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Discussion Starter · #1 · (Edited)
I have done a search through the forums but I'm not finding anything that covers my issue. I picked up an '02 GT MT recently for a good deal and have been fixing it up (My son picked up a '99 AT recently, 300k miles and he loves it!)

From the moment I picked up the '02, it has had some hesitation. I will get random P017X P030X codes but the cylinder is not consistent. I also get P0420 codes sometimes. If I drive gently, it runs just fine. I have done all of the following:
1. Replaced plugs with new NGK Iridium
2. Tried 5 different OEM MAF sensors (cleaning each one before trying)
3. Installed new air filter
4. Checked for loose intake plumbing (have not done smoke test yet)
5. Removed cat and soaked in detergent, cleaned O2 sensors with Elec Contact cleaner.
6. Picked up 6 additional used coils and tried moving them around but code doesn't follow.
7. Run seafoam through engine oil and fuel.
8. Fuel Injectors have been cleaned and tested and all their seals replaced.

I'm kind of lost at this point, anybody have any more ideas ?

update 4/25/2022
In addition to the items above, I have now done the following:
9. Run engine flush, replaced oil with fresh oil, run 2nd flush and replaced oil and filter.
10. Replaced front O2 sensor with new denso unit.
11. Cleaned up back O2 sensor.
Car does not appear to be throwing CEL but still has slight hesitation

Wheel Tire Car Vehicle Automotive tire
 

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Discussion Starter · #5 ·
Look for a leaky intake manifold gasket (common issue on early GT) or other vacuum leak. You were on the right track with the MAF. Most hesitation as described is MAF reading related. Any unmetered airflow will cause issues.
sigh I suppose I'll try and get a smoke test. thanks for the suggestion.
 

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Discussion Starter · #11 ·
Noting to do with your issue..

But the rims..

All the 86’s I thought had 5x114 pcd where celica are 5x100

don’t doubt you though. Just odd
The guy I got them from assured me that they were 2017/18 Toyota 86 wheels. The back of the Toyota center caps actually say Subaru (because of course the BRZ is the sister car)
 

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Discussion Starter · #14 ·
That's odd - I had JDM 16" Toyota 86 wheels when I had my FR-S. They were definitely 5x100, because I thought about keeping them for my '00 Celica when I was selling the FR-S. The bolt pattern fit fine, the offset is the issue and I didn't have spacers. Incidentally my '02 Celica had spacers when I bought it 10mm F and 25mm R I believe. I gave/sold the wheels to my brother, but if he lived close by I'd probably stick them on just to see how they look/fit.

I did need to use 8mm spacers on the wheels because they hit the front calipers otherwise. I also removed the little inserts that came in the wheels out so that I could use regular round end lug nuts. The bore is 1mm larger on the 86 wheels. I may put a spacer in there sometime. I have a tacoma that's had wheels with 6mm too large bore for years now with no issues so not too worried.
 

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Discussion Starter · #15 ·
Depending on the actual DOM, you've likely got an oil burner. P0420 is a common issue w/them fouling the cat. P0171 is a common CEL for MAF/vacuum issues (unmetered air entering). P03XX would misfires (which can be the result of P0171).

While it won't actually solve the cat problem, you can mask it by adding a spark plug defouler to the post-cat sensor.
Thanks for making me re-read my original post, I'm getting random P030X codes (cylinder misfire), not P017X codes.
Going to try the brake cleaner suggestion mxaustin suggested this morning.
 

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Discussion Starter · #17 ·
Oh yeah, I forgot about the lug nuts. I think the FR-S had a different thread/pitch than the Celica if I remember right, so I would've needed to buy new lug nuts as well. I did buy hub/bore spacers for the tiny difference, even though it wouldn't really matter once all the lug nuts are torqued properly. I do remember the hub popping out the center caps because of the offset when I did a test fit with no spacers.

Sorry for the off-topic discussion!
Initially I just used the lug nuts that came on the Celica but I have a whole garage full of crap from my countless car projects so I found some acorn nuts that had the correct thread pitch and worked well.
 

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Discussion Starter · #20 ·
Hesitation and random misfring at high speed are more likely caused by ignition.

Like someone posted, check the plug wells and clean out oil and contaminations. Oil will cause ground faults with the plugs therefore misfiring.

NGK plugs may not be compatible like DENSO plugs. Check gaps to make sure. Clean out contacts.

Check and test ignition cables for potential shorting to ground.

Off brand and counterfeit ignition coils will misfire intermittently.
The car runs smoothly at WOT, and at lor RPM, just not in the middle.
There is no oil pooling in the plug wells.
 

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Discussion Starter · #21 ·
Just an update, replaced the intake manifold gasket, no change.

Sprayed brake cleaner all over the place while it was idling and nothing changed the RPM.

The coils I have are all OEM and of course because the 7th gen is coil on plug, I don't have any ignition leads.
 

