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Discussion Starter · #1 ·
I went to the dyno yesterday and am I am confused about why my car will not tune at all. I have a major lean spot and adding fuel did nothing to the fuel curve. Steve Jarvis said he has saw this happen a few times. I really need some help with this problem. I dont know much about tuning, but to me it looks as if the car is running out of fuel because the a/f curve is proportional to the hp curve. Mods are: BCC,DP/Exhaust,Profec B,VPC/S-AFC,FMIC,Cam Gears,Max Air/HKS SMF, Auto w/stock stall.Anyways here is the dyno sheet:

 

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Old School
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It's the OBD1 ECU causing the lean spot.

You have two choices:

1. Buy and install an OBD2 ECU. Only downside to this is it won't heat the front O2 sensor, so the car goes rich when it's idled for a long period. During normal cruise the hot exhaust gas will keep the O2 sensor hot enough to work properly. You'll need to re-pin the DLC under the dash to use the diagnostics.

2. Install an e-manage with the additional injector harness. You can add injector duty cycle post-ECU and completly cure the problem. It worked perfectly for me.

If you plan to someday dump the stock MAF go with option #1.

Edit: Should have looked at your mods more closely. While you can use the e-manage with a VPC, were I you I'd do the OBD2.
 

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TRACK CAR
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3. You can also modify the stock fuel rail, add an aeromotive fuel pressure regulator, and bump up your base fuel pressure. Worked great for me.

Mike
 

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I'm in process of going true OBD2 also...already have the ECU installed and now need to do the heater re-wire, diagnostic port enabling, and O2 sensor simulator. I'm doing this mod to prevent exactly what you're seeing on the dyno. OBD1 controlled A/F curves are downright scary at BPU power levels!

That is one stout automatic Supra TT there! This "lean spot" issue seems to become exagerrated the harder you push the stock twins, and your power is very impressive indeed.

Steve T.
 

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I've been running with my primary O2 heater on all the time for about 4k miles now without any problems.

Steve, bring your camera tomorrow and we can document the heater and diagnostic port mods.
 

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Boost Junkie
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Wow,

Good numbers, but welcome to the world of OBD 1 lean spot. I had the exact same problem. I ended up getting an OBD1 G force with a better fuel curve (I'm assuming like the OBD2) and the problem went away totally. A/fs went from mid/high 13s from 4500-5500 to 11s. My recommendation is to add an aftermarket regulator and turn up the base pressure slightly. You can then take out fuel with the afc up top. Don't forget you are modifying the stock tables and unfortanetly, even with an AFC, you can only add so much fuel. By adding more pressure you are going around the pcm. Keep in mind you can only add so much.

Also, great numbers. I made 417whp at 19psi with my BPU++++ auto (front mount, cam gears, VPC/GCC, pulleys. It was too hot that day (nearly 90 degrees) for my old stock twins to pump out more then 19psi. I'm confident that if I could have run more boost I would have had similar numbers to yours. Twins are gone now though in favor for the SP63 kit.

Steve K.
 

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How man pounds of boost did you dyno at?

I need to go dyno my car soon and I dont want to push it to where I'll experience the lean spot.

Josh, i remember you mentioning to me that it only occured when you dynoed it at 24psi, but not when you did 20? (correct me if im wrong)


Thanks.
 

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Original Owner
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i had a similar lean spot also, the higher i boosted the more lean that spot got,

from 17psi, ratio was around 11.5:1 to 12:1 but when running 21 psi it was closer to 13:1
 

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HotSoup said:
I went to the dyno yesterday and am I am confused about why my car will not tune at all. I have a major lean spot and adding fuel did nothing to the fuel curve. Steve Jarvis said he has saw this happen a few times. I really need some help with this problem. I dont know much about tuning, but to me it looks as if the car is running out of fuel because the a/f curve is proportional to the hp curve. Mods are: BCC,DP/Exhaust,Profec B,VPC/S-AFC,FMIC,Cam Gears,Max Air/HKS SMF, Auto w/stock stall.Anyways here is the dyno sheet:


same problem



im going to try dumping in more fuel at those pts with my afr
 

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::MySTiK_[SuPRa]:: said:
How man pounds of boost did you dyno at?

I need to go dyno my car soon and I dont want to push it to where I'll experience the lean spot.

Josh, i remember you mentioning to me that it only occured when you dynoed it at 24psi, but not when you did 20? (correct me if im wrong)


Thanks.
Phu, I wouldn't know I never run over 16psi, LOL...

I can't remember exactally, but I think I was fine up to about 18-20psi, and started to go lean after that. Let me know when/where you plan to dyno, I might do some pulls too.
 

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Josh S said:
If you are OBD1 adding more fuel with a afr, sfac, etc. will not help.
Amen, brother. The options to fix this are all in this thread.
 

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Discussion Starter · #15 ·
Thanks for the info everybody. Where the hell am I going to find an OBD2 auto ECU at? I have searched the for sale section, and I only saw a couple that have been for sale over the last couple years. I really want to cure this problem and get the car properly tuned asap, because I had NO idea it was running this lean. What needs to be done once I put the OBD2 in? I know O2 sims will be needed, but does someone have a schematic or diagram for changing the pinout for diag and the O2 sensor?
Thanks,
Jason
 

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Boost Junkie
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There is a post on here on how to change it to OBD2. Do a search, it should show up. As for the leanness-yea it is bad. I had no idea either until I put my wideband in and hit the highway a few times. Just turn down the boost until you get it fixed. You might want to try calling Boostlogic, I just borrowed an OBD2 ecu from Kean 2 months ago. Also, other shops such as Titan, Powerhouse, Sound Performance. They have have some OBD2Auto ecus just laying around.

Luckily, my G-force OBD 1 ecu solved my problem.

Steve K.
 

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On a side note...my bpu had same thing....and i ran it for 30-40K on stock twins....never hurt anything. And i obviously raced the piss out of it. Just adding my .02 and experiences. And i ran at bpu 11.4-11.7s at 122-124s.
 

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Silver Bullet said:
On a side note...my bpu had same thing....and i ran it for 30-40K on stock twins....never hurt anything. And i obviously raced the piss out of it. Just adding my .02 and experiences. And i ran at bpu 11.4-11.7s at 122-124s.
Tony,

I had the problem for a while before I fixed it also.

IMO, if you use 100 octane and don't do a lot of all-out 5th & 6th gear freeway pulls you'll never hurt anything.

Pump gas + high boost + high EGT's from timing retard and lean AF is likely to toast the exhaust valve stem seals sooner than they otherwise would be.

The same pump gas conditions as above + 5th & 6th gear freeway racing *may* lead to a hole in a piston.
 

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Has everyone who has had this problem, had it with an S-AFC? I would first trying completely unhooking the S-AFC.

The issue with making adjustments with an adjustable fuel pressure regulator is you won't be able to fix the curve, you will just make it richer everywhere.

Later, Steve
 

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Steve Jarvis said:
Has everyone who has had this problem, had it with an S-AFC? I would first trying completely unhooking the S-AFC.

The issue with making adjustments with an adjustable fuel pressure regulator is you won't be able to fix the curve, you will just make it richer everywhere.

Later, Steve

i have it with an afr
 
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