Yet another LTFT diagnosis...
 

I've been researching and I think I need somebody to sanity check my logic and next steps. Also, I'm wondering if I'm just suffering from automotive hypochondria.

Problem:
I'm seeing LTFT in the +14-17% range (measured via a PLX Kiwi3 using OBD Fusion and DashCommand).

Additional Info:
My buddy teaches auto tech classes at a local school so we used his smoke machine (removed air filter and placed it in a plastic bag before reinstalling it). Injecting through the brake booster port on the intake, the only place it came out was around the MAF (bad o-ring, replaced) and eventually through the airbox.

My MAF reads ~5.5g/s at idle (recently cleaned, new o-ring installed). I left the battery disconnected while we were working on this to make sure the fuel trims were cleared. They started at 0% and slowly climbed back up.

No CEL.

STFT's move around a bit in the +/- 3% range though I've seen spikes as high as +/- 5%.

Ignition coils, plugs and wires are about 8 months and 8k miles old.

Bone-stock intake system, OEM air filter.

The engine was rebuilt in 02/2016, all sensors were re-used.

Fuel economy is good with a combined 19.5 mpg with mostly highway but some Chicago city traffic and 10 autocross runs.

I put a MityVac on the VDI and SSV actuators and they open/hold vacuum fine.

Does the following section make sense?
It looks like the next likely candidates for troubleshooting are the front O2 sensor and fuel pump.

To check the fuel pump, we're mostly interested in fuel pressure. To check that, install an adapter (Aeromotive 15120, Aeromotive OEM and Specialty Adapters | Aeromotive) in the fuel line near the brake booster with a gauge attached. Do I just turn the key to run to let the fuel pump prime the system or do I need to let it idle?

To check the O2 sensor... No ideas. How do you do this without just swapping parts? I guess just log sensor data and see if everything makes sense.

Or am I just being an automotive hypochondriac and should stop worrying so much?

Are the trims over the whole load range?..or just at Idle?

I don't know about the whole range but definitely at idle and highway cruise.

There seem to be different profiles depending on whether it's idling, throttle tip-in (accelerating from stop) and steady part-throttle (highway cruise).

Do you have an aftermarket intake?

:wavey:

It doesn't take much of a vac leak for a 14% trim at idle.

Cold be a funky injector as well....if it is spraying less than expected it will add fuel to make it up.

What is your fuel pressure?

If the car runs I wouldn't freak out too much anyway 😊

Update on the troubleshooting...

I smoked it again yesterday from two different points (to make sure that the part I disconnected wasn't the source of the leak. The only place smoke came out was through the front of the airbox. That confirmation plus the normal MAF readings make me think is isn't an intake leak.

We also put a pressure gauge on the fuel line and it idled at ~62 psi and dipped to 59 psi when I rev'ed it. It's right in range of 54-65 psi.

We measured the resistance of WBO2 heater circuit (3.1 ohms, normal range is something like 2.16 - 2.9 ohms) before we realized that it needed to be cold. Oops.

The service manual states that the WBO2 output should be positive when "accelerated" and negative when "decelerated". I assume that means that the current out of the sensor should be positive when I push on the go pedal and negative when I lift off.

I bring this up because when we were using my buddy's expensive-ass SnapOn MODIS Ultra diagnostic scanner, we saw the opposite of this. That is, it seemed to dip when I depressed the go pedal and then spiked when I lifted off. This could have just been the response lag between reality and the scanner displaying the information.

The MAF will indicate normal airflow even with the presence of an intake leak. The MAF only measures what comes in through the normal intake path, not what comes in through a gap after the MAF, which would be additional air for which the car needs to add additional fuel.

I'd suggest going over all the vacuum lines to make sure they're plugged in correctly and over all bands/connection to make sure they're snug. Vac leaks can be a b!tch to nail down, but they're also easier than replacing injectors and things like that.

With the scanner you should be able to log throttle position and WBO2 output to see what it's doing. In fact log everything at idle and at highway cruise and post it here.

I had easy 10s after I re-built mine. Computer looks at air flow, then at RPM, then a few tables and guesses on how much air actually made it into the rotor. TRIM is where it looks at the O2 sensor and says "Damn, I was wrong by xx%". Kane says it much better in his tuning class. I tuned mine to get them back to +- 3%.

If you have the original O2 sensor, might want to change, just because. Or, see if someone has one in the area. From what I understand, as they get old, they get sluggish.

Fuel economy says engine is running right, it just has to adjust to get there.