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Discussion Starter · #24 · (Edited)
I reset my codes today after doing an engine flush, followed by a clean oil rinse and another flush. Didn't solve the hesitation but half way through the second flush, had some good carbon come out of the exhaust! I also pulled all the 10 coils I had aside, separated the OEM Denso's and checked for the ones that had the closest resistances across all the terminals and installed them in back in the car.

When the code pops up again, I'll see if I can get the trim in the freeze frame.

To be clear, the the car does not hesitate on gentle throttle or at full throttle regardless of RPM. In other words, if I shift early so my revs are still quite low and put my foot flat, she pulls smoothly, she only start hesitating when I back off the throttle a little.
 

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Discussion Starter · #25 ·
Pulled my live data on the drive to my first appointment today. I believe the system I am using uses
Rectangle Slope Font Plot Line

the terms "short and long-term fuel replenishment storage" instead of "fuel trim". The interesting thing to me is that short and long term, bank one and long term bank 2 make sense but short term bank 2 is drastically higher (I think. It's far enough off to be distorting the graph).
What would that imply?
 

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Discussion Starter · #27 ·
Lol... Far as I know, we don't have a bank 2 for fuel trims.
I was thinking the same thing, clearly in reality we only have 1 bank, one exhaust header with 1 set of sensors but something is creating readings for "Bank 3"!

Short term fuel trims should usually show in low numbers, as they are just corrections from/to the long term trim. Long term trims should usually be less than 10. Anything > 20 should trip a CEL. Long terms in the 10-20 range may indicate a potential issue that is being corrected by the ECU.
In doing a little research yesterday evening, my best guess is that the pre-cat sensor may be bad so I ordered one of those. (Which was it's own learning experience! Denso lists the sensors for the specific car vehicle on their website but if you are willing to swap plugs, you can save a fortune on the sensor! Got a new Genuine Denso for $19, Advance Autoparts wanted $125!)
 

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Discussion Starter · #29 ·
I usually recommend the direct-fits. If you're going to splice in another sensor, I recommend making the splice in the cab where it will be dry.
I'm splicing in the exactly correct sensor (According to Denso), and yes, I will of course try to keep the wiring splice inside the car out of the elements.
A conveniently fitted plastic plug is just not worth $100 to me.
 

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Discussion Starter · #32 ·
Well, I replaced the preheat O2 sensor with a brand new denso and the problem persists. Back to thinking I'm going to have to get a smoke test done to look for for vacuum leaks.
 

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Discussion Starter · #34 ·
What did you use to clean the MAFs? MAF cleaner or something else?

Is the butterfly operating correctly in the OEM airbox? Air filter condition?

What brand fuel are using?
Cleaned the MAFs with electrical contact cleaner, have used it for 20 years. I now have 5x MAFs, switching between them makes no difference so I don't think it's the MAF sensor.

I have noticed several threads talking about the butterfly in the OEM airbox, even seen some pics but mine doesn't seem to have a butterfly or even a place where one looks to be missing.

Brand new air filter.

I will say this, I did another 50 miles in the car today and I'm pretty sure I can tell the difference that performing the 2 sequential engine treatments made. It's definitely seems to run smoother.
 

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Discussion Starter · #35 ·
Cleaned the MAFs with electrical contact cleaner, have used it for 20 years. I now have 5x MAFs, switching between them makes no difference so I don't think it's the MAF sensor.

I have noticed several threads talking about the butterfly in the OEM airbox, even seen some pics but mine doesn't seem to have a butterfly or even a place where one looks to be missing.

Brand new air filter.

I will say this, I did another 50 miles in the car today and I'm pretty sure I can tell the difference that performing the 2 sequential engine treatments made. It's definitely seems to run smoother.
What brand fuel are using?
I live in a big city in Texas, there are only mainstream gas stations around. The first tank I put in, I used premium but since then I've been using standard (87) from mostly Shell.
 

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Discussion Starter · #37 ·
The 3 wires from the Denso O2 sensors are about 5 inches long, so the splices have to be under the car.

You can solder the wires, but I used the splices and taped up with electrical tape. Have not had problems with those splices in years.
Just as a follow up to this comment, the wire that came with the Denso O2 sensor was plenty long enough to pass through the rubber seal so I could do the join inside the cabin. It also came with 4 crimp connectors and heat shrink tubing to do the job well. Having done this now, I would always do it this way in future if it saves me as much.
 

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Discussion Starter · #40 ·
The MAF may be clean and tested good, but it could still malfunction at certain speeds.

I had this problem before with my GTS. The MAF was cleaned and electrically tested good a few times but still showing the P171 code. I swapped in the same MAF from a Lexus, the car ran smooth again, code was clear. So it was the bad MAF.

Replaced with a DENSO MAF and the car has been good ever since.
I have 5 x Denso MAFs, car behaves the same with all of them.
 
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