Overcoming 2LTE fuel cut limit

GTSSportCoupe

2LTE whisperer
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As most 2LTE owners know, the ECU goes into fuel cut when it sees more than about 14psi at the boost sensor. Once intercooled, its desirable to run more than 14psi. I've found personally the motor really comes alive around 17-19psi. In order to achieve those boost numbers, the computer has to be tricked in some way (along with a manual boost controller, and turning up the fuel at the spill control valve).

For years I've used a bleed valve that @pradocruzer came up with. In fact he sent me the valve I was using. The theory behind it was it lowered the entire boost curve as seen by the ECU by bleeding off air before the boost sensor. It was adjustable, so could be adapted to the range of boost I was running. Well, recently my trusty bleed valve crapped out.

So I installed a cheap Chinese Ebay one that I'd bought quite a while ago. ASC-08 off ebay. Well, it turned out to be a total piece of garbage. In about 1/16" of a turn It went from no boost to full over boost as seen by the ECU. So it was nearly impossible to adjust. Not sure why. I guess you get what you pay for. Now I know why a number of you have struggled tuning your motors with these.

So, I rememebered @Waikato Willy mentioning he'd used a 4.3V zener diode to clamp the Boost sensor output to 4.3V. He had measured the boost signal voltage output to see at what level the ECU went into fuel cut. So I tried the same zener diode today that Willy had used. It's a 1N4731. On my engine, it dropped the boost signal way more than the 4.3V rating. After a bunch of testing, I realized the zener diode was loading the map sensor output. The 1N4731 begins reverse conducting significant current long before it gets to the 4.3V rated voltage.

So, back to the drawing board. I looked more in our inventory at work, and found another 4.3V zener diode that had less reverse leakage and thus should load the map sensor less. It's a 1N749A. Installed that sucker, and voila! I could hit my old boost levels again with no over boost issues. Tons of power with no pissing around adjusting a bleed valve etc.

I connected the 1N749A between the MAP sensor output (Yellow wire) to ground (Brown wire). I'll make a little schematic at some point and post it up here along with ordering information for the part incase anyone else would like to do the same.

Now keep in mind, I've only been running this setup a couple hours now. I will report back and let you guys know how it goes in the long run though. Once concern I have is that when the zener diode is reverse conducting at 4.3V, it does still load the map sensor output. In theory I should have a limiting resistor at the sensor output to keep from damaging it.... However I think Willy's sensor is still working ok after months of this?
 
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Waikato New Zealand
yep 600ks a week or more on average , really cold mornings i do see spikes of 22psi but generally sits around 18-20psi, my soldering still looks terrible and its been no problem since October 2018, so far no weird map sensor issues or anything

this is the exact zenner diode i used


here is the old pic

zenner diode.jpg
 

GTSSportCoupe

2LTE whisperer
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Thanks Willy for the zener idea and getting me started playing with my truck more. I'm glad to hear yours is still working well.

I just got back from a 2hr drive. Although the diode I used works pretty well, I think there is room for improvement. It is giving me a narrower power band than what I had before; for some reason missing some low rpm torque I used to have. Also the boost sensor output does not play nice with my alcohol injection controller anymore (signal is less progressive now, so hard to tune). So I'm going to do a little more R&D as I have time. Luckily I design electronics for a living, and work in a lab that has a great parts inventory. So I'll play around with more zeners to see if I can find one that is 'perfect', haha. Might make a fancier active circuit too... Or might just clean/fix my air bleed valve and put that back on, haha. Anyhow, no matter what, I'll report back here.
 
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Definitely keep us posted Nick.
The issue you described with the speed controllers is exactly what you and I had talked about. I have 3 different versions of the cheap ones. They have absolutely no adjustment to speak of. Closed and 1/16th of a turn goes from nothing to wide open essentially. It's been a very frustrating pursuit. The one I have in my truck currently is the best of the 3 and will still let too much boost by on a long grade. It's almost like it's variable and it shouldn't be. Works ok for a bit and then after a long pull it will suddenly allow too much boost by and hit my fuel cut. All while holding a steady 18-19PSI boost on my gauge.
Naturally that always happens when I have people behind me in my slow ass truck. Now ,I am dropped to 40 mph and they are getting pissed.
 
