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Discussion Starter #1
Hello everyone I'm new to the forum so don't mind me, my 2000 Toyota Corolla is giving me problems, I bought an obd reader and I saw in the beginning when I got the car it was running lean, got a new air flow sensor, properly gapped the spark plugs, it ran great ltft and stft were perfect for a few days, then it started running rich, like -18% ltft rich.
All I know is I might have a bad cat because both o2s match each other while driving, when idle the downstream reads 0.02 volts. I recently cleaned the old mass air flow because there was dirt on the wires, it got a little better but still lean, I'm not sure if it's a shitty after market air flow or something else please help! I'm not sure which sensor to believe
 

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If you're using an aftermarket MAF sensor, that's a mistake. Go back to oem and delicately clean the MAF wires. You have to use OEM or equivalent electrical parts. Vacuum leak would say +10%. If the fuel trim is -10% or something like that, means too much fuel is being dumped in or the maf sensor is screwed up. Could have bad injectors. Make sure your oxygen sensors are Denso and the MAF sensor is factory equivalent such as Denso and not some off brand aftermarket. Even Bosch would be a mistake in this application.

Fuel trims are usually based upon the oxygen sensors giving back data saying something isn't right. Positive fuel trims (+10%) can be caused by numerous things, especially a worn out oxygen sensor. However a negative fuel trim (-10%) is caused by a very tiny handful of things so should be easy to figure out fairly quickly.

MAF/Map sensor, Injectors, fuel pressure regulator (if the car has one), bad ignition coil, fuel pump (somehow causing too much fuel pressure), Valve lash, are pretty much the only things to cause a negative fuel trim. Oxygen sensors, when they fail, fail "lean", (+%) so they're telling the car to add fuel. It'd be exceptionally rare for an oxygen sensor to fail "rich" (-%).
 

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Discussion Starter #3
When I did clean the OEM it just caused my car to run excessively lean, Stihl trimmers were at about 22% combined, so far in a last-ditch attempt I tried cleaning the aftermarket one which is a duralast by the way and so far what I've got is that the numbers are perfect while driving down to the 0.8% range but when I idle it gets to the negative 18% range, originally before cleaning the oem one I just put it back in after buying the duralast one and the car started bogging horribly like barely getting past 20 miles per hour and multiple misfires, I bought another denso from RockAuto and it did the same thing, now that I cleaned the old one I can put it back in but it's just causing my car to run lean instead of rich
 

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Huge difference between the sensors it seems. Check the gasket around the MAF sensors when you install it, perhaps it has a massive air leak. Maybe swap maf sensor gasket or something. Loose wiring at the harness connector should be checked. Try wiggling it or adjacent wires to see if that causes the car to behave differently.
If you're 100% certain there are no leaks near the MAF sensor, but the OEM and Denso sensors both lean towards lean when idle, then I'd trust those sensors over the duralast sensor.

Disconnect the brake booster line and plug it up, see if the fuel trims drop. The vacuum hoses on these cars are installed with constant tension clamps that eventually fail over time. I suggest inspecting the brake booster vacuum line for cracks and if there aren't any, use some worm screw clamps and tighten the hose down that connects to the engine side. Then, with the hose still disconnected from the booster, plug the hole and see if the trims go down at idle. If they do, that means your brake booster has a vacuum leak.

I bet you do indeed have a massive vacuum leak. Just find all the vacuum hoses, inspect for cracks and if there aren't any, tighten them down with worm screw hose clamps.
 

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Discussion Starter #5
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I know the one hoses going to the engine is more frail then rubbery but to my knowledge it has no leaks or openings on it, I'll try what you recommended, also ive been using the old oem one after thouroughly cleaning it and now it's not bogging or anything and the fuel trims are slightly better (20% vs 25% before at idle) and the fuel trims combined while driving are around 12%-14% and it kinda goes to normal % when I barely press the gas and throttle position is at 13% (11% throttle is with no foot on the pedal), upstream o2 mainly stays low at idle and goes up to .7 volts 40% of the time, driving gets it to .7 - .8 volts and sometimes dips to .2, downstream stays low a majority of the time 0.025 and fluctuates from .7 to .2 while driving and sometimes starts steady at .7 ish, I'm thinking that it's a bad cat from running lean for however long it was before I got it because I get the p0420 ocassionaly and sometimes rarely I get evap code p0440
 

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how many lbs of air is the MAf reading at idle vs at 2k? Usually torque for android will output that info. Post that to see if the MAF is reading at least 2 grams per second at idle.
 

