Lightweight flywheel throwing off A/F ratio ?

[debrelezebre]

So I have a 2008 Spec B and the previous owner installed a lightweight flywheel with a stage 1 exedy clutch and pressure plate. The car has never been tuned or anything. Couple months ago, I felt the engine was jumping up and down in rpms whenever I press the throttle and just hold it at the same position. Now I feel it when I'm driving as well. Sometimes the engine just shuts down for less than a second as if it just stalled mid acceleration and then it bucks and accelerates back again. Suddenly, I got a code for my upstream O2 sensor that it was faulty so I changed the sensor and it helped for a little bit but now it falls back to the same issue as before. I've been digging my brain for quite a while to try and figure out what it could be and my conclusion keeps coming back to the flywheel being lighter than supposed to be and the ECU is freaking out trying to add fuel and air but can never reach a certain ratio as it isn't tuned. My current mpg average is 14mpg which is terrible and I have not been going into boost because those damn gas prices be too high at the moment If someone can chime in and let me know if indeed a lightweight flywheel does reduce mpgs, please let me know it would be greatly appreciated. As I said, I've been picking my brain for quite some time anything can help. Thanks in advance.

 

PS : the car is still bone stock except for that mod.

Edited April 17, 2022 by debrelezebre

[Bobsyouruncl]

coils, plugs?

[boxkita]

why not get it tuned? the 14mpg would be enough to suggest a tune to help you out. mid-20's is more common than not

[JoeInOregon]

The symptoms in regards to stallling and air fuel chasing while not under load, are exactly what my car does after a battery disconnect and needed a throttle body relearn.

 

No idea if this is at all related to your issue but what you are describing sounds awfully familiar

 

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Edited April 18, 2022 by JoeInOregon

[JoeInOregon]

The symptoms in regards to stallling and air fuel chasing while not under load, are exactly what my car does after a battery disconnect and needed a throttle body relearn.

 

No idea if this is at all related to your issue but what you are describing sounds awfully familiar

 

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Also worth noting I have a stage 1 clutch and WRX aluminum single mass flywheel, significantly lighter than OEM and I have no issue , but do require a throttle body relearn Every time I disconnect the battery

 

Stage one OTS tune

 

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[debrelezebre]

coils, plugs?

Brand new plugs and coils were tested, they were previously replaced and are still good

[debrelezebre]

why not get it tuned? the 14mpg would be enough to suggest a tune to help you out. mid-20's is more common than not

My plan for the car is to keep it as stock as possible but since the flywheel and clutch are already replaced, i don't really plan on replacing them to OEM which would cost me around 900$ total and the closest subaru tuner around here is abt 3 hours away from me so im holding off on that for the moment.

[debrelezebre]

The symptoms in regards to stallling and air fuel chasing while not under load, are exactly what my car does after a battery disconnect and needed a throttle body relearn.

 

No idea if this is at all related to your issue but what you are describing sounds awfully familiar

 

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Yea I'm familiar with the relearn, I did 2 times already just to see if it would help in any way possible but it comes back to the same issues

[Bobsyouruncl]

You may have fueling issues. The old sensor may have been giving bad readings before it failed having degraded and now that you have a brand new one it may read different than the previous?

[JoeInOregon]

Yea I'm familiar with the relearn, I did 2 times already just to see if it would help in any way possible but it comes back to the same issues

Ya as others have stated probably a fueling issue , caused by what I can't say.

 

What I'm pretty sure of is if I'm my 177k mile stock car tolerated the lighter single mass flywheel your car should as well.

 

I also didn't mention but I ran the same setup on bone stock no tune before I flashed and didn't have any issue either

 

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[Fabsx]

Clean your maf sensor, relearn ecu.

 

Might be worth checking for vacuum leaks as well. Especially around your tmic(splitting endtanks) and tmic to throttle (known to leak given your mileage)

 

 

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[debrelezebre]

Thank you all for the insight it really helps me have a plan on what to diagnose. I’ve spoken to many of my mechanics friends and most of them are stating the same thing, fuelling issue. Most of them think its my throttle body thats going bad since the TPS and the IAC valve are integrated within the TB. My next step is pulling spark plugs to check if they have a lot of dark buildup which would mean theres too much fuel going in. I will post an update whenever I have the results for that. And to answer Fabsx, the MAF is cleaned and good, that was my initial thought to the whole issue im having so i replaced with a known good MAF and problem was still present. I will definitely check for vacuum as well.

[debrelezebre]

Hello again, update on the car. I replace my throttle body with a remanufactured OEM one and now the car is not stalling anymore so thats one issue out of the way but thats all that it did, im still having the same hesitation issue. I took a video of it to show what im talking about. You’ll see theres a little delay between when i press the pedal and when the rpms start rising. When i press the gas you hear the engine making a noise as if it wants to stall and reducing rpms but it jumps back up. Here is the link.

 

08 Subaru Legacy GT Spec.B throttle delayed

 

 

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[Max Capacity]

Yeah, I'm thinking a smoke test of the vacuum system is next on your to do list.

 

Most of us have zip tied all the vacuum hoses on our cars. Here's an example.

 

 

[Max Capacity]

Hint, the fix for that yellow TPMS light is a piece of black electrical tape. I've had that in place for over 6 years now.

[debrelezebre]

Hint, the fix for that yellow TPMS light is a piece of black electrical tape. I've had that in place for over 6 years now.

 

 

Dully noted [emoji23] i will zip tie the vaccums when i have a chance and let you know if it does anything

 

 

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[Max Capacity]

Make sure you do a boost leak test or smoke test.

 

If you have a air compressor, take the hose off the air filter box, plug that with a pint size paint can. Pull the small hose off the blow off vale, give that hose a short blast of air, put your thumb over the hose, listen for leaks. You don't need much air in that hose.

[debrelezebre]

Hey all i have another update on the car. As you guys were mentioning to check for intake leaks and vacuum leaks, i checked every single hose and line and nothing was leaking. I also checked the intercooler and nothing is leaking from there either. I zip tied all vacuum and pcv lines. I ran like that for some time and nothing has changed. So at that point i did a fuel pressure test and my gauge was giving 32 psi at idle. When i would press on the gas, it would unstably jump up to almost 50 psi and then drop back down to 32. Im thinking my next step is to replace the fuel pump but before i do that i wanted to know if i should replace the fuel pressure regulators first (one on pressure line one on return line). Would this be a symptom of a bad regulator or fuel pump? I also wanted to know if there is a fuel filter that i need to change because i dont see any anywhere. TIA!

 

 

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