Straight to the point…The Long Term Fuel Trim on my RX8 shoots straight to 15% when the rpm reaches around ~2200, and goes back down to 0% when the rpm fells below ~2200. I observed that in neutral. It seems to me that the behavior is something programmed in the ECU, since it appears to do that independent of what the O2 sensor says, although at around 75mph in 6th gear the O2 sensor will just stay at low voltage (lean) and not come back up, while the LTFT tops out. Does it do the same thing on your cars? Or this is abnormal?

Right now there is no CEL triggered. I do have three occasions in the past that the CEL came up with P0171. Twice was when I was doing autocross, and once was when I was cruising at 82mph on the highway. I can’t reproduce this code in any way, but I don’t think the high LTFT is normal.

The engine has 22k miles on it, so I hope it’s not done yet…My guess is that it is something that triggers around 2200rpm (SSV)? But why does it do that without O2 Sensor first goes to lean? And I think SSV only opens a new intake path but not a new possible vacuum leak location?

Anyway any suggestion/idea is welcome.

Thanks!

 

this is the difference between open and closed loop

this thread may help explain open/closed loop

https://www.rx8club.com/tech-garage-...d-loop-170331/

 

An LTFT of +15% means vacuum leak! Both LTFT and STFT will read zero when in open loop.

 

A high positive fuel trim at cruise rpms could be a vacuum leak although the fact that the trim is not present at idle suggests something else .
Do you have an aftermarket intake ?
Clean your maf to start with, as that is easy, before you do anything else .

BTW : In open loop you should see a ltft but stft will be zero.

 

Thanks for all the input! When I maintain an rpm around 2200 the status says closed loop IIRC.

Everything on the car is stock, other than the BHR coils and wires, which was installed 10k ago along with new plugs.

The car idles with closed loop fine. It seems it's only a problem above 2200rpm, at least in the neutral gear case.

I will clean the MAF and see how it goes.

 

By the way, I was running this strange function "vehicle test" on my scan tool, and two of the values came out red:

MID: $b0 TID:$99
Min: 25.197% Max:-74.383%
Current: -79.938%

MID: $f0 TID:$cc
Manufacturer defined MID
Min: 8,273 Max:2
Current: 52,464

Most of the items have description but these two do not. I tried to run a quick google search but I guess being some random letters don't help with the search.

 

What is your AFR when the trims are +15?

At that RPM you should be in closed loop....so the o2 sensor/MAF relationship is very important. Not sure if you are still in CL at 80mph, possibly in 6th gear and low throttle position

 

Commanded AFR: 14.78 Measured AFR: 14.91 RPM: 2061.5

LTFT1 is around 15% MAF is around 12 g/s Closed Loop

At highway speed (50-80 mph) the LTFT is around 12.5% to 17.5%. The measured AFR still follows commanded AFR. This morning I was not able to get the constant low O2 sensor voltage like I did during the trip three days ago, but I guess I might have been going downhill hence the lean condition.

So the weird thing is LTFT is high anytime the rpm is above 2000, in gear or not, in closed loop.

Now I am thinking it's either MAF or fuel system. I have not cleaned the MAF yet, so I will do that first. Working a lot overtime this week so hopefully I can get to it soon. The airbox in my rx8 is a pain to push out...

 

There are 3 LTFT's depending on the maf being covered. Everything about ~25 gm/sec should have only one LTFT that is quite stable. The STFT will vary with throttle on/off, load, etc. - are you sure you're not confusing the two?

 

cleaning the MAF is easy, it comes out with 2 screws.

mine is doing the opposite, it seems there are two different (or more) fuel trim tables. mine @idle is 10% LTFT, and if you rev it to 2500 its %2, so i think mine is a vacuum leak.

yours being the opposite is something else. i'd suggest looking at the coils, and wires

 

OK...this might be it...THREE LTFT's???

What I found today coming back from work is, the LTFT shown seems to be some mapped value. This time I try to see the exact value instead of to plot it. It seems like with cruise control on highway speed MAF and the LTFT has the following relationship:
MAF: < 7 g/s LTFT: -2.3%
MAF: 7-20 g/s LTFT: 13.3%
MAF: 20-50 g/s LTFT:15.6 %
MAF: 50+ g/s LTFT: 16.4 %

Very strange. Then I thought about what HiFlite999 said, and flipped through the menu again, there is this "Fuel Trim Bank 1 Sensor 2", and it fluctuates around +/-2%...I wonder if my scan tool is confused?

