Wandering AFRs and crazy fuel trims at partial throttle

David Buckwalter

David Buckwalter

Greenie N00B Member
Greenie Member
Looking for help diagnosing a weird issue with crazy fuel trims and a wandering AFR under light throttle. There is not an obvious vacuum leak causing this. This isn’t my daily anymore so it is not the end of the world but I am at my wits end trying to figure this out so I can work on my 6th port install and tuning.

Year/Make/Model: 2008 Speed 3
Mileage: Body:150,000 Engine:~60,000
Location: Southeast Pennsylvania

Concern: AFRs are not hitting the target under light load, throttle, most commonly under cruising conditions and partial throttle, idle seems to have issue settling at 14.7 at random times as well. LTFT is generally around -10 and STFT is all over the place anywhere from -25 to +25. Logs are provided below. Also twice I have had the car start and be stuck at 9 AFR until shutting off and restarting or until letting it warm up. The car’s fuel economy is also absolutely trash now, it was never great but I maybe get 150-175 miles out of a full tank, I used to be able to get like 250 as long as I didn’t romp on it. There has been no additional engine/power mods on the car since the tunes were dialed in just regular maintenance and occasional repair.

DTC's: None related, one code for a dead heater circuit in the rear O2 sensor and the codes from my EGR delete.

Modifications: CPE TMIC, 3in highflow catted DP, 3in HKS Catback, TIAL VTA BOV, CS injector seals, Damond EGR Delete, HTP 3in intake, CST4, JBR intake manifold TIG, MAC valve, Autotech HPFP internals, NGK 6510 spark plugs, dialed street tunes for both e30 and 93 (issue observable on both), not really a mod but I did the timing chain 10K-ish miles ago.

ECU/Tuning Software: Cobb Accessport

Tuner: Andale927

Is the concern intermittent? Not really, the issue is observable to varying degrees every time the car is driven.

Diag already performed: Boost and vacuum leaks searched for and eliminated with pressure and smoke tests. Valves and injectors were cleaned and injector seals replaced. Spark plugs were replaced. I have tried both tunes and reset the ECU/ fuel trims for them multiple times. MAF connector was replaced. Reseated and checked on almost all electrical connectors in the engine bay. Purge solenoid valve was tested and confirmed working. Wastegate vacuum lines and MAC valve were checked. Oil cap seal was checked.

Datalogs: The CSVs are also uploaded

Idle: https://datazap.me/u/buckwdav/idle?log=0&data=3-15

2nd gear: https://datazap.me/u/buckwdav/2nd-gear?log=0&data=3-15

3rd gear: https://datazap.me/u/buckwdav/3rd-gear?log=0&data=3-15

Cruising and upshifting: https://datazap.me/u/buckwdav/cruising-and-upshifting-2-3-4?log=0&data=3-15
 

Attachments

I think it will be impossible to fix/rule out anything until you replace the O2 sensor. That bad heater cell can be the cause of your issue.
 
Mfinlay04 said:
I think it will be impossible to fix/rule out anything until you replace the O2 sensor. That bad heater cell can be the cause of your issue.
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Fair enough, I figured since that was the secondary it didn't matter that much besides emissions codes. I have replacement primary and secondary O2 sensors on the way, they should be here Friday so I can update if that helps this weekend.
 
David Buckwalter said:
Fair enough, I figured since that was the secondary it didn't matter that much besides emissions codes. I have replacement primary and secondary O2 sensors on the way, they should be here Friday so I can update if that helps this weekend.
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If you have a plug, just pull the second O2 sensor and see if it makes a difference. The second sensor in theory calibrates the primary sensor.
 
Unplugging the rear O2 had no effect, I should have the new sensors coming in tomorrow I will update if that helps.
 
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Mfinlay04 said:
If you have a plug, just pull the second O2 sensor and see if it makes a difference. The second sensor in theory calibrates the primary sensor.
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Update: I have now replaced both O2 sensors with brand new units from NTK and clean both my map and maf sensors. Neither change made had any effect on the symptoms my car has been exhibiting. Really not sure what could be going on here it's very strange still seeing crazy fuel trims and an AFR that seems to have a mind of its own
 
You have a leak somewhere in letting air in
 
Easter Bunny said:
You have a leak somewhere in letting air in
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Any tips as to where I should look? I have smoke tested the whole intake and intercooler tract at around 20 psi and fixed everything that was leaking which was one one clamp on the intake for the tube going to the valve cover. I have tested multiple times since and found absolutely nothing. All seals are in good condition and the only sort of leaks of any kind I have at the moment is a small oil seep above the water pump on the timing cover and the back side of my valve cover has an oil seep. I have put like 15 hours into diagnosing this, this week and am really not sure what I could be missing
 
Start squeezing all the rubber hoses a lot of times there is a crack that won't leak until the engine moves under load
 
Oh good idea I didn't think of that. I'll try and give that a shot tomorrow. I also wonder if there may be a check valve besides the PVC I am missing checking or something like that aswell
 
One thing I did notice which is weird is that when I am idling the AFR becomes erratic and goes lean if I hit the brake. Should I investigate possible leaks or issues with the brake booster?
 
Yes, plug the connection point at the engine and see if your issues improve.

Edit: GP9A43490A is the part number for the hose with built in check valve, which is regrettably discontinued by Mazda.
 
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SyntheticAtmosphere said:
Yes, plug the connection point at the engine and see if your issues improve.

Edit: GP9A43490A is the part number for the hose with built in check valve, which is regrettably discontinued by Mazda.
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I will try that out tonight, I will clamp the line to the brake booster and see if that solves the issue entirely if it does I'll be thrilled but I can't imagine a brake booster will be cheap or easy to replace.

As far as that check valve goes, any chance you can find a picture of it? I looked up the part number but I'm not seeing any pictures of it I'm pretty handy and an engineer by trade, I can probably come up with a replacement check valve off a different car that is an equivalent or something like that if need be. Just need to know what I'm looking for and where it's located in the engine bay.
 
I dont have a picture of the valve, its built into the hose. I would block @ the intake manifold first. If you symptoms improve, the hose, check valve or booster are the issue. If no change, hose/check valve/booster are not your issue.
 
Like it's literally in the hose you can't see it
 
Oh okay, I don't have time tonight but I will check it out tomorrow or some other time this week, is this just a normal check valve? If I find it and cut it out of the hose do you think I can just replace it with a universal?
 
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