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Turbo Prelude H22 Deceleration Stall

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Hello all, and thanks in advance for the help. I'm very new to tuning (this is my first endeavor) so if I'm missing anything or leaving out important information bear with me. I have a 2001 Honda Prelude SH that is completely stock, minus the Treadstone turbo kit I've installed. I'm running 6lbs of boost at the manifold, 600cc injectors, and monitoring using an AEM failsafe boost/AFR gauge, tuning using an AEM FIC (I need to keep the OBD system intact for the time being). I have managed to get the car running very well and stable across the board (few tweaks in VTEC needed for fuel, running rich), closed loop AFR is stable with a +/- fuel trim in the 5-10% range.

My issue is this, during warmup the vehicle idles perfectly without issue. Once hot, more often than not during a deceleration the car will stall in neutral (clutch pressed) unless I let the engine rev down under load to around 1500 before pressing the clutch. I'm not sure if the issue I'm having is air or fuel related, being that it's a deceleration my AFR gauge is pegged lean (decel fuel cut) so I can't accurately monitor if the fuel isn't coming on quick enough or if I have too much/too little air at the point it stalls.

I'm looking for some guidance of what to check or adjust to try to dial this in, I know I'm limited to what I can see and do since I'm running a piggyback and the factory ECU is handling all of its corrections on its own.

I had no issues at all before modifying the vehicle, and the engine was refreshed (stock rebuild except for deleting the balance shafts) 10k miles ago. I have tried changing the injector response time in the AEM FIC (this is unclear as to what the values should be, many differing opinions and no official documentation) but I have it set at 350ms and that's where it runs the best (I used new injector latency minus stock injector latency to achieve this), I can achieve smooth operation both increasing or decreasing the response time but 350ms has been the best setting, others do not solve the issue I'm having.

The vehicle will idle stably at 750 (factory setting) at all times, I only have the deceleration stall issue.

I haven't had anything to do with the FIC so I'm not sure how much help I can offer. My gut feeling is that this will be related to the fuelling on over run. The two situations that would create this stalling issue would be the engine running excessively rich on over run, or potentially an over run fuel cut that isn't re-enabling fast enough. Given that the FIC piggybacks the stock ECU, I'm not sure what level of control is available in these areas though.

I was considering the same thing, so at least I may be on the right track. I wonder if the fuel cut isn’t coming off “fast enough” for the larger injectors and the engine is stalling because of a lack of fuel. If I leave the engine loaded on decel “engine brake” down to around 1000-1500 rpm, until I feel the load change (I don’t know how to describe it other than you can feel the decel load change) it doesn’t stall.

The FIC doesn’t give control over anything other than fuel and ignition maps. I can’t adjust compensation maps at all. I have been trying to figure out a way around it, such as adjusting throttle body minimum opening or anything to trick the ECU into disabling fuel cut a bit sooner, but I haven’t tried anything yet. I prefer to learn why something is happening before I blindly chase a fix. I’ve also considered trying to find a faster reacting injector if indeed it is due to lag time. I need to use a scope and try to see if I can watch when the ECU commands fuel vs when the car stalls to verify my theory.

Can your ECU be modified with a Hondata module, allowing you access to the factory maps and settings. You might be able to then change the necessary parameters. This help from Hondata gives me a hint it might be possible:

http://www.ktuner.com/KTunerHelp/fuel_cut_settings.htm

No, Hondata doesn’t have a setup to allow me to interface with the factory ecu. I’d have to convert to an obd 1 setup to run it. I went the route I did because it’s the only setup I’ve found that lets me piggyback without disrupting the obd 2 ecu. It works great other than this issue, which is a pain because I can’t alter the factory ecu to compensate, I have to find another way around.

As you've noticed, there are some tradeoffs involved in running a piggyback. The stock ECU is still in control, it doesn't know or care what you're trying to do, and it can adapt its fuel and timing based on the measured knock and O2 readings. If you're not already using an OBD scanning app like Torque to watch what the stock ECU is doing, that's probably the first thing to do. You said it yourself "can't alter the factory ECU to compensate" so it's important to see exactly what it's doing. Also learn to view the FIC datalogs in a log viewer program, like AEMLog or MegaLogViewer.

Remember the stock ECU is trying to keep the AFR near stoich (14.7) at idle and light loads. Most engines tend to keep running when they are running 10-20% richer than stoich but tend to stall when they are running 10-20% leaner than stoich. So my best guess is the engine is stalling from being too lean as opposed to being too rich. Try adjusting the fuel trim near the RPM & MAP points where the engine tends to stall, and watch the OEM ECU's learned short-term and long-term fuel trims to make sure it's not fighting you.

The injector response time is used as part of the math for the piggyback's fuel trims. For instance, if the stock ECU's injector pulse is 2.0 ms and the FIC's fuel map is set to -50% , leaving the Injector Response Time at zero would result in the new pulse of 1.0 ms from the FIC. The problem is the injector didn't actually spray fuel for the entire 2.0ms; there's a tiny valve inside, a spring forcing the valve closed when the power is off, and a little electromagnet coil that charges up and pulls the valve open using magnetic force. After being commanded to open by the ECU, the injector might spend 0.4ms - 1.0ms just charging the coil and moving the solenoid valve against the spring before it actually sprays fuel. If the FIC ignored the injector's response time and changed the fuel pulse from 2.0ms to 1.0ms , this would change the actual fuel delivered by much more than -50%. By telling the FIC that the OEM ECU is probably assuming the fuel injectors take 0.9ms to open, it will be more accurate when it tries to trim -50% from the amount of fuel delivered to the engine.

If you work through some math examples, injector response time is a relatively small factor at high load / big pulsewidths and a very big factor at idle and light loads. If your FIC's fuel map is set to a flat -50% everywhere and the Injector Response Time is set too low, it will reduce fuel too much at low pulsewidths and the engine will run too lean. You can either adjust the Injector Response Time in software until the engine acts consistent at both small & large pulsewidths, or you can tune around it by using different trim numbers for different RPM/MAP cells in the FIC's fuel map.

This post is getting longer than I intended, but hopefully it helps a bit. Good luck, piggybacks can be tricky.

Ok, just as an update to this concern. I've installed a faster-reacting set of injectors, that are within .2ms of the factory injectors, and adjusted the tune/settings to match. The deceleration stall is all but non-existent at this point, so I was correct in my theory that the old injectors couldn't react quick enough when the ECU requested fuel cut to come off. I'm now just working on dialing in the idle fuel amounts, as with A/C on it idles perfectly, no loads it's a little rough, I need to richen it up a bit and tweak the o2 offset to keep the factory ECU happy. Thank you all for your help and input.