So there are three values I think might be Fuel Trims (Now I'm confused). The names in the scan tool are:
Fuel Trim Bank 1 Long Term (The LTFT I have been thinking)
Fuel Trim Bank 1 sensor 2 (The mysterious +/-2% one, does not show up in my Protege5)
Fuel Trim Bank 1 Short Term (Gotta be the STFT)

One thing I know is, when I let go the gas pedal in gear, Fuel Trim Bank 1 Long Term goes to -2.3% while Fuel Trim Bank 1 sensor 2 goes to 0. I need to confirm this but this might be a way to tell which one it is.

Also I cleaned the MAF and it doesn't change the behavior.

 

Sensor 2 is likely the rear O2 sensor..it is a narrowband and fluctuates up and down to signal the ECU of its values

I think I would check the fuel pressure...and then the injectors....it seems like you have a fuel defecit and that the ECU thinks you should be getting more fuel than you really are....

 

and that brings up another of my question. In the list I have three things:
Air Fuel Ratio(measured) - tries to follow commanded AFR in Closed Loop. Pretty straight forward value
O2 Sensor1 Equivalence Ratio(alternate) - a value fluctuate around 1
O2 Volts Bank 1 sensor 2 - in real Voltage value...I can see it does not jumps up and down drastically like narrow band O2 sensor on my Protege5

I don't know if you need another value for the O2 sensor...

I am looking up something called EVAP Purge Percentage...Does RX8 ECU support this value? Because it is not in my list and it seems like it would follow MAF...

I do want to clean the fuel socks though...at 120k it has not been cleaned.

 

Equivalence ratio is Lamba 1 = approx 14.7 for gasoline

Evap purge is an emissions thing...it will not be your problem. If you have an evap leak you will get a CEL quickly...just leave your gas cap off for a few minutes and you will see

If you are on the original fuel pump at 120K miles you should think about a new one. The filter is integrated into the pump assembly...and the pump is barely adequate when new

I would bet your FP is down......

 

Maybe that would explain the two times I got P0171 during Autocross? (although how hard the engine is being driven during autocross is debatable) That the Fuel Pump can't keep up with the load?

What are the symptom of a weak FP? Maybe I should look into that.

Actually what I really need to figure out is what those two PID's stand for...I can't the info anywhere!!

MID: $b0 TID:$99
Min: 25.197% Max:-74.383%
Current: -79.938%

MID: $f0 TID:$cc
Manufacturer defined MID
Min: 8,273 Max:2
Current: 52,464

 

and I did a few more search around, get another scan app, and I think the LTFT I have been looking at is indeed the LTFT.

and I think what HiFlite999 meant by 3 LTFT is that there are three different 'modes' of LTFT...I found this info on Racing Beat website. Hence the 'mapped output' behavior of the LTFT with relationship to MAF.

Anyway...I think the Fuel Pump is the target now...

 

I would be very interested in finding out what these are as well:

MID: $b0 TID:$99
Min: 25.197% Max:-74.383%
Current: -79.938%

MID: $f0 TID:$cc
Manufacturer defined MID
Min: 8,273 Max:2
Current: 52,464

I have the EXACT same reading coming from my torque app. I just picked up the car yesterday after an engine replacement. I'm contacting the the service manager to see what they are, he's been very good working with me. I'll post up once I know more.

I wasn't paying attention to the LTFT at the time, I'll watch it on the way home tonight and see what they are.


**update**Rec'd an email back from the Mazda Service Manager and he had never seen the MID's before. He had no clue what they were related to. If anyone knows anything about what these are, I'd appreciate the info!

 

What would cause a cl/ol fault.

 

Bump..

 

Problems with the O2 sensors, ECT, IAT, MAF, anything that causes limp mode...like MOP
Injectors or fuel delivery system...after the max trim is set it likely goes into open loop
ECU fault, wiring problems....etc

 

I thought limp mode was sluggish and everything. My car functions as it should.

 

You asked what could cause open/closed loop problems......that is one of them

Obviously your car isn't in Limp Mode

 

Oh LOL.. Yeah I did say that.

 

Actually cleaning the MAF did help. Now my LTFT is 9.4% instead of max out, and the system is back at adjusting the STFT against the O2 voltage.

9.4% still seems high, but acceptable. Hopefully a new fuel pump can fix the rest..

 

Replaced the fuel pump. No change to the LTFT...still around 9.4%-12%. Hmm...