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Discussion Starter · #1 ·
Got the smurf back together and now my fuel trims are consistently around -10. That's adding long & short term together. I do have a P1443 code, but that was there before the repairs and is the only code.
Last repair was the intake manifold, a used stock nPI has been installed.
Over the past few months I've replaced: radiator, water pump, upper/ lower rad hose, heater hoses. With the latest repair being the intake because the heater nipple cracked.

Any thoughts on why the fuel trim is low and should I be concerned?
 

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97 Thunderbird 4.6, 98 Mark VIII LSC
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Is it the STFT or LTFT that is showing up -10%? They mean opposite things, so knowing which is important. :)
 

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1995 Thunderbird LX 4.6 red
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Kidd, Replace the purge valve and the purge valve sensor. And check all those 2" pieces of hose under the passenger side wheel well. IF the purge valve is stuck open the fuel vapor will enrich the mixture and confuse the fuel mixture calculations.
 

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Discussion Starter · #5 ·
Brandon, see screenshots below for fuel trims:


Gordo, that makes perfect sense. I'll get those parts on order and check the EVAP lines out. About 3-5 years ago I went through and replaced all the VAC lines, except the EVAP near the gas tank, thanks on my to do list whenever I replace the fuel pump.

As a slight side bar, what is the fuel trim sensor?
 

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97 Thunderbird 4.6, 98 Mark VIII LSC
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Fuel trim is a correction factor. The MAF sensor is the primary sensor - fuel is delivered based on the amount of air detected, then the burned mixture is measured by the front O2 sensors to tweak the fuel delivery. Injector duty cycle is adjusted from what the MAF sensor says it should by the correction factor, STFT. In SCT's logging software the STFT is lambda - numbers under 1 indicate a lean condition (PCM has to enrich) and numbers over 1 indicate rich (PCM has to lean). Other software shows fuel trim as a percent (positive or negative) so I can't say what it's doing...
 

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Scott, you might want to check if the all the EVAP hoses got hooked back up. There is a strange black plastic hard line on the passenger side that goes under the spark plug wires. IIRC, one side goes to the throttle body, the other side goes to a metal line sticking up from the inner fender.

Al
 

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1995 Thunderbird LX 4.6 red
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Fuel trim is a correction factor. The MAF sensor is the primary sensor - fuel is delivered based on the amount of air detected, then the burned mixture is measured by the front O2 sensors to tweak the fuel delivery. Injector duty cycle is adjusted from what the MAF sensor says it should by the correction factor, STFT. In SCT's logging software the STFT is lambda - numbers under 1 indicate a lean condition (PCM has to enrich) and numbers over 1 indicate rich (PCM has to lean). Other software shows fuel trim as a percent (positive or negative) so I can't say what it's doing...
I my experience with scanners that show the percentages. In relation to the positive and negative numbers I look to see the STFT and the LTFT basically average to zero. It's never exactly zero, but somewhere in that area. Considering he has long term numbers in the -5.50 range and short term numbers in the 25 range. He's got a problem for sure.
 

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Discussion Starter · #9 ·
Gordo, exactly the way I look at fuel trims. I use the Torque app and it spits the data out in a csv so excel becomes my friend. see attached for a graphical view of the summation of the fuel trims for each bank. You can see it tends to be negative once she warms up.

I have the EVAP parts on order, should be here to today and I'll be going over that system probably Saturday.

 

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Discussion Starter · #10 ·
Scott, you might want to check if the all the EVAP hoses got hooked back up. There is a strange black plastic hard line on the passenger side that goes under the spark plug wires. IIRC, one side goes to the throttle body, the other side goes to a metal line sticking up from the inner fender.

Al
Double checked and do have that one hooked back up. Valid question though. I always think I'll remember how to reassemble while I'm taking things apart and it'll only be apart for a few days... Thank God for goodle pics & diagrams.
 

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Sounds like you are well on your way to fixing the EVAP problem. The solenoid should show which port goes to the canister and which goes to the engine vacuum.

On your fuel trims question, I wouldn't be immediately concerned unless its driving bad or throwing codes that need to be fixed for registration renewal. Supposedly messed up fuel trims indicate a fuel system problem that are likely to use up the catalytic converter prematurely. So should be fixed.

Bank 1 and Bank 2 fuel trims are not additive. I don't see anything that adds up to -10. Those numbers look strange to me, What I've seen when watching fuel trims seem to be real numbers rather than fraction conversions. Don't know why your scan tool maxes out at 5 15/32.

I don't like the high short term fuel trim numbers. Looks like a MAF sensor problem. In my experience I had a MAF sensor that gave me good performance but also gave me high short term fuel trims. Cleaning didn't help the numbers. After you clear the EVAP problem if the STFTs are still high I would consider replacing the MAF sensor.
 

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Discussion Starter · #12 ·
When I bought the car ~5 years ago it was throwing a lean code for both banks, high fuel trims at the time. Ultimately, it was the MAF. One of the POs installed a K&N air filter, as far as I can figure the oil contaminated the MAF, cleaning it wouldn't do anything. I had a used one sitting on the shelf and that fixed it. I need to look for that old data and compare MAF readings to what I have now.

Jim, if you look at the graph after she warms up the overall fuel trims hover around -5 to -10. The screen shot of the numbers were when she was first started, which you can see in the graph are closer to 0 and then fall into the negatives.
 

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The screen shot of the numbers were when she was first started, which you can see in the graph are closer to 0 and then fall into the negatives.
When the car first starts it stays in open loop, so fuel trims aren't indicative of what the O2s are detecting - just what the commanded mixture is (rich). There should be a flag in the logging software to tell you if it's running in open or closed loop.
 

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Discussion Starter · #18 ·
Think I found the problem. It looks like a file or something was drug across the charcoal canister. I have no idea how that would have happened.

Would that cause a faint fuel smell around the vehicle? I though I smelled gas on the way home from work yesterday, but was hard to tell in traffic, it was very faint and not consistent.
 

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That will absolutely cause the fuel smell. And when the purge valve opens to the engine it's going to suck in fresh air rather than the vapor from the fuel tank.

Could be a rodent of some sort did that.
 
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Discussion Starter · #20 ·
That's what I thought. With un-metered fresh air coming in, shouldn't my fuel trim levels go positive to compensate for the extra air? Here's my temp fix (RTV) till I can get a new canister shipped. I'm going to hold off on replacing the purge valve and the purge valve sensor, as I suspect this is the root cause.

 
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