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So, what does it feel like when it hits the fuel cut? Is it just a reduction of power or is it more dramatic than that?
 

GTSSportCoupe

2LTE whisperer
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Definitely keep us posted Nick.
The issue you described with the speed controllers is exactly what you and I had talked about. I have 3 different versions of the cheap ones. They have absolutely no adjustment to speak of. Closed and 1/16th of a turn goes from nothing to wide open essentially. It's been a very frustrating pursuit. The one I have in my truck currently is the best of the 3 and will still let too much boost by on a long grade. It's almost like it's variable and it shouldn't be. Works ok for a bit and then after a long pull it will suddenly allow too much boost by and hit my fuel cut. All while holding a steady 18-19PSI boost on my gauge.
Naturally that always happens when I have people behind me in my slow ass truck. Now ,I am dropped to 40 mph and they are getting pissed.
So the good one I have is a: Chelic Speed Controller ASC 150-02 . If you can get your hands on that one you'll be happy. I've taken mine apart to clean it, and it is not as simple inside as I'd assumed. Probably why it works well.

It is frustrating when loosing power on a hill for sure. I've had boost lines pop off before resulting in nothing seen at the boost sensor and instant loss of about 30hp, haha.

So, what does it feel like when it hits the fuel cut? Is it just a reduction of power or is it more dramatic than that?
Check engine light comes on and an instant reduction in power. Not a big deal if you're just in town. But if you're pulling a big highway grade or towing it sucks big time!
 

GTSSportCoupe

2LTE whisperer
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I found the datasheet for the air speed controller that worked really well for me. I've attached it to this post. Again, the one I was using is: Chelic Speed Controller ASC 150-02 . The sweet spot was about 3-4 turns out. There is actually a graph in the datasheet that shows the orifice size for a given number of turns.
 

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Perfect timing, i just had a ASC -08 arrive but will hold off on installing after reading this, or is there a way I could test the unit before installing to see if its worth using?

Regarding the resistors, im in NZ and can get the 1N4731 that wikato willy used from Jaycar. But not sure on the 1N749A as its not in their listings. Do you have links for tracking one down? and is the change of diode worth it as apposed to the 4731? would like to avoid the boost spikes if possible.
 
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Seems as though I had trouble finding that Chelic one. You sent me the product code on it. I can't remember why I never bought that exact one you have.
As you know, it may not matter. If I can ever find the time to pull the head of my engine and I find it cracked, you know what I am about to do.
 
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New Zealand
yep 600ks a week or more on average , really cold mornings i do see spikes of 22psi but generally sits around 18-20psi, my soldering still looks terrible and its been no problem since October 2018, so far no weird map sensor issues or anything

this is the exact zenner diode i used


here is the old pic

View attachment 2049445
Regarding the boost spikes, is this only when really going for it and it spikes from 20-22 on climbs etc? or are we talking motor run away spikes when it should only be boosting 12 on a motorway on ramp and it gets a mind of its own and over boosts/ fuels?
 
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Waikato New Zealand
usually spiking under big load up a hill or towing , usually when i turn off overdrive, it will drop to third then back off throttle so it goes into 3rd lock then back on the peddle, thats when I can make it spike up, my bleed off is by no means accurate, really rough as guts but doesnt seem to bother it, when it fell right off i managed 25psi but without the fuel it only made the egts way cooler
 

GTSSportCoupe

2LTE whisperer
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Regarding the boost spikes, is this only when really going for it and it spikes from 20-22 on climbs etc? or are we talking motor run away spikes when it should only be boosting 12 on a motorway on ramp and it gets a mind of its own and over boosts/ fuels?
Remember you're talking LJ78 vs KZJ78 here. I don't think you'll ever get boost spikes on a 2LTE that will cause any harm.

