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fuel trims

B

BrApple

No Life But CEG
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Joined
Dec 23, 2001
Messages
21,731
Location
Gales Ferry, CT
can someone explain what goes into determining the fuel trims? I have been doing alot of data logging for Joey and he is saying that bank 2 is running 20% richer then bank 1. so I would like to try to understand myself what goes into determining the fuel trims and what could cause them to be different from one bank to the other.

at one point I did get a cel for bank 2 to rich.
 
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is your car a returnless? i have noticed that even on my basically stock SVT, if i watch the LTFTs that bank 2 is always about 10-20% richer than bank 1, even with new O2s. it seems to be the worst (closer to 20%) at idle, which makes sense since the IAC is at the back of the intake near bank 1. cruising it sits about 10% richer.

Its one of those things that really bugs me too.
 
striker2 said:
is your car a returnless? i have noticed that even on my basically stock SVT, if i watch the LTFTs that bank 2 is always about 10-20% richer than bank 1, even with new O2s. it seems to be the worst (closer to 20%) at idle, which makes sense since the IAC is at the back of the intake near bank 1. cruising it sits about 10% richer.

Its one of those things that really bugs me too.
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Well I have a returnless car that we just dynoed 360whp and I sent him the datalog on the 2000 SVT contour Returnless and the banks are less then 5 % off at any rpm or load so I would scratch off the theory because I have some more datalogs of newer cougars supporting that they should be less then 5% of each other. The reason they give 5% as a limit is for turbulance, heat spots etc, but when you have 20% difference that is alot especially when the PCM trip the code at 25% as a problem. The big issue with a FI car is that the wideband is reading of combined banks so if I dial in his car to 12.1 A/F as a sum of two banks then one bank will be 13.0 A/F and the other 11.5 A/F netting him a 12.0 after the turbo. Now would you feel comfortable going to WOT with one bank running leaner than others. I knew there was something wrong here so I asked him to pull his plugs to show me a pic of the tips I noticed right off the go that the number 3 cylinder was new looking compared to the 1 & 2 & 4 & 5 & 6 where 4 was a nearest color but was not even close to the same burn color. but before I did this I noticed his O2 were reading different as far as response time but still were working right as they should. So suspecting my tuning experience I asked him which one was new because different brands and newer sensorr vary from the OEM's. Well After nailing that on the head I wanted him to switch them to see if I can make the older sensor move the rich over two bank 1 and if it did a simple new sensor would fix this. Well it didn't make any difference so I knew it was electrical or mechanical at this point. He did do some CR readings prior but I think he spun the motor over with the other plugs still in the motor making it harder to get quick pumps to build pressure.

So right now the car run great as a whole but no engine is suppose to run 20% off from the other side without some explaination but I do know the PCM will adapt the long term fuel trims to make it right by making a 18% change across the boards so now it bandaided the problem to make the engine run smoother but you are leaving power on the table because this will hurt you when you plan on pushing the boost up to the top where you will have one bank running much leaner than the other then you will pop the motor due to detonation on the leaner bank. I know it gets aggravating but I have to as a tuner make customers know there is a issue that might cause issues down the line by washing cylinders or leaning out to detonate.

Datalogs do not lie if the sensors are working correctly but if it was my car I would install another o2 bung on the rear bank and move the wideband to it get a reading under load then swap it over to the front header which already has a spot on the header for the downstream and then make a determination if the car is safe enough to run higher boost with one bank running different. joey
 
I agree with you that running one bank so much leaner is a bad thing, espicially on a boosted car, but that doesnt explain why my stock SVT is like that. if your willing to look at datalogs PM me what you want and ill get them for you. i dont want a tune or anything, just want the banks to run close to even LTFTs so i need to know whats wrong and how to fix it.
 
yes one O2 sensor is new, the other is a few years old and is a bosch also. I may replace that older one anyway just so I have 2 new sensors.

I have noted the difference in trims myself in the datalogging.

as for the plugs I would say that bank 1 plugs where pretty similar and bank 2 plugs where also fairly similar. However bank 2 was a bit darker then bank 1. I did note that some of the plugs only seemed to havea decent amount of coloring on one side of the plug. I'll pull the plugs and check again.

I'll run another compression check with all the plugs out. Also when I did it yesterday the ingnition wasn't on so there should have been no fuel being sprayed and the coil pack was already disconnected.

I also think I might swap back to a stock set of plugs. It has bothered me recently that on my current set that wires 5&6 don't clip onto the plugs as the others do.

what else should I check? I want to fix the issue if I can, not just have Joey tune it out.
 
striker2 said:
I agree with you that running one bank so much leaner is a bad thing, espicially on a boosted car, but that doesnt explain why my stock SVT is like that. if your willing to look at datalogs PM me what you want and ill get them for you. i dont want a tune or anything, just want the banks to run close to even LTFTs so i need to know whats wrong and how to fix it.
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hey striker send me a datalog I will look at it for you and see if I can fix the problem for you even stock car have problems come about spark plugs, CR , valve seat, iginition issues, harness issues, dirty injectors, etc that will cause this problem

send me your datalog at burritasvt@yahoo.com

I need this

engine RPM
engine load
engine coolant temp
battery voltage
STFT banks 1 &2
LTFt banks 1 &2
pulsewidth fuel injectors bank 1 & 2
MAF counts
MAF lbs/min
absolute throtlle position
upstream o2 both banks
downstream 02 both banks
Intake temps
vehicle speed
spark
open /closed loop
 
I would look at the O2 voltages themselves and see if the 2 banks agree with each other.