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Discussion Starter #7
Mad at idle was 2 grams per s, and at around 2000 is 9.5 per s, so good and bad news, I found a leak at the air intake hose while spraying carb cleaner and I sealed it as best as I could the clamping it harder and pushing it in more and the fuel trims looked fantastic (1.6 ltft, 7.8 stft) and it idled better, it stayed like this for around five minutes of driving buti pulled to my house and shut off the car and grabed something I came back out turned it back on and now it's stuck at 13% ltft at idle and lowers a bit while driving, now maf at idle is 1.7 grams per sec and 5grams per sec at 2000 rpm. Wth. It did this when I properly gapped the spark plugs and the fuel trim was even better bit it only lasted like 3 days
 

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Mad at idle was 2 grams per s, and at around 2000 is 9.5 per s, so good and bad news, I found a leak at the air intake hose while spraying carb cleaner and I sealed it as best as I could the clamping it harder and pushing it in more and the fuel trims looked fantastic (1.6 ltft, 7.8 stft) and it idled better, it stayed like this for around five minutes of driving buti pulled to my house and shut off the car and grabed something I came back out turned it back on and now it's stuck at 13% ltft at idle and lowers a bit while driving, now maf at idle is 1.7 grams per sec and 5grams per sec at 2000 rpm. Wth. It did this when I properly gapped the spark plugs and the fuel trim was even better bit it only lasted like 3 days
was the cause of the leak a torn hose or an actual loose clamp? Clamping a torn hose really wont solve the problem.

In addition, can you clarify the following:

1) Burns any oil between changes?
2) Any other CELs
3) Try a full reset of the system (at your own risk of course) by unplugging the battery and leaving it off for a while (preferably overnight) but even 15 minutes should be enough, drain the system by turning on the dipper.
 

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Discussion Starter #9
I'm guessing it was a loose clamp because I pushed it in more and tighten the clamp more and now when I spray there's no sign of a leak, the oil was in between the two dots about 2,000 miles ago and now it's right beside the bottom dot, I just filled it to the middle again today, this car has 137000 miles so I'm guessing that's normal, the only other cel I'm getting is a p0 420 and I'm guessing that's because it's been running lean for so long and it killed the cat (bottom O2 fluctuates like upstream while driving), originally before cleaning the mass airflow sensor it was at 25% ltft, I unhooked the battery for about 15 minutes and cleaned all the connections to the sensors with the correct spray, so far fuel trims look a little better especially at idle 4.7%ltft but when I press on the gas it gets up to 10% and combined with long-term and short-term it's sometimes gets to around 15% while driving and acceleration from a stop. it's a whole lot better than what it was but I still think there might be something going on
 

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Discussion Starter #10
I should also mention that at idle when the long-term fuel trim is at 4.7% sometimes the short-term fuel trim will jump to 9.4% but then go right back
 

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Assuming you fixed all vacuum leaks, cleaned up all your electrical grounds, and replaced your oxygen sensors, if slight variations in pressing the throttle causes the short term fuel trims to move up, that could mean a worn out throttle body and not a lot you can do besides have it rebored. One way to verify this condition is if you're stationary and you warm up the engine to at least 180f and leave it there for at least 15 minutes, check the air intake temperature and see if it matches the outside or if it's hotter like the engine bay. Typically try to get the air intake temps to like 120f or so. Note the fuel trims, then take it for a drive, the air intake temperature should drop to ambient temps, monitor the fuel trims and if they go up (adding air) that means it's definitely the throttle body.

Here is what happens: On a hot or cold engine, if the vehicle is stationary, everything warms up to the same temps, so the butterfly valve and throttle body expand evenly. However, if the engine is hot and you start moving through the air, so the engine bay cools down, and you get a fresh cold air charge into the intake the throttle body is still hot from the engine, but the air intake charge is cool and it hits that butterfly valve, causing it to slightly shrink. Now there is a very fine gap and air moves into that gap, appearing as a vacuum leak. This problem can appear either as a high idle or as a low idle. I suppose the cars that learn the idle with a cold air intake charge, such as driving the vehicle soon as the battery is attached will have a low idle (since the air charge is always cool) while cars that learn their idle by idling the engine while stationary so the throttle body, butterfly valve and engine bay are all at similar temperatures will have a high idle. A lot of Toyotas/Lexus of these years suffer from this issue.
 

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Discussion Starter #12
The air intake is accurate and heats up with the engine when idling, the idle rpm is around 650-720 never really higher or lower. I will however check the temperature while driving and also what do you mean by ambient temperature, if you mean the ambient air temp sensor my app doesn't have a reading for it and my car doesn't either maybe because the sensor is not there, I think it only affects the cabin ac or something so I decided not to get it, if you just mean the air temp outside then yeah that I can do
 

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Discussion Starter #13
Also if this is the problem the only thing I can do is rebore it or get a new throttle body? How does this not affect every single engine due to cold air hitting the butterfly valve?
 

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Ambient temperature is basically the air temp that is surrounding the car, outside air, something you'll have to measure manually if your car doesn't. (most cars don't). Ambient temp vs air intake temp, air intake temp is something that is measured in the air intake tube and all modern fuel injected cars do this.

