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Low fuel trim

2K views 38 replies 8 participants last post by  Grog6 
#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?
 
#3 ·
Is it the STFT or LTFT that is showing up -10%? They mean opposite things, so knowing which is important. :)
 
#4 ·
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.
 
#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?
 
#6 ·
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...
 
#8 ·
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.
 
#7 ·
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
 
#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.

 
#11 ·
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.
 
#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.
 
#17 ·
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.
 
#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.
 
#19 ·
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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#28 ·
Yeas, it's just a cannister full of activated charcoal. You might want to blow thru the ports to figure out what's what. The function is to let the fuel vapors adsorb on th charcoal, so engine vacuum can burn them later, when the purge valve opens.
 
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#29 ·
Would a regular rubber cap hold up? Everything used on the EVAP should be fuel rated hose.

Maybe that's why the EVAP hoses always break down after a number of years. It's basically just vacuum hose. Ford didn't expect us to still be driving these cars after 25 years, lol! The hose probably lasts for a typical lifespan, 10-15 years.

Al
 
#30 ·
I understand the basic operation of the EVAP system, as Grog explained above. With that hole in the canister and the charcoal, etc being exposed to the elements, is it possible/ probable that the charcoal has deteriorated?

BTW: the CEL went off today, I did fill up with gas yesterday.
 
#31 ·
I understand the basic operation of the EVAP system, as Grog explained above. With that hole in the canister and the charcoal, etc being exposed to the elements, is it possible/ probable that the charcoal has deteriorated?

BTW: the CEL went off today, I did fill up with gas yesterday.
The charcoal can get contaminated over time. I know some people clean them out with mass amounts of wd40. But I've never tried it. Also being open to the air can bring in dirt that will help the purge valve fail.
 
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#32 ·
They should be ok; iirc, the pores are too small for water, so they shouldn't get saturated with humidity. You have to mix it with a liquid, and it's not easy, lol.
 
#33 ·
Oh, T6: you want edpm or silicone, not rubber. Viton type stuff, not buna.
 
#34 ·
I used "fuel" hose from the local store.

Al
 
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#37 ·
Replaced the charcoal canister, purge valve and sensor. Cleared the CEL and it did not return on the test drive.
FWIW, The new canister was significantly lighter than the old one.
However the fuel trims are still hovering around -10%.
-10 is still a pretty good improvement over the -20s I saw on your charts. Besides, if you aren't getting codes for being lean. You really don't have to worry about it too much. Unless you are of course trying to make the engine air tight. Maybe because you want to build some kind of 007 submarine MN12. In which case I say, carry on.
 
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