Positive Fuel Trim, Throttle Open At Idle

[Dima]

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!

Topic

[Dima]

New development. The thing keeping the throttle body open is called the throttle opener (if I remember correctly), according to the sticker beneath the hood describing the EGR system. So I suspect the EGR system is at fault. I switched out the round EGR valve (circled in red) with one from a Camry (identical), and behavior didn’t change. The only other part of the EGR system it could be is the solenoid (circled in blue), right? Because the throttle opener is connected only by one hose to the EGR system, so I don’t see how anything else could keep the throttle body open. Any ideas?

[college man]

The egr has nothing to do with the throttle plate opening
or closing. That solenoid is what tells the egr when to open
or close. The only thing that controls the throttle plate is
the throttle cable to the linkage.

[Dima]

Are you sure? Take a look at this image:

The throttle opener is pushed up against the part that moves the throttle plate, and it’s keeping the throttle plate open at 10.6%. When the car is off, the throttle opener can be pushed in and the plate can be closed. With the car on, with vacuum, it cannot be closed.

[college man]

If you pull the vac line to that plunger and cap it.
will the throttle plate close? You may need to test
that plunger with a hand held vacuum pump.

[Dima]

Unfortunately I don’t have a hand held vacuum pump. But I can say this. With the car off and the vacuum line undone, the plunger’s stock position is holding the throttle plate open. It feels like there is a spring inside, which holds it open but can be compressed when there is no vacuum, but pushes right back up.

[college man]

Try and suck on the end of the hose to create vacuum.
see if it moves. if not you may have found a vac leak.

[Dima]

I’ll try that right after work, thanks.

[Dima]

So I appear to be very wrong. The throttle opener depresses when the engine starts, keeping the throttle open at 10.6%. When the car is off, it pops up. So the throttle opener works fine. I sucked on the vacuum tube to it, it goes down by itself all the way just fine.

I read that some Toyotas have their throttles open at idle too. http://www.yotatech.com/f2/throttle-position-8-idle-normal-137497/

I took it to a shop and a mechanic just took a quick look. Both of us could hear a vacuum leak, but I still can’t find it. I think I’ll do a smoke test soon, blowing smoke into a vacuum line and seeing where it comes out. Because spraying brake cleaner is proving ineffective, as the leak might be in the EGR lines deep in the engine.

I also bought a new PCV grommet, just in case.

[college man]

keep us posted on your progress. 😉

[Dima]

Installed a new PCV grommet. Didn’t help, but I didn’t expect it to. Needed a new one anyway, old one was torn up. Still can’t find the vacuum leak.

Replaced the TPS just in case with a spare I found, no change either. Still hunting. Planning a full clean of the TB and intake manifold for tomorrow.

[Dima]

So I’m sure it isn’t the IAC. Took apart the IAC, throttle body, intake manifold, and fuel injectors for cleaning. Didn’t do it to solve the fuel trim issues, just as maintenance. Horrible mess inside the intake manifold. IAC is now cleaned out and seems to work fine, as far as I can tell. I think the only issue it could be is a vacuum leak now, but carb cleaner sprayed everywhere reveals nothing. I’ll find it eventually.

[college man]

Try using a spray bottle filled with water to search for vacuum
leaks. soak everything.

[Dima]

Would I be listening for an idle change or trying to stall the engine with water?

[college man]

Idle change. you will hear the water suck in.
Take it to about 8minutes.

[Dima]

Tried the water spraying trick around the intake manifold gasket and the air chamber gasket. Nothing.

Also took the car for a drive after cleaning out the inside of the intake manifold, throttle body, fuel injectors, and idle air control valve. Fuel trim is still erratic, but less so. It’s about -11 to +10. I don’t like it, but it’s better. Maybe a weak IAC? Clogged injectors? What do you think?

[Author]

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