Also, you can get the 1N749A from digikey: 1N749A TR Central Semiconductor Corp | Discrete Semiconductor Products | DigiKey
 

GTSSportCoupe

2LTE whisperer
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yew! might have to look at a local option... will cost $26 to ship the 90 cent part haha. cheers for the link though.
Ouch! Could try the NZ version of Digikey (https://www.digikey.co.nz/)? There are lots of other electronics parts online distributors that have them too that might have cheaper shipping....

There might also be better and newer zeners that do an even better job. I'll look into it when I find some time. Busy working on my house this week.
 
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GTSSportCoupe

2LTE whisperer
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Took a second to find a better 4.3V zener. You could see if you can find this one easier: https://www.digikey.co.nz/products/en/discrete-semiconductor-products/diodes-zener-single/287?k=BZX55C3V9-TAP&k=&pkeyword=BZX55C3V9-TAP&sv=0&pv7=2&sf=0&quantity=&ColumnSort=0&page=1&pageSize=25

Manufacturer part number is: BZX55C4V3-TAP

Maybe I'll bring in a bunch on my next order at work. I'll do some experimenting and let you guys know which one is the winner.

I've also thought about using a 5V rail/rail op-amp to attenuate and buffer the boost signal to the ECU. It could be made adjustable. It would appear as a high impedance to the boost sensor (so as not to load/damage it), would reduce the signal a given percentage (adjustable), but maintain the boost curve shape (not sharp cut off), and buffer the modified signal to the ECU. The boost sensor gets 5V from the ECU, so it would be easy to make a little circuit board to do this that sits near the boost sensor. Could put it in a little plastic case and put some little terminal blocks on it..... Ahhh... I should stop procrastinating on what I should be doing.
 
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Do you have more info on the issues you also had with the 1N4731 willy used? and do you think that was due to your set up being modified in other areas, or more of a 2lte vz kz ecu difference with his being the kz? just with the 1N4731 being so accessible here it would be good to know more on wither the issues would cross over to a standard 2lte set up like in my LJ.
 
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Always a good thing to see you improving the 2lte.
I bought the speed controller from Princess Auto not sure if they still sell them.
May be a good idea to have a new one as a mechanical backup for the glove box
 

GTSSportCoupe

2LTE whisperer
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Do you have more info on the issues you also had with the 1N4731 willy used? and do you think that was due to your set up being modified in other areas, or more of a 2lte vz kz ecu difference with his being the kz? just with the 1N4731 being so accessible here it would be good to know more on wither the issues would cross over to a standard 2lte set up like in my LJ.
It was just a tiny bit better than disconnecting the boost line from the sensor all together. The MAP sensor can only source so much current, and on the 2LTE sensor, the 1N4731 loaded it down too much. So in a nut shell, the computer was not sensing even close to 14psi. Probably closer to about 5psi.

Why don't you try ordering the one I linked to above? This time I used Digikey New Zealand...maybe check to see if the shipping is cheaper. And honestly, even if you have to pay $25, it's a very worth while mod.
 

GTSSportCoupe

2LTE whisperer
Joined
Mar 23, 2012
Messages
5,539
Location
Victoria, BC, Canada
Always a good thing to see you improving the 2lte.
I bought the speed controller from Princess Auto not sure if they still sell them.
May be a good idea to have a new one as a mechanical backup for the glove box
Hey! You're still checking the forum once in a while! Nice to see you post again. Good to know where you got the speed controller from; thanks. I'm going to have to check Princess Auto to see if they have more. Maybe I'll buy a bunch and mail them to people, haha.

Fellas, pradocruzer is the original guy who started pushing the boundries of the 2LTE engine and the 70 series Prado's in ways people had not even thought of before (and before most of you even knew what a 70 Prado was). I learned a lot of what I know by reading his posts when I first bought my truck. He's also since gone to a 1UZFE and has done some pretty nuts suspension modifications (linked). Also installed a front e-locker from an 80 series to compliment the factory rear locker? Definitely my favorite LJ78 out there!
 
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