@ Loki, you are right, but.... If there is a leak, the MAF will read low, because the ECU will adjust idle speed back down, then add trim to get mixture. A 5.0 gps at idle speed means the air is coming from someplace else, and trim means it had to add gas to get the commanded mixture.

Basically, you are getting more air, or less fuel, than you think, on average. You could have both rotors running lean, or one fine and one really lean. I think the plugs will tell that story, I would pull them and look at them. If they look different, start looking at fuel injectors. If you have a fouled plug, that can be the problem as well. Less spark is less burn which uses less O2, which makes the sensor think you are running too lean..

Good luck

I'll do that as soon as I get my rear suspension sorted out. It's sagging a bit low after installing OTS Koni Sports.

Thanks for the extra diagnostic ideas. Running rich will cause the plug to have black carbon/soot build-up on the plug whereas lean will cause the plug to be more brown, right?

Well, this is interesting. It's been about 100 miles since I smoked the intake and checked the O2 sensor. The fuel trims were reset when we disconnected the battery.

Aaaaaaand, they're normal.
Idle: 7.0%
Tip-in: 5.5%
Cruise: 3.3%

Please help!
 

I'm trying to nail down a problem with my car and I think it's the fuel injectors. In the rear rotor. I initially experienced MASSIVE power loss about a month ago (like suddenly while I was driving, no noise, no clanking, just power loss and misfire). Since then I've been doing extensive amounts of research in this forum and trying different troubleshooting things that I found in my free time. The reman engine was put in this car in March of this year and only put about 5k miles on it since then so I highly doubt compression is an issue, but if you think I should, then I will get a compression test done.

I replaced the coils, plugs, and wires with oem replacements. This brought back the power for only a few days. Then all of a sudden the power left again. With a rear rotor misfire CEL. So I replaced the plugs in the rear rotor, cleared the codes, did 20 brake stomp, and there was no difference. Then a p0171 (system too lean), p0300 (random misfire) and a pending p0302. So I moved on from the ignition system.

The STFT was +25 and the LTFT was 0. It stayed that way. Never fluctuated. Found a vac leak, fixed it, and now the STFT is around -.8%-.8% and the LTFT is +22.6% (all of this is at steady idle). When the A/C is on, the idle fluctuates from 800-1.3k. It drops down to 800, then jumps up to 1.3k then slowly goes back down around 900 then immediately drops to 800. It doesn't die at all, but it does do this loop as long as the A/C is on. It's better when the Headlights are on with the A/C. It goes from about 850 to 1k very gradually in both directions. Cleaned the MAF sensor with CRC Maf sensor cleaner, no change.

I moved on to checking the fuel pressure. When primed it holds at ~60psi. At idle it's about the same(the needle moved back and forth quickly between 59 and 62 almost as if the vibrations from the engine were interfering somehow). So the fuel pump isn't the problem.

So last night I noticed that the car ran the exact same without the rear ignition coils connected to the spark plugs. So I switched around the rear and front ignition coils this morning to see if it would make a difference. Didn't change. Car starts up and runs the exact same with or without the rear plugs connected. Can the rear fuel injectors be completely pooped out to where they are not working at all? I thought they were replaced when the guy put the reman in, but I just looked at the invoice paper and fuel injectors were not purchased, only fuel injection seals.

Sorry for the long write up, but I figured the more details I provide, the better someone would be able to assist me. BTW my car is an 04 MT. It has 142k miles on the body and about 5k at best on the rebuilt engine.

Do you have an issue with the coil plugs? Sounds like you may have a broken wire?

Could be an ECU problem as well.

You could swap the primary injectors and see if it follows the injectors.

I think the coils are working as i shocked myself doing something just...so terribly stupid I'm ashamed of myself (car was running while the rear 2 coils were unplugged...attempted to plug them back up not really thinking about the fact that the car was actually running. How could I not have actually processed that thought? I have no idea, but the shock def put me in my right mind and that mistake won't happen again I'll tell ya that). I'll double check both wires though.


https://cimg9.ibsrv.net/gimg/www.rx8...37fc02f0d4.jpg

Swap both primary 1 and 2 (4 total being moved) or just one of them (2 total being moved)

If it's an idle issue then swap out just the Red injectors......

Where do you live? Someone local might have a set to try??

Sorry for the late reply. I've been super busy the last 2 days. I'll swap them around tomorrow and see where that gets me. It's not just an idle issue though. The loss of power is constant all the way to redline. Then I realized why (running on only the front rotor). The main issue now is finding out why the rear rotor isn't firing.

