Dodge Challenger Forum banner
1 - 15 of 15 Posts

·
Registered
Joined
·
49 Posts
Discussion Starter · #1 · (Edited)
Hey guys,

I've noticed my long term fuel trims can get as low as -12.5 at times, on both banks. Is this normal for a stock tune? I got a catch can and corsa exhaust at the moment - soot on the tailpipes but doesn't look concerning to me.

Just wondering if this is something I should further investigate, or just live with it. Also have mode6 data if needed. Thanks!
Font Technology Screenshot Electric blue Parallel


Edit:2020 RT 16k miles
 

·
Registered
2016 Jazz Blue A8 Scat
Joined
·
1,394 Posts
It's not unusual if you are coasting or making on/off throttle transitions.

What happens at steady state cruising or idling?
 

·
Registered
Joined
·
49 Posts
Discussion Starter · #3 · (Edited)
What happens at steady state cruising or idling
I don't think I've noticed any significant changes during idle/cruising. Just hovers between -8 and -10 majority of time when driving/idle.

Edit: slight tap on the pedal and LTFT goes to -7/-7.8, then reverts to -10 as I ease off the Pedal.


I've tried resetting the adaptives already and I'm hesitant to do that again since it took forever for the o2 sensor test to complete.
 

·
Registered
2016 Jazz Blue A8 Scat
Joined
·
1,394 Posts
What fuel are you running? Are you maybe running non-ethanol? What is the elevation where you live?

Negative trims means removing fuel.
 

·
Registered
2018 Plum M6 Scat Pack with Shaker
Joined
·
272 Posts
As a general rule, 10% LTFT (Long Term Fuel Trim) is not bad. 20% definitely needs investigation.

My truck came with 25% positive LTFT plus some additional STFT (Short Term Fuel Trim). Still "in regulation" but would code every time I went up a hill.

In my case, the MAF was reading consistently off across the entire RPM range (I calculated the theoretical reading based on engine volume). My MAF was otherwise working fine but the IAT was also wrong enough to throw the calculated density off a few more percent.

I also pulled my plugs to verify normal color (not lean or rich); this inspection confirms the system is still "in regulation."

"In regulation" simply means the system is ending up with the correct amounts of catalytic gases for the catalytic converters to function properly. Not too much or too little fuel.

The computer tries to push STFT to zero by adjusting LTFT to compensate. The computer remembers the previous LTFT each time the engine shuts off, and applies it on the next start.

If LTFT and STFT ever are both at 25% (pegged), the system is no longer in regulation, there is too much or too little fuel going through the engine. The engine will likely go into limp mode.
 

·
Registered
Joined
·
49 Posts
Discussion Starter · #6 ·
Standard stuff (up to 10% ethanol???) mainly 92 octane from Costco. Temporarily 2150' above sea level, however I mainly live close to sea level and had similar Ltft's then as well.

but the IAT was also wrong enough to throw the calculated density off a few more percent.
Didn't even think that the IAT could be a potential cause!!! I noticed a significant decrease in MPG when I first installed my hellcat air box, which lead me to believe I had a vacuum leak for probably 2-3 weeks. Maybe that could have potentially brought in debris and contaminated the sensor?

I usually have the mind set of not fixing things if it ain't broke, but I don't see the harm in spraying some electronic cleaner on the IAT sensor....very interesting. I won't be able to do it for a while but will update later with results.

Couldn a dirty Map sensor be a potential cause as well?

Edit: 2020 RT 16k miles
 

·
Registered
2015 RT 5.7 M6
Joined
·
11,647 Posts
Reset your adaptives (basically zeros out learned fuel trim). If your interface does not have that option then pull the PCM fuse or disconnect your battery. Then try and relog your fuel trims. Generally when I start up my challenger in spring I see the PCM pulling fuel as ambient temps are warmer (air is less dense) than when I store the car away late in fall.
 

·
Registered
2018 Plum M6 Scat Pack with Shaker
Joined
·
272 Posts
Make sure you spray the right cleaner on the right sensor. And I would not be quick to spray at all.

I looked at your numbers again and while I did not look up the spec, I don't see a problem.

MAP is Manifold Absolute Pressure, my problem was mostly the MAF. You may have just typoed it above. [The car under discussion is MAP -- not MAF --equipped. See post by Chally Tatum below]

My sensors had the unusual characteristic of still working precisely, but not accurately. It looked like it was working.

I plotted the actual readings (bottom line with dots) compared to the theoretical draw (upper line).

