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Hello!
I currently own a 1996 Toyota Corolla with a 1.8L 7A-FE engine. Car is essentially bone stock. I inherited this car from my parents, who left it without service for 8 years (5 of those it was not driven). I’ve taken over and done all the maintenance myself (mostly!). Still runs great, feel wise. I’ve been driving around with a Bluetooth OBD II scanner lately, hooked up to my Android phone running Torque and displaying information. And I’ve noticed some very strange behavior.
At idle, my long term fuel trim reads 7.8%. Hitting the pedal makes the fuel trim go down to 4, and pushing it more and more makes it rise. Flooring it gives about 8.6%. Letting go drops it to 1.6 then it rises to 7.8 immediately. I tested the same scanner and phone on my father’s Miata, and his long term fuel trim is a consistent 0% with slight variations up and down.
Another thing is throttle position. At idle, it says that my throttle body is 10% open. Flooring it makes it go to 77% or so. Tried it on the Miata, 0 to 99.2% so I know my scanner is working. I checked the throttle cable, it’s perfect. Barely loose, just enough as to not pull at the throttle body. Car idles fine at 650-800 RPM at all times, even with AC on. Normal idle without AC is around 700.
My fuel mileage in the city has been pretty bad too. Last fillup netted 19 MPG, but with highway driving, it’s a lot better. Here is a list of maintenance I have performed in the last few months:
- Engine oil and filter (10W-30 as far as I remember, did wonders to stop the car from burning oil. Still full after 1000 miles) plus some Lucas Oil Stabilizer
- PCV valve
- Tires about a year and a half ago (also re-inflated to 32 recently)
- Rotated tires a few months ago
- Air filter (OEM Toyota)
- Distributor cap and rotor
- Spark plug wires (Import Direct, NGK was out of stock)
- Spark plugs (NGK, gapped by me)
- Front oxygen sensor (OEM Denso)
- Fuel filter
- Front brake rotors and pads with full bleeding
- Power steering flush
- Coolant flush
- Transmission flush and filter
I also ran some Seafoam through the gas tank a few times. Doubt that made much of a difference, but it keeps the CEL off on my father’s Miata (bad pre-cat).
Today I’ll go for a good drive and check short term fuel trim, as I’ve read today that it’s more important than long term.
I’m not sure what it is, but I think it’s either clogged fuel injectors or a vacuum leak somewhere. Any advice would be much appreciated!
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Drove today, stopping once in a while to check on the fuel injectors not spraying. They’re dry. Stupid gaskets, returned a pack for being defective. Fuel trim wasn’t good, at one point hitting 14% in the city. After my trip, I decided to test the car how you asked me to. When fully warm and after 15 minutes of driving, I put it into park. Opened the hood, watched my phone for short term fuel trim, and saw that it was at 0-8%, mostly staying in the 4-8 range. When keeping it at 2,500 RPM, the STFT drops to +/-3, constantly switching between the two, which is normal. So it’s a vacuum leak. I can hear whistling, but I’m not sure if it’s a vacuum leak or just a squeaky belt. I can’t figured the freaking thing out.
Also, when driving and accelerating up to about 30 MPH, fuel trim does get high. When going a constant speed on the freeway, it’s fine. Sound like a vacuum leak to you? I don’t know, but I’m thoroughly frustrated.
Lol it all sounds normal. Ur chasing a problem that doesn’t exist. Unplug a small hose somewhere to induce a vacuum leak then see what a real leak does to ur numbers.
Dude. First of all, watching STFT is stupid. Hitting 17% for a milisecond is irrelevant (and your scan tool cant even sample as fast as stft changes.)
Look at the average – aka LTFT.
Secondly. U think u have a vac leak? Do a propane enrichment test. And induce a small vac leak to see what a vac leak really does to fuel trims.
thirdly. Take advice like above comment and “mechanics” with a grain of salt. Trust hard evidence and published information.
The thing I sent u specificall says +- 20% is normal.
[quote=”crypkilla” post=62316]Dude. First of all, watching STFT is stupid. Hitting 17% for a milisecond is irrelevant (and your scan tool cant even sample as fast as stft changes.)
Look at the average – aka LTFT.
Secondly. U think u have a vac leak? Do a propane enrichment test. And induce a small vac leak to see what a vac leak really does to fuel trims.
thirdly. Take advice like above comment and “mechanics” with a grain of salt. Trust hard evidence and published information.
The thing I sent u specificall says +- 20% is normal.[/quote]
Hey… you… stop being so right. Yeah I guess I’m stressing over nothing. I’ll do the propane enrichment this weekend most likely. Till then, I’ll watch long term fuel trim which is a bit wonky. Might reset the ECU and start fresh, because the first thing I taught it is revving high (checking for fuel injector leaks).
And engine is absolutely unmodified. Even has an OEM Toyota air filter in it.
I think this will be the last update. Cleaning the throttle body and fuel injectors helped the issue, but didn’t fix it. However, I decided to just drop $17 and get an intake manifold gasket kit. Replaced the intake manifold air chamber gasket, tightened it really well (it was a bit thicker than the original), and noticed that the pipe attached to the bottom of the EGR vacuum modulator valve was very loose. Replaced it with a new hose I had laying around and put tiny clamps on there to keep it firmly in place. Afterwards, I took it for two and a half hours of driving. The fuel trim stayed solid between +/-3 on the highway, occasionally going to +/-4. Rarely did it even reach +/-6.2. Even in rough uphill stop and go driving through a speed bump laden area, it did not go above 7.2%. Went really low only once, when I let go of the accelerator and braked and put it into park, spiked to -14% but didn’t go that low again (probably the IAC opening). All and all, I’m very happy. It’s not only acceptable, but very good.
Also, 26.5 MPG last fill up after a lot of idling and revving to test my various repairs. And a good amount of fuel lost to spilling (my fuel injector gasket broke and sprayed a substantial amount of fuel). I consider this a success! I wanted to thank all of you for helping and weighing in! It’s very much appreciated.
hurray! and congratulations. this has been very educational. i’m now motivated to go outside and diagnose poor fuel econ on my 97 Ford Escort! maybe i’ll start a thread about it! thanks for the thorough and frequent updates!
[quote=”crypkilla” post=62989]hurray! and congratulations. this has been very educational. i’m now motivated to go outside and diagnose poor fuel econ on my 97 Ford Escort! maybe i’ll start a thread about it! thanks for the thorough and frequent updates![/quote]
It’s a pleasure providing these updates. Someone might find them useful. That hose below the EGR vacuum modulator valve is really inaccessible, many wouldn’t think to look there. Glad I noticed it.
And I’d suggest it! Though frustrating, it’s been a lot of fun. Plus the car will last a good while longer with new gaskets and a thorough cleaning inside the intake manifold.
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