Fuel trim that gets better with higher RPMs points to a vacuum leak. Clamp off the vacuum hose going to your brake booster and observe your fuel trims. It is often overlooked.

 

I did try clamping off the brake booster but to no avail. One interesting thing I found was when I open the air filter housing while the car is running, I can hear something that sound like a hum with a tonal quality like a horn. Not really loud but its obvious. When I shut the air filter compartment back, I can still hear it but only faintly. Is this normal?

 

Humming is harmonic beat from two different air-flow streams making two different frequencies of sound.

Smoke test is bust because intake is always under vacuum (unless you have turbo). Any smoke you put into engine will never leave out vacuum leaks because engine is sucking air IN through leak.

Proper way to find intake vacuum leak is to plug exhaust with potato, banana or cork. Then pressurise intake to 2-3 psi so there is positive presssure in intake. Then squirt soapy water around every joint, junction, pipe, hose-clamp after MAF. Don't forget every injector O-ring and intake-manifold gasket. Especially on back side underneath intake-manifold. Need to crawl under car with strong flashlight with mirror-on-stick to squirt and inspect that area.

Without measurement numbers, MAF cannot confirmed as bad. Vacuum-leak will throw off MAF readings because a lot of air the engine's sucking in isn't being measured by MAF. Replacing perfectly working MAF (or any other part) with brand-new perfectly working part will change nothing.

 

* Fixed the misfire code and the cat code. *
Do you recall the cat code and how did you fix it.


Can you post the MAF in g/s along with 02v/ engine load/fuel trims at idle/ 2500/3000?


Did mechanic provide any diagnostic info that led them to believe changing MAF and upstream O2 would fix the lean condition?
Personally i believe he owes you, at least for the misdiagnoses.

 

*If I push the rpms beyond to 3000+, the LT fuel trim goes back up to around 35…sometimes even 40. * So fuel trim improves around 2500 rpms then gets worse again around 3000 rpms. *

Sorry I missed this on the first read. Looks like you have restricted injectors. Fuel pumps aren't typically an issue on these Corollas.

However, I would check two things before replacing injectors:

1. MAF: Check your MAF values at 2500 and idle. They should be:

Condition . Air flow rate (gm/s)
Ignition switch ON (do not start engine) . 0 gm/s

Idling : 4 to 6 gm/s
Running without load (2,500 rpm) : 13 to 20 gm/s
From idling to quickly accelerating : Air flow rate fluctuates

2. o2 Sensor: Make sure your upstream o2 sensor flips between rich and lean at .8 and .1 volts on live data

 

Fixed the misfire code with new sparkplugs. They were all pretty bad. Also, the sparkplug wells were full of old black oil which I think was also causing areas for the spark to find a different path so I replaced the valve cover gasket. Not sure if it is what fixed the catalytic converter code but I had read to run a couple tanks of high octane fuel to clear up a cat code. That cat code has gone away but I have no way to prove the high octane fuel is what did it.

Edited for clarity

 

Thanks Doctorbee, I will make a note of the MAF readings tonight. Could I test using a single new injector before replacing all four? Would a single injector provide any improvement that could determine whether or not to purchase the rest?

 

That is possible. I'm not sure how much of a difference you would notice. If you're tight on cash I would try used DENSO injectors. This seller seems highly rated:

https://www.ebay.com/itm/Toyota-Cor...:KJoAAOSwP~tW44sJ:sc:USPSPriority!92649!US!-1&LH_ItemCondition=4&frcectupt=true

 

I'd disagree about the fuel pump not being an issue, most of the 8th gens with high mileage that I've come across have had a failing OE Denso fuel pump, and replacement with a new Denso pump was an easy fix.

I haven't seen a fuel pump be the sole reason for running lean or a P0171 code, but it is possible for a weak pump and dirty/clogged injector to combine and be a limiting factor to getting enough fuel to one cylinder, at least enough to throw a code, even if it's no happening consistently or just under heavy acceleration.

Dependiing on mileage, a new fuel pump can be worth it anyway, just for the restoration in performance/acceleration, plus you can keep the old one as a spare.


Usually the P0171 is an air/vaccum leak, so you really need to check and double check and triple check ALL the hoses going to the throttle body, air cleaner, etc. Sometimes there's a cut in the rubber or even just cracks in the end of a hose that allow air in. Just like with the intake manifold gasket, sometimes the paper thin factory gasket for the throttle body needs to be replaced with a newer one to give an airtight seal.

You might also make sure you're running OE parts as necessary. As in, only Denso or NGK Iridium spark plugs. OE Toyota PCV valve. Maybe even Toyota or Denso air filter in case a cheapo brand one is overly restrictive. Sometimes aftermarket parts for fuel/ignition/etc. give weird problems and codes that go away when the OE stuff is used instead.

