higher than 70 counts, shut off the engine and adjust the TB screw again. Repeat this process until the IAC is in the proper operating range.

Keep an eye on the TPS voltage as well. You can't exceed 0.82V on your application. If you exceed that voltage, you'll lose control of the idle completely. It's at that point that you may consider Step 2... drilling a larger hole in the TB blade.

[Alvin]

I wouldn't even bother touching it until it at least has a IAT. There is a going to be a big fueling difference as the temp changes.

[Mingus]

Your "Base Running Airflow" looks pretty high. I normally see numbers in the low to mid teens.

[SolarSupremacy]

@kevin87turbot, that's one of the first things I tried with the car and it both made the stalling worse and, when the RPM was high enough, it wouldn't drop back down. The minimum air screw is completely disengaged. (Commenting more on this at the bottom.)

I just took it for an hour and a half drive, all the way from cold start to as heat soaked as possible.

It only stalled once but I don't blame it. I did a hot start and jabbed the throttle to intentionally disable the idle adapt functions and tried to drive off. It stalled a few moments later when I stopped. Started it again, didn't fuck with the throttle, and it was perfectly fine.

On the way back home with the engine as hot as could be, I had a couple low-speed deceleration moments where it would drop down to about 400 RPM but it recovered.

Looking at the logs, the ignition timing keeps going down with the RPM past 800 and all the way down to 370 (!). Then the ignition timing gets bumped back up and it recovers just fine. For whatever reason, the ECU was intentionally cutting timing until it seems like it "realized" it was stalling and jumped back up. But there's no stall saver on this and nothing I can think of that would account for it being at 7 degree advance at 370 RPM and then in an instant decide to go up to 26 degree advance.

(Continuation on minimum air screw.) Although it would be proper to not have the IAC handling all of the airflow needed for idle, most of the idling in the last trip I just took wasn't pegging out the IAC. The car has plenty of control over the idle with ignition timing now.

I think it's good enough as-is. I didn't expect it to stall at any point (other than when I intentionally screwed with it) and it was very pleasurable to drive. I think adjusting the air screw would set me back a full day's work on needing to compensate for it and the engine's already in some precarious balance of barely running.

[SolarSupremacy]

I have made a major discovery.

The Idle Air Follower (IAF) proportionally matches the Throttle Position Sensor (TPS) as long as the TPS is greater than 0%.

In the specific circumstance where you let off the throttle so fast that the ECU reads the TPS as going from X to 0, the IAF doesn't match and instead decays slowly (as it's supposed to) down to 0. However, something must be wrong with the ECU or the tuning though because if that's the purpose of the IAF, in the circumstance where the ECU happens to catch the TPS at 1% instead of 0% as it slams shut, the IAF drops to 1% as well and in the next instant, would decay and not matter at all.

While the Idle Follower Airflow is decaying, the Short Term Idle Adapt is disabled, seemingly along with all other idle functions. The car seems completely indifferent to the idea "idle" until the Idle Follower Airflow decays to 0. In this car specifically, decaying from 20% throttle down to 0 on the IAF takes about 3 seconds. That 3 seconds without any idle control is the reason this car dropped all the way down to 370 RPM. The instant the IAF decayed to 0, the STIT kicked on, timing jumped up by 20 degrees, and it went up to idle.

Now for the questions...

Why? Why does the Idle Throttle Follower work this way? I feel like it's not supposed to.

I'm going to try multiplying the decay by 100 so its effectively disabled.

[SolarSupremacy]

Well, that's it. Giving the car back to the customer this afternoon.

The issue/bug with the Idle Air Follower locking the ECU out of going into idle mode was... interesting. But with that solved (by cranking up the decay rate), it's fine now.

The adaptive idle PIDs are dialed in pretty well. You can let off the throttle anywhere anytime and it'll settle down at 800 RPM.

It's still running terribly inefficiently though. Well enough to spin the tires but poor enough that it overheats in slow traffic after being heat soaked.

But... two days ago the car would sometimes start and even having the thought of putting it in gear or turning on the AC would immediately stall it. And now you can drive the car around all day doing whatever you want and have a great time and it runs and idles so much smoother you won't slowly develop a concussion. If I wasn't a tuner and never looked under the hood, I'd be convinced the car's in pretty good condition. I'm happy with how much I've managed to improve its drivability, ESPECIALLY with the lack of almost every sensor I've taken for granted. I really hope this customer gets it some much needed maintenance (really, just the sensors would be enough) and a proper tune. I don't think they will though. I think they're going to run this 1988 Camaro into the ground with the only expense being gas.