You rebore it because used throttle bodies have this issue anyway. A fuel trim variance of less than 20% isn't enough to justify getting the throttle body rebored. Anything above 20% however will cause a check engine code and you'll have to get the issue fixed at that point.

This is the company I used:

only company that I know of that rebores existing throttle bodies.

As for why this doesn't affect every engine... well, it sort of does. For example on my V6 Tacoma 5VZ-FE, the uneven wearing of the butterfly valve caused the valve to get stuck just before the throttle stop, so a TPS of about 11/12. However, when the engine is warmed up and the butterfly valve is cooled so it shrinks, the valve can close slightly more and hit the throttle stop showing a TPS of 10. 10-12 TPS is considered 'idle', changing the idle stop to 12 usually causes the engine to rev too much due to the opening so it never idles. If the throttle body and butterfly valve are perfectly even, then the valve always hits the throttle stop first before closing entirely. The shrinking of the butterfly valve can be compensated either with the idle control valve or in the case of DBW, they just adjust the butterfly valve opening slightly to adjust the idle.

Some cars wear out their throttle bodies more than others. Supposedly on the Toyota's of these years, the springs add an uneven tension that causes the valve to torque in the throttle body, causing uneven wear. I believe the newer DBW throttle bodies don't suffer from these issues, but I'm not 100% certain.
 

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I'm guessing it was a loose clamp because I pushed it in more and tighten the clamp more and now when I spray there's no sign of a leak, the oil was in between the two dots about 2,000 miles ago and now it's right beside the bottom dot, I just filled it to the middle again today, this car has 137000 miles so I'm guessing that's normal, the only other cel I'm getting is a p0 420 and I'm guessing that's because it's been running lean for so long and it killed the cat (bottom O2 fluctuates like upstream while driving), originally before cleaning the mass airflow sensor it was at 25% ltft, I unhooked the battery for about 15 minutes and cleaned all the connections to the sensors with the correct spray, so far fuel trims look a little better especially at idle 4.7%ltft but when I press on the gas it gets up to 10% and combined with long-term and short-term it's sometimes gets to around 15% while driving and acceleration from a stop. it's a whole lot better than what it was but I still think there might be something going on
two dots represents a single qt of oil. If your burning that much, that isnt normal in my book. Explains why you have a p0420 code and the cat problem.

Now how this ties into your LFTT and STFT a more mechanical savy poster will certainly have to chime in.
 

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these cars are notorious for burning oil anyway, especially if the oil changes were neglected at any point, car is run hot or they didn't use synthetic oil from the beginning. It's a design flaw with the oil control rings being 'low tension' and not enough oil cooling ports in the piston head to prevent caking of the piston rings.
 

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Discussion Starter #17
So burning a little less than a quart for 2000 miles bad? I thought that was acceptable? And also yes when I originally bought it the oil was black and very low bit still drive just fine no noises anything, I was thinking about cleaning it out somehow with maybe mmo or seafoam not I'm unsure of it. But the fuel trims, the only thing happening now is elevated fuel trims while accelerating and after a second the stft goes negative to somewhat even it out are we sure it's the throttle body or is it normal for fuel trims to rise while driving or accelerating. At one point when I had the aftermarket mass air flow sensor still in the car I realized my spark plugs might not be properly gapped and they weren't so I properly Gap to them and for about 3 days the fuel trims wer always steady at 0.8% or -0.8 percent. After the couple days it just started running high negative fuel trims but again this is the aftermarket mass air flow sensor and the old oem one which I cleaned was reading high fuel trims.
 

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Verify the soot in the tail pipe. If there's oil, it's leaking into the exhaust stream and your O2 sensors are coated in oil. Your fuel trims are going nuts because of this.

You will need to remedy this by doing a wet compression test to find out which cylinder(s) are leaking and replace them. Mmo will only help for a while. It's a band-aid and will get worse over time.

 

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Discussion Starter #19
Verify the soot in the tail p
Hope do I verify the soot? When I've touched it it seems a tiny bit wet but seems normal considering it's hot and I do get a little residue left on my finger, inside the pipe is black but it seems normal, also how do I do a wet compression test and can I come the o2's and do the cylinder replacement myself or is it too complicated? I'm more of a diy guy and I've done everything to it with no mechanic
 

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If the soot has oil, then you are burning oil. You shouldn't do it when it's just driven, instead do it as it's cooling down. This will prevent you from getting injured.

A compression test can be done as DIY. The tester can be rented from AutoZone or O'Reilly's. The way it's done is to remove the spark plug, and thread the the gauge into the spark plug tube. From here, you crank the engine. The value will be in psi. Then you repeat it for the remaining 3 cylinders.

 
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