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fuel trims question

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
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850oldschool
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fuel trims question

Post by 850oldschool » Sun Nov 03, 2019 1:02 pm

Hi, I recently acquired a scan tool and I've been looking at all the available data.

Something which seems odd is that the short term fuel trim is always exactly zero. When I graph it it's a flat line, with min and max values of zero. Is this normal? I was expecting a jagged line continuously moving about a percent either way. That's how the MAF looks.

Also, the long term fuel trim is -18.0%. So, tending rich, but not yet setting a code. Would a bad fuel pressure regulator be contributing to this? Leaky injectors? The air filter is clean.



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Re: fuel trims question

Post by Ozark Lee » Sun Nov 03, 2019 4:08 pm

Most ECU's are programmed to trip the CEL at a 20% departure from the default.

...Lee


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850oldschool
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Re: fuel trims question

Post by 850oldschool » Sun Nov 03, 2019 8:06 pm

Okay, so what I seem to have figured out here is that my P1308/EFI-444 is causing the engine to run open loop. I cleared the code, started the engine to see if it came back instantly (it didn't), then flipped down to short term fuel trim and found it working normally. Reading "fuel system 1" returned "CL-FAULT". When the code returned fuel system 1 changed to "OL-DRIVE". So the P1308 is preventing it from shifting to closed loop operation.



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Re: fuel trims question

Post by jimmy57 » Sun Nov 03, 2019 8:12 pm

That is not right and I suspect the scan tool is not interpreting it. You can have fuel trim inhibited for faults for O2 sesnor and for some other components where they know fuel trim correction will not be possible.



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Re: fuel trims question

Post by greg850r » Mon Nov 04, 2019 6:00 am

When my fuel trim flat lined it was the front 02 sensor.


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Re: fuel trims question

Post by 850oldschool » Mon Nov 04, 2019 8:27 am

I've spent hours reading about OBD2 over the last 2 weeks, and I have to say that the fuel system status readout in the generic OBD2 mode is by far the least documented part of OBD2. What I know so far is that "OL-DRIVE" is when the ECU makes a decision to temporarily suspend closed loop operation during WOT or ? I can't really find any specific info about the exact trigger.

"OL-FAULT", it turns out, is when the system is unable to to enter closed loop, due to something like no signal from the front O2 sensor.

And what does "CL-FAULT" mean, and what triggers it? I still don't know.



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Re: fuel trims question

Post by abscate » Mon Nov 04, 2019 4:16 pm

OBD “ standards only cover basic diagnostics. The specifics of engine management are proprietary code for each manufacturer. That’s so they can defeat emissions testing and hope no one will discover it.
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Re: fuel trims question

Post by 850oldschool » Mon Nov 04, 2019 8:10 pm

If it's this hard to figure out what is throwing the system into open loop operation, I would say that the system doesn't
even qualify as "basic diagnostics".



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