2025-05-1 : The Issue
This is usually something that always pops up by the end of the year for 1zz corolla. You’re due for an inspection, then the ambiguous lean code comes up and you find your long term fuel trims at positive 30%. I’m not going to pretend to have all the answers, because it could be anything, and 1zz engine that run lean is as natural as birds flying. It’s common and I suppose most old 8th gen corolla probably don’t have updated registration stickers due to this. So, let’s just make a huge check list of what I tried and hopefully this can help your situation out.
What Mods I did:
- 7th celica plastic manifold
- 9th gen ebay short ram
- tried to clean up the evap and vacuum rat nest
- took head off to redo piston rings
Efforts to lower LTFT:
- fuel filter
- fuel sock
- fuel pump (Chinese tester)
- plugs
- matrix blue injectors (3 years ago)
- matrix coil over plugs (3 years ago)
- stock maf housing with short ram
- switched the EVAP lines
Fuel Limit, too much Air, EVAP
My diagnosis for fuel lacks using the fuel pressure gauge, because I wanted to change the fuel filter and sock, also had an extra pump laying around. Furthermore, I didn’t want to spend money to make a fuel pressure gauge hook up port. The missing wildcard here is I have no way to measure fuel pressure now and I have to finish up my civic, before I add more ‘eyes’ for my corolla. Later I want some data logging and basic gauges.
The ‘too much air’ theory is plausable, since I upgraded to the celica plastic manifold and throttle body. I also can’t seem to use my stock air box. I had it setup before, but can’t seem to replicate what I did before. So, I have a 9th gen corolla short ram, but I suspect it’s slightly too large for the maf housing. There’s a lot of things in this area that could throw off the air/fuel mixture. There’s extra Celica specific vacuum ports, a intake air temp valve that’s not too healthy, the short ram (corolla ecu hates these), and numerous gaskets.
The EVAP confusing is another factor. When I tried to get all the EVAP solenoids to one side of the engine bay and mounted on actual brackets, the corolla rebelled. I became confused where the input and output of the evap canister was.
Possible Fixes
Celica 7th gen ecu Swap
Disadvantages:
- Downtime for my daily
- Wiring complexity
- Inspections: You have a celica ecu in your corolla?
Advantages:
- Tuned for Intake manifold, TB
- More aggressive tune
- Could be less strict with short ram intakes
9th gen Intake Box
Disadvantages:
- Maf housing might not be compatable with 8th gen ecu
- May not fit engine bay perfect
- More restrictive
Advantages:
- Should be easy and cheap
- Should reduce air and lower LTFT’s
- Can use performance filters like Apexi
- OEM looks
- Road trip to California?
2zz Injector Swap
Disadvantages:
- at least $60 used
- ecu not specifically tuned for this
Advantages:
- ecu can change trims to accommodate increases flow
- LTFT should drop within -10% to 10% due to more fuel
- Many vouch for the increased performance running 2zz injectors in their 1zz
- Great base for later turbo and standalone tuning
2025 - 05 - 04 : Some Progress
To fit in engine bay...
I had to cut the bottom off. The Celica 7th gen throttle body is wider than the oem 8th gen one, so the air box has to be moved and there's not enough room for the air box unless it's at the stock location. I was able to use the 9th gen ebay intake pipe, but no have a vacuum port I need to cap off.
Unfortunatly...
This did not completely elimate the positive LTFT. It's around 15% now at idle. So, one of the above ideals need to be implemented unless I find a vacuum leak. Also, the O2 sensor is not Denso and still slightly lazy. If the 2zz injectors can finish this off, then we can get this car inspected.
2025-05-06 : Wait,Before 2zz injectors
Before we buy used 2zz injectors to richen the mixture, we need to try some basic maintenance to ensure we have no vacuum leaks. Since our smoke machine died, because it’s made in China, let’s just redo all rubber gaskets on intake side. The gaskets are cheap on rockauto. Let block of some unneeded ports too.
I believe this will be easy process. I would like to go to junkyard to find the proper throttle cable bracket and may try to route the cable from the left side if possible.
The red vacuum host is for the evap air intake and I went back to stock air box. Now there’s a bolt and clamp blocking it, but maybe the nipples I get will be better.