Also, try swapping O2 sensors on the banks and see if the fuel trim variance follows the O2's. If it does, then you know where your problem lies.

If it doesn't then you'll have to figure out why one bank is burning better than another.
 
Barge said:
I would look at the O2 voltages themselves and see if the 2 banks agree with each other.

Also, try swapping O2 sensors on the banks and see if the fuel trim variance follows the O2's. If it does, then you know where your problem lies.

If it doesn't then you'll have to figure out why one bank is burning better than another.
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Both upper O2s are brand new Bosch 15717 sensors from advance auto parts installed July 27th (front) and 28th (rear). after reseting the PCM the LTFTs are still not even.


Joey, ill send you an email tomorrow with a datalog.
 
Barge said:
I would look at the O2 voltages themselves and see if the 2 banks agree with each other.

Also, try swapping O2 sensors on the banks and see if the fuel trim variance follows the O2's. If it does, then you know where your problem lies.

If it doesn't then you'll have to figure out why one bank is burning better than another.
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swapped sensor and no difference. I replaced the one that was a few years old and still no difference. I used 13115, which are actually suppose to be for a zetec but I don't see why that would make a difference. either way both are new and where installed with a month of each other.
 
Banks that far off... 2 likely reasons.

1. Intake leak
2. Exhaust leak

If one bank is drastically pulling fuel trim then a leaking injector could be the problem.

I concur with the above statements because my long term fuel trims are within 5% of each other. Now since being forced to run ethanol blended fuel after the move they are seemingly always LEAN but that is another story... (Blended fuel sucks!)
 
DemonSVT said:
Banks that far off... 2 likely reasons.

1. Intake leak
2. Exhaust leak

If one bank is drastically pulling fuel trim then a leaking injector could be the problem.

I concur with the above statements because my long term fuel trims are within 5% of each other. Now since being forced to run ethanol blended fuel after the move they are seemingly always LEAN but that is another story... (Blended fuel sucks!)
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well I can check the headers again to make sure they are not leaking but I don't hear an exhaust leak like I did when I first put them on my 2.5L engine. Also I have already resealed the buzuiki plates to the heads and all the intake gaskets where new at the time the 3L was built.

As far as I know there are no leaks as the car idles fine, sometimes alittle high but fine. I do know that the turbo xs type H-rfl does leak past the o-ring in the top where the vacuum line attaches.
 
What condition is the injector harness in?

When I did my swap my harness was brittle and 2 of the rear injectors ended up arching off the manifold causing the injectors run fully open. End result was a 11.5:1 AFR out the tailpipe which seemed normal, but the SCT software said otherwise. Once we put a wideband on each manifold the AFR was below 10:1 on the rear bank and like a 12:1 on the front bank. Got all new harnesses put on and it fixed the problem. Maybe 1 injector is arching? Just a thought.
 
beyondloadedSE said:
What condition is the injector harness in?

When I did my swap my harness was brittle and 2 of the rear injectors ended up arching off the manifold causing the injectors run fully open. End result was a 11.5:1 AFR out the tailpipe which seemed normal, but the SCT software said otherwise. Once we put a wideband on each manifold the AFR was below 10:1 on the rear bank and like a 12:1 on the front bank. Got all new harnesses put on and it fixed the problem. Maybe 1 injector is arching? Just a thought.
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harness should be fine, I never noted any issues with the harness. I do plan to install my wideband sensor on each bank to see what each is doing. I also think that swapping the injectors from between banks would be a good idea to see if it is an injector. I did check them when I got them and they where all identical.

however if a injector was running full open would I be able to still get 24 mpg on a highway only tank of gas staying out of boost?
 
yeah, i just pulled ~29MPG almost entirely highway this past weekend. its really baffeling me cause the car seems to run fine, even though the LTFTs are too far apart.
 
striker2 said:
yeah, i just pulled ~29MPG almost entirely highway this past weekend. its really baffeling me cause the car seems to run fine, even though the LTFTs are too far apart.
Click to expand...

yeah mine ran very well last night also. however I was running a tune with the adaptive learning turned off.
 
beyondloadedSE said:
What condition is the injector harness in?

When I did my swap my harness was brittle and 2 of the rear injectors ended up arching off the manifold causing the injectors run fully open. End result was a 11.5:1 AFR out the tailpipe which seemed normal, but the SCT software said otherwise. Once we put a wideband on each manifold the AFR was below 10:1 on the rear bank and like a 12:1 on the front bank. Got all new harnesses put on and it fixed the problem. Maybe 1 injector is arching? Just a thought.
Click to expand...

This is where I am at right now I want him to put the wideband on each bank in the position of the downstream because I know this will tell it all for sure and then we can really do some real hunting for the problem but until we bakc this up with some secondry readings we will wait to check further.
 
striker2 said:
yeah, i just pulled ~29MPG almost entirely highway this past weekend. its really baffeling me cause the car seems to run fine, even though the LTFTs are too far apart.
Click to expand...

Well the reason you are running fine is becaused th PCM corrected the problem by balancing the banks by adding or subtracting fuel but the real problem is not fixed but you are loosing power because the WOT is no longer monitored by the PCM in open loop so you can be leaving power on the table.
 
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