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Fuel trims won't stay in memory on ecu, resets everytime I turn the key.

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cupa-joe

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Discussion starter · #1 ·
As the title states my ecu won't save my fuel trims. I toated an ecu from a bcs short and I fixed the short. Just picked up an Eprom ecu threw it in and car runs fine, but my fuel trims won't stay in memory.

What I checked so far:

1. Fuse 19 (battery) in driver side footwell.

2. I checked for constant 12v in pinout 103 on the harness. Checked out ok.

3. Checked the wire going to the harnes they seem fine.


I don't know what else to check, it kinda hard to set my maft up when my trims reset everytime.

What can I try or check? any help would be appreciated. Thanks.
 
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Discussion starter · #2 ·
Ok, I noticed last night that when I turn on my head lights and heater my battery voltage drops down to low 12.xx volts. Could this have anyting to do with my ecu not saving fuel trims?
 
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Discussion starter · #4 ·
I'm watching them on my logger. Everytime I turn the key my fuel trims reset back to 100%.
 
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Discussion starter · #6 ·
Just picked this one up. Last ecu popped the BCS fuse, so I tracked down the short and fixed it. That ecu wasnt holding memory either. Then picked up this Eprom and threw it in and car runs fine but my trims still are not holding. So I thought It popped a cap or something so I opened it up and still looks good. Could my battery be so low that its not giving my ecu enough power to save the trims?
 
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#7 ·
12v when off should be fine, the car should run with anything over 10v even tho the starter might not. Oh, theres another fuse in the engine bay that the ecu relies on, its sub fusible link 6, closest to the fender. Do your dome lights, interior lights and radio work? Those are also on the same circuit.
 
Discussion starter · #8 ·
Yea, my dome lights work. When I start my car my voltage will sometimes drop to 11.xx volts. The sub fuseible link 6, Is that the 40volt battery one?
 
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Discussion starter · #10 ·
93_GSX said:
Your system shouldn't drop below 12.xx when running, but if it drops to 11.xx when starting the car thats probably normal as your drawing power from the battery to start the car.
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Yea, I was thinking the same. Do you know if the ECU need 12+ volts to store memory? Maybe when it drops below 12v it loses everything it learned?
 
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#11 ·
The ECU doesn't need a full 12-volts to hold memory. Probably more like 9 or 10 and it will still do as long as it stays constant. If voltage is dropping low enough during starting, it may be resetting at that time. You could test that theory just by having the logger on and turning the key only to the on position with out starting the car and see if the old fuel trims are still there. That would rule out a low voltage issue.
 
Discussion starter · #13 ·
MadBeachDSM said:
The ECU doesn't need a full 12-volts to hold memory. Probably more like 9 or 10 and it will still do as long as it stays constant. If voltage is dropping low enough during starting, it may be resetting at that time. You could test that theory just by having the logger on and turning the key only to the on position with out starting the car and see if the old fuel trims are still there. That would rule out a low voltage issue.
Click to expand...
Ok, well last time I watched it my low trim was at 91% and I turned my key to the on position and the trims jump back to 100%. So I guess the low voltage isnt my problem. I really don't know what else to check.
 
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#14 ·
Thats gotta be a bad ecu, as in its likely not a power issue, now as to what in the ecu is bad is another story. Can you log and see if the trim gets reset in the moment between when you turn off the key and the ecu shuts down. I'm wondering if theres an issue with saving the trims.
 
Discussion starter · #15 ·
Hmm, thats lame. This is a new ecu, the last ecu had the same problem. Next time when my trims change ill check and see if they reset after I turn the car on. Ill update this tonight.
 
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#16 ·
The majority of electrical related problems and things shorting out that I have come across on cars is usually do to bad ground wiring. Possible that your ecu might short out on you because it can't find a ground and reset your codes. As an extra measure, you can even splice an additional ground into your ecu harness just to make sure, it won't cost you anything and is worth a shot.
 
Discussion starter · #17 ·
aferraro21 said:
The majority of electrical related problems and things shorting out that I have come across on cars is usually do to bad ground wiring. Possible that your ecu might short out on you because it can't find a ground and reset your codes. As an extra measure, you can even splice an additional ground into your ecu harness just to make sure, it won't cost you anything and is worth a shot.
Click to expand...
Should I add a ground to both pinout 101 & 106? or just one of them?
 
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#18 ·
cupa-joe said:
Should I add a ground to both pinout 101 & 106? or just one of them?
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By looking at this http://www.vfaq.com/mods/ecu-harness-1G.html

I would recommend grounding both to a clean metal to metal chasis related part inside your car. Since you know you are getting power, and since you will be able to rule out your ecu shorting out because of a bad ground wire, your last and final resort is replacing the ecu.
 
#20 ·
Oh, and I don't see it as being ECU related at all. In the rare possibility that both ECU's are bad and doing the same thing, I guess it could happen, but doubt it.

Sounds more to me like a bad ignition switch, or something shorting out from the time the key is in the on position to the start position clearing out power to the ECU.
 
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