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Discussion Starter #1
I've been having issues with my fuel trims being all over the place but I can't find any leaks anywhere. At idle LT fuel trims are steady from +10 to +12. But once I start driving the fuel trims dive to -20s. Before I go into all the diagnostics I've already tried I wanna hear what some of you guys think is causing this. My tuner thinks it's most likely a leak or something and not a bad tune.
 

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I've been having issues with my fuel trims being all over the place but I can't find any leaks anywhere. At idle LT fuel trims are steady from +10 to +12. But once I start driving the fuel trims dive to -20s. Before I go into all the diagnostics I've already tried I wanna hear what some of you guys think is causing this. My tuner thinks it's most likely a leak or something and not a bad tune.
LT (Long Term) fuel trims? By my observation and granted with an otherwise healthy engine these don't change all that much. They are to compensate for things that don't change all that much over a short span of time like temperature, altitude, "engine wear" or degradation in other areas like intake air filter restriction.

What are the short term fuel trims doing?
 

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Discussion Starter #3
LT (Long Term) fuel trims? By my observation and granted with an otherwise healthy engine these don't change all that much. They are to compensate for things that don't change all that much over a short span of time like temperature, altitude, "engine wear" or degradation in other areas like intake air filter restriction.

What are the short term fuel trims doing?
STFT stay low +/- 5% when the LT fully learn. That goes for both when it's at +10LTFT at idle and when it's -28LTFT cruising. I reset the pcm and also tried reloading the tune to see if it changed but the STFT bring the LTFT back to -28 everytime.
 

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Whats the fuel pressure when driving? You might want to hook up a gauge and see. You might have a pump starting to fail. Its got enough pressure and volume at idle but under acceleration its dropping the volume out and hence the - fuel trims.
 

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Discussion Starter #5
T
Whats the fuel pressure when driving? You might want to hook up a gauge and see. You might have a pump starting to fail. Its got enough pressure and volume at idle but under acceleration its dropping the volume out and hence the - fuel trims.
That was my next move, gonna rent a gauge tomorrow
 

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Discussion Starter #6
Whats the fuel pressure when driving? You might want to hook up a gauge and see. You might have a pump starting to fail. Its got enough pressure and volume at idle but under acceleration its dropping the volume out and hence the - fuel trims.
Don't negative fuel trims mean it's pulling fuel out, so it's rich?
 

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Don't negative fuel trims mean it's pulling fuel out, so it's rich?
Ya, sorry had brain fart, was late and long day and was thinking backwards and dont have a scanner in front of me to see the 02 data. Look at the map sensor. See what its reading. Check for vacuum leaks, ( highly doubt it, but check).
 

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Discussion Starter #8
Yea I checked everywhere for vacuum leaks and the map sensor seems to work fine as far as I know, 4-5psi at idle and up to 14.5 on throttle
 

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MAP at idle(no load,park or nuetral) is 10.8-11.2 typically. MAP increases with load(on accel) as vacuum is low and decreases on decel as vacuum is high.
You need to check the vacuum on the intake because something dont seem right with your #'s. Is it n/a or boosted?
 

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Discussion Starter #10
MAP at idle(no load,park or nuetral) is 10.8-11.2 typically. MAP increases with load(on accel) as vacuum is low and decreases on decel as vacuum is high.
at idle a number like 13.5 indicates a small vacuum leak (1.5 ish).
You need to check the vacuum on the intake because something dont seem right with your #'s.
So map should be higher than 5 at idle? I always thought it should be 2-5 but I could be wrong, I'll look into it thanks
 

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Ya it should be a little less than vac on the intake. Put a vacuum gauge on it and see. Step on the brake multiple x's quickly while watching the #'s also. Should go up, edited the last post cause i forgot to ask if its n/a or boosted.
 

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Discussion Starter #12
Ya it should be a little less than vac on the intake. Put a vacuum gauge on it and see. Step on the brake multiple x's quickly while watching the #'s also. Should go up, edited the last post cause i forgot to ask if its n/a or boosted.
N/A until I know my fueling is safe. If I can atleast find out if its a bad injector or something I'll use it as an excuse to order the blower since it comes with injectors, that's how I like to warrant modding my car lol
 

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Definitely look at the map sensor then. Somethings wrong. N/a motor should have a higher reading then that. Check the vacuum pressure of the intake. If its low also then youve got other issues like cam timing or a monster cam thats not creating enough vacuum for the sensor. If it was an injector then one bank would read off and other would be right with the ltft.
 

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Discussion Starter #15
Can you read voltage at the map or just pressures?
Honestly not sure, I know my logs on the Diablo are reading pressure tho. It's just a 392 cam and my tuners saying the old logs were looking good 2-3 months ago with the same mods. If it's a map sensor then that's another reason to get the supercharger lol
 

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The 392 cam should make good vacuum. Manufacturers like easy brakes that wont get hard after a single pump. You can make sure that its got 5v for the reference and voltage changes on other wire when vacuum is taken away but my $$ says the sensors bad. Does it read 0 with KOEO?
 

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Discussion Starter #17
The 392 cam should make good vacuum. Manufacturers like easy brakes that wont get hard after a single pump. You can make sure that its got 5v for the reference and voltage changes on other wire when vacuum is taken away but my $$ says the sensors bad. Does it read 0 with KOEO?
It reads about 14.7 key on engine off
 

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Discussion Starter #19
Ya go buy a map sensor. Should read "0" engines not running so shouldnt read nothing.
It's reading pressure not vacuum so wouldn't 14.7psi mean it's good? Barometric pressure is about 14.7psi cuz I'm at sea level
 

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Map sensors should read 0 when KOEO. Im at sea level and ill double check tomorrow on mine. The more we talk just tells me its wrong. It should read about 25/30% less then vacuum while running at idle no load. 5 is not a good # at idle.
 
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