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

[college man]

Try searching for vacuum leaks.

[Dima]

Was just going to do that today. Quick question: Eric suggest carb cleaner. Is brake cleaner also acceptable?

[exceptionalchris]

I would use b12 not brake cleaner. you can get a can of it a any auto parts store like 8 bucks.

[Dima]

Sorry, but what’s come b12?

[exceptionalchris]

Sorry take that “come” out. type to fast. its call b12 this a blue a red bottle. thats what i would use

[college man]

CRC has MAF cleaner. Brake clean will work.

[Dima]

Thanks everyone. I think I found the vacuum leak. The ventilation hose was cracked, pretty big crack too. Bought the only hose my local auto shop had that has an identical inner diameter, but it was too thick to clamp down even with strong clamps. Went to a Toyota dealership and ordered the vacuum hose there. I’ll install it, reset the ECU, and report back!

[Dima]

Alright, so I tested for vacuum leaks. Couldn’t find a thing. The only cracked hose I found I replaced, a ventilation hose. After an ECU reset, fuel trim still bad. Noticed that my throttle body is stuck open at 10% at all times, even when the car is off. It can be shut by hand, but goes back to 10%. Could this be a stuck IAC valve? Or is it a faulty coolant temp sensor? Just tested the intake air temp sensor, it’s fine. Don’t think the 96 Corolla has either a MAF or a MAP. This issue is really starting to frustrate me. Why is my throttle body open??

[college man]

The throttle body does need to be open a little to get air
into the engine. does the car idle at the correct rpm.

[Dima]

My father’s Miata has the throttle body fully closed at idle. Shouldn’t the idle air control valve be providing the air at idle, not the throttle body?

The car idles fine, around 700-800 RPM. Stock value is 800 according to my manual.

Could it be the TPS keeping it open?

[college man]

[quote=”Haloruler64″ post=60898]My father’s Miata has the throttle body fully closed at idle. Shouldn’t the idle air control valve be providing the air at idle, not the throttle body?

The car idles fine, around 700-800 RPM. Stock value is 800 according to my manual.

Could it be the TPS keeping it open?[/quote]

The IAC is used for when accessories are turned on to help
keep the target idle. ac ect.. The throttle plate may looked
closed. but its cracked. Is your throttle cable to tight?
If you disconnect the throttle linkage does everything move freely?
Is the throttle body clean?

[Dima]

Sorry, but I’m a little confused. All the things I read state that the IAC controls idle, because keeping the throttle body cracked would be inefficient. http://www.ehow.com/info_12225663_idle-air-control-valve-fuel-system.html

My throttle cable is actually quite loose. When I fix this issue, I will tighten it. But it has quite a bit of slack in it, so I know it isn’t keeping the TB open.

I will clean the throttle body this weekend. But dirt isn’t the issue. I can push the throttle body closed really easily. But there is a piece right below the part you can twist to control the TB that pushes it back open. The TPS, which as far as I know, is computer controlled. I was told it could be offset after time and I should adjust it? Because at the moment, a part is keeping the TB cracked even when the engine is off and I think it might be the TPS.

[college man]

see if this helps. according to this. your fuel trim
numbers should be at zero% but can be + or minus 10%

http://www.justanswer.com/toyota/2ye7r-2000-toyota-corolla-hesatation-problem.html

[Dima]

Yeah, I get as much as 18% higher in my short term. It’s abnormal. My O2 sensor is brand new, the 96 Corolla doesn’t have a MAF (uses an IAT sensor, which I confirmed works last night). No error codes from the ECU, so no problems it can find.

Cleaning of the TB, IAC, and intake manifold will happen this weekend.

I suspect it’s the TPS that’s become unadjusted and is pushing the TB open. But I can’t be sure.

Ninja edit: No hesitation problems either. Car is pretty speedy.

[college man]

The positive 18% means your adding fuel. usually unmetered air.
check the tps and keep us posted.

[Author]

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