Rectangle Slope Font Plot Parallel


After accounting for most of the difference (MAF 20% low) there was still another 5% total error. Another way of looking at it is I might have found 80% of the problem. The engine was not reading all of the air going into it.

More sensor readings... My IAT was reading high, enough to account for the remaining error. The actual air was denser (cooler) than the IAT indicated, so still more air than measured.

Had to buy the part to confirm my diagnosis. The MAF and IAT are in the same replacement part. After replacing the part, here is the new plot.

Rectangle Slope Font Parallel Symmetry


Theory and practice are within a few percent. I went for a drive and my LTFTs plummeted.

First my STFTs went way negative. That made the O2 sensor readings correct and the engine was again in regulation. Then over some minutes driving at various speeds and under different loads, all of the tables adjusted.

The O2 sensor readings matched (drove) the trim changes and ended up looking exactly like when the truck had the "problem." I put this in quotes because the engine was running correctly the entire time. The sensor errors caused codes but the fuel trim tables were actually doing their jobs and keeping the engine in regulation.

I don't see a "problem" in your readings. I do encourage you to learn more about how these systems on the car work; sometimes you have to think your way through a problem to solve it.
 

·
Registered
2018 Plum M6 Scat Pack with Shaker
Joined
·
272 Posts
... I noticed a significant decrease in MPG when I first installed my hellcat air box, which lead me to believe I had a vacuum leak for probably 2-3 weeks. Maybe that could have potentially brought in debris and contaminated the sensor?
That is two additional pieces of information.

A manifold vacuum leak will show up in LTFTs at idle and part throttle with low load, show up less under heavy throttle and load.

Considering negative fuel trims can indicate a dirty air filter, a slight positive on your car may be showing you have a clean filter and a better-flowing airbox.

Now as to the fuel mileage drop... Did your driving style change with the new airbox? ;)
 

·
Registered
Joined
·
49 Posts
Discussion Starter · #12 ·
Updating this in case someone comes across a similar issue in the future.

TLDR: LTFT is fixed and within normal limits. Checked PCV valve, cleaned MAP sensor, cleaned IAT sensor, cleaned engine air filter followed by resetting the adaptives (disconnecting negative battery terminal). Would recommend checking engine air filter first.

1. MAP sensor - This was the first thing I checked since I didn't have access to my tools, sensor had some oily residue but nothing too concerning. Used some electronic cleaner and re-checked fuel trims but no changes.

2. Checked PCV valve, I assumed this wasn't the issue because I could hear a rattle whenever I shook it - thus it was not locked open (which would potentially create a negative LTFT?).

3&4. Cleaned the IAT sensor (w/ electronic cleaner) and whacked the air filter several times. I didn't notice any initial changes in LTFT.

5. Disconnected battery to reset the adaptives. After 500 miles post-reset, I think I can conclude that my LTFT is fixed. It now lies within 0 to -1.6, and idles at -10ish while parked. Only downside is that my stupid O2 sensor won't complete the emissions monitor/test.

If I were to guess it would have to be the air filter. That thing was pretty dusty even though it was installed in May 2021 (Mahle LX 3482 Air Filter). Good stuff.
 

·
Registered
2015 RT 5.7 M6
Joined
·
11,647 Posts
Not sure how a dirty filter would cause that much of a change in LTFT as aircharge also dictates fuel requirement as it is measured (actually calculated from) by the MAP sensor? So you reset the adaptives but how long did you drive the car? Mixed driving for about 20miles should be plenty long for PCM to adjust to current conditions. Speaking of which, was the ambient condition the same or was it hotter/colder from when you first noticed your fuel trims?
 

·
Registered
Joined
·
49 Posts
Discussion Starter · #14 ·
Not sure how a dirty filter would cause that much of a change in LTFT as aircharge also dictates fuel requirement as it is measured (actually calculated from) by the MAP sensor? So you reset the adaptives but how long did you drive the car? Mixed driving for about 20miles should be plenty long for PCM to adjust to current conditions. Speaking of which, was the ambient condition the same or was it hotter/colder from when you first noticed your fuel trims?
I drove the car the next day after resetting, it's been 500 miles after reset now and LTFT hasn't changed much since reset. Ambient temp's were the same before and after
 

·
Registered
2015 RT 5.7 M6
Joined
·
11,647 Posts
I drove the car the next day after resetting, it's been 500 miles after reset now and LTFT hasn't changed much since reset. Ambient temp's were the same before and after
If you still have the old filter, you can try tossing it in again and see if your LTFT change to negative again to confirm, but then again why futz with it when it is working fine now?
 
1 - 15 of 15 Posts
Top