 

Thank you everyone for your input. I've not yet been able to do more testing but hopefully I will get to this weekend.

Sent from my Moto G (4) using Tapatalk

 

Ok. I was able to do some testing.

At idle around 750rpm it was averaging 2.3 g/sec

At 2500rpm it was averaging 7.3 g/sec

At 3000rpm it averaged 9.2 g/sec

These figures are in grams per second based on the car scanner app I am using.

I couldn't seem to get a reading with just the key turned on without the car running.

The O2 sensor voltage seemed to be switching high and low at the rate it should.

What are your thoughts on these figures?

Sent from my Moto G (4) using Tapatalk

 

MAF reading at 3000 should be at least 20 g/s.


One interesting thing I found was when I open the air filter housing while the car is running, I can hear something that sound like a hum with a tonal quality like a horn. Not really loud but its obvious. When I shut the air filter compartment back, I can still hear it but only faintly. Is this normal?"



Low MAF reading can be from restricted intake.
Intake to/from filter clear?


Restricted exhaust can also cause low MAF.
Back pressure test?


Finally MAF itself can be bad.

 

There's an error in the FSM - MAF should be 13-20 g/se at 2,500 RPM.

with fuel trims so high it is unlikely to be a restricted exhaust.

Make sure your air filter is clean.

if it is then you probably need to replace your MAF.

 

- clean the MAF
- check the air filter
- spray water all over the engine to check for vac leaks

if you still have the code, move on to getting some FP readings

 

I had the same happened to me, I bought refurbished injectors and clean where the injectors sit, and installed everything back and turned car on and check engine light was gone, before all that I changed fuel pump, intake gasket, air filter, new vacuum hose, clean the throttle body, changed the maf sensor and did smoke test, no leak. If you haven't change filter or injector, I would change injectors first.

Sent from my SM-G965U using Tapatalk

 

Purchased some remanufactured injectors which I hope to install this week. I'll let you all know what the results are.

Sent from my Moto G (4) using Tapatalk

 

Update: Installed some Ebay remanufactured injectors and the lean condition is fixed. LTFT hovers around 0 to -3 or +5 depending on how I'm driving. Unfortunately, now I'm getting the random misfire code: P0300. The car is also hard to start...the engine has to turn over about 10-15 times before starting. I think I've got a leaky injector or two. Anyone have a hack or trick to find which injector might be leaking?

I'm also nervous that the catalytic converter might be shot. I'm getting an average .8 volts on the downstream O2 sensor and the voltage fluctuates as I drive.

 

Check the following for p0300, The bolded bullets should be easy to check with a scan tool


Open or short in engine wire

Connector connection

Vacuum hose connection

Ignition system

Injector

Fuel pressure

Mass air flow sensor - what are your new MAF values at idle and 2500rpm?

Engine coolant temperature sensor
Compression pressure

Valve clearance

Valve timing

PCV hose connection
PCV hose
ECM

 

MAF flow-readings should match vacuum-level in intake-manifold at idle and 2500rpms.
Very simple test and very simple fix once you get those two numbers.

 

The car ran fine with no starting issues prior to the injectors being replaced. It only had the fuel trim issue being way to high but no misfires. I don't think the misfire is compression, valve clearances or anything that deep but I might be wrong. Seems like something directly related to the injectors. Would the ECU record a misfire on starting being that it takes so many turnovers to get it started? When starting, it kinda sputters to life (misfires) then it runs good. Am I way off in my thinking here?

 

Did you get flow-test charts with your "new" injectors?
Have you done a compression-test? What are the numbers?

 

Ask the seller to swap them outt for another set

 

Thing is, there's no definitive evidence original injectors were bad, they were never tested.
Then using refurbed untested injectors introduced yet another variable.

So at this point, you need to test either original injectors or new refurb injectors to determine what their specs are. If they (old or new injectors) are indeed good-as-new in flow-rate and pattern, then problem is else where. So much easier to test and measure parts to figure out what's good or bad, rather than just randomly throwing darts.

 

Here's example of how numbers will help pinpoint why MAF-flow readings are low. You want to compare MAF-flow with actual physical flow as indicated by intake-manifold vacuum (mechanics have been using this test for +100yrs).

1. MAF@idle/2500 (high) + manifold vacuum 20-24 in.Hg (high) = everything OK

2. MAF@idle/2500 (low) + manifold vacuum 15-19 in.Hg (low) = MAF OK, you have vacuum-leak, MAF only records 1/2 of actual flow, other 1/2 flow comes in through leak. This condition is supported by high fuel-trims to make up for air not measured by MAF

3. MAF@idle/2500 (low) + manifold vacuum 20-24 in.Hg (high) = MAF bad, high-vacuum means no leak, yet MAF is not showing high flow, replace MAF and repeat test measurements


Without intake-vacuum measurements, MAF readings are useless.