I think it is unlikely to be an injector from the description. Once the Sec and P2 injectors stage it should fire.

Have you done a compression check...or at least turned it over without the L spark plug so you can listen for pulses?

Not yet. I was hoping it didn't have to come to that since the engine is so new, but it seems like it's getting to be the only thing left. I checked the coil harnesses and they look to be fine. I'll contact my local mazda dealer and see if they can do a compression test. In the mean time I'll check the wire resistance and listen for pulses.

New develpment. I couldn't find anything like this issue on the forum. It's still running bad, but while driving, the rpms very slowly and steadily started dropping. Like in about 5 seconds it dropped a couple hundred rpm. I downshifted and shifted back up and it started climbing again. Sometimes keeping it floored kept the rpms the same. But every now and then there was what seemed like a louder than usual back fire and then a noise that sounds like a gallon jug snapping back in place after having one of the top corners pushed in. Except deeper and more metallic. It's a really weird and unsettling noise. Wherever this noise came from, it was coming from behind me. The first time i heard it i immediately thought it was the fuel pump because of the location of the noise, but I don't think a fuel pump can make that noise so I doubt it was that. After I parked it and put the ebrake on, I noticed smoke coming up through the ebrake area. It wasn't a whole lot, but smoke of any kind coming from anywhere but the exhaust is not a good sign. Guess mazda will be diagnosing my whole car instead of doing just a compression check when I get it to them.

Well, any kind of backfire means one thing: too much fuel in the exhaust, which implies a few possible causes:
- failed ignition, so fuel goes into the rotor and right back out and ignites in the hot exhaust. You shocking yourself is not the same as making effective spark over the spark plug gap. Shocking you doesn't take much, jumping that gap takes thousands of volts.
- leaking injector that's constantly dumping fuel (DISTANT second probability)

If you're seeing SMOKE from your ebrake area, that could be plastic cooking on hot metal sufraces, which means your catalytic has bought the farm and become a container for hot exhaust gases. You'll want to stop driving the car until you can fix/deny this condition, unless you enjoy buying new engines.

Your LTFT is maxed out, which is also no good. I get that you fixed one vac leak, but you have created or missed others. Go over it again and/or reset the computer and have it learn from the beginning.

Ok. Thanks for that info about the spark. The coils are fine as I swapped them around with the front 2. So could both the rear coil harnesses or plug wires be shot to hell at the exact same time? This problem literally came out of no where. I drove it one day, running fine. The next day immediately upon starting I noticed it was off. It didn't purr lol. Drove it and noticed the massive loss of power and then it died at a red light.

The fuel trims were reset about 3 times (and relearned) after the vac leak fix and the LTFT always finds it's way up t 22.6%. So you're probably right about another leak that was missed, but I just thought something else was to blame since the STFT was no longer stuck at 25%.

Also, the guy I bought the car from a few years ago gutted the cat, so could the fuel igniting in the exhaust alone keep it at sustained temperatures to cook the plastic? Or it only happens when the cat "hot boxes" the exhaust when it's shot. Maybe something crawled up there and died and is clogging up the thing :suspect:.

Any insight on the RPM dropping while I was driving? Also a detail I may have left out, upon starting the car cold, it coughs like huge misfire. It revs up to 2k like normal then drops immediately to 1k. Then it tries again and drops and then bounces between 1.5 and 1 until it gets it together after about 15 seconds. Then it's as smooth as 1 running rotor can be. If I try to assist in getting it stable, it reaches about 3k (i don't rev it any higher because its cold) then backfires and drops to 1k and does that for the ~15 second period. So now I just let it be and let it do it's thing. After that it doesn't really back fire unless i rev it up to ~6k...until today that is.

Edit: I just want to thank you guys for your help. I know it's not fixed yet, but you are taking time out of your day to help me out a little.

Update: Finally got around to doing the pulse compression check. Took a plug out of the front rotor and cranked (fuel cut of course) and out came super loud constant air bursts (I didn't know it was supposed to be that loud it scared me at first). Put it back in and did the rear rotor and nothing. Like not even intermittent bursts. I heard nothing (this was from the inside of the car with the hood and drivers door open though so it could have made some type of noise, but it is obviously no where near as healthy as the front rotor). Would sea foaming help this? Or is this case too extreme?

Reman engines have a pretty average reputation... I wouldn't be surprised if it's cactus. Forget seafoaming, let Mazda sort it out.