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Discussion Starter · #1 · (Edited)
This is really infuriating!

I bought the car with a CAT code, pitch black trans fluid AND engine oil.
-R&R trans fluid & filter (shifts BETTER)
-R&R engine oil
-Got 30 MPG

CAT code RE-APPEARS
-Replace CAT
-Replace PCV
-Replace spark plugs
-Shifting reverts to OLD pattern
-Replace timing chain tensioner
-Got 24 MPG

Start getting less than 21 MPG
-Start considering shift solenoid(s)
-First trans shop said "If it were the solenoid(s), they'd throw a code"
-Second trans shop says "Use the OD switch".
-Replace engine air filter
-Use Lucas injector cleaner

What's especially infuriating, is that my 2003 Corolla, is only getting 5 more MPG than my 1950's truck... WTF?!
 

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What color are the spark plugs now? If black, then you're running rich. You have no codes now? Does the engine run normally? Try disconnecting the battery for 15 minutes so the computer will re learn and hopefully your mileage and shifting will improve. Weird problem....
 

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A plugged up catalytic converter can cause the transmission to shift strangely, which could cause low MPG. It's easy to drop the exhaust (four spring bolts, a bracket with two nuts, and two rubber hangers) and shine a light through the downstream O2 sensor bunghole to see if the light is diffused and shining through all the little holes in the mesh media of the cat. If light is not shining through all of the tiny holes in the mesh media, that will cause transmission problems, along with power problems, which in turn will cause low MPG. If your replacement cat is already plugged up, then you are running with a lot of burned oil or fuel into the exhaust. Replacing the cat will only be a temporary fix.
 
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Discussion Starter · #7 · (Edited)
Kevcules46- Not sure, I'll have to check. Yes, there are no codes at the moment, and the engine is running fine. Regarding the re-learn... Looking for a mystery?

Junzilla- Don't know. Looks like I'll have to stop by AutoZone.

John Anthony- I get what you're saying... But if the cat &/ downstream O2 sensor were plugged up, it would be throwing a code(s), right? Because at the moment it's not throwing any codes, and it doesn't seem to be struggling for power. It just takes FOREVER for it to shift (even with O/D on).
 

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マズダスピード3
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Clean maf sensor
Clean throttle plate
Clean fuel injectors and replace o rings
Check brakes for dragging
Check compression for blow by
 

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2007 Silver Corolla CE Auto
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The engine temp (or what the engine thinks is the engine temp from the sensor) can also be an issue for mileage. They richen the fuel mixture by double pulsing the injectors. See below.



 

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You said the transmission fluid was "pitch black" before you replaced the fluid. That means the transmission was very neglected, and possibly had never been changed, or had been put under abnormal stress. Which exact fluid was used in the replacement? When you say the fluid was "replaced" (you used the term "R&R") do you mean it was a simple drain, pan drop w/ filter replacement, and refill, or was the fluid actually completely flushed out? A complete flush requires around 10 to 13 quarts to get all the fluid replaced (yes, that's more than the total system capacity). A flush should only be the low pressure flush using the transmission's own pump, NOT the higher pressure flush that is used with some of the machines at the lube places. This is especially important not to use high pressure flushing machines when the transmission presented with black fluid, or was having problems. If it was just a drain, pan drop w/ filter replacement, and refill, I suggest that you do it again one or two more times, or do a cooler line flush using the tranny's own pump (which is low pressure). Check the replacement filter to see if it's already dirty/plugged-up, which can easily happen if you only drained and refilled when you replaced the screen filter, and you had black fluid to start with. Make sure to use the correct fluid, which is T-IV compatible. When the engine is hot and you've driven for a while on a freeway, check the fluid level on the dipstick while the engine is running with the tranny in Park or Neutral. It should be within the full range, not more or less. When the engine is cold, you can also check the dipstick level, which should show it is within the cold range.

Taking forever to shift can be due to a plugged up screen filter or just dirty fluid. A one-time drain and refill on a transmission which had pitch black fluid is nowhere near good enough. About a year and a half ago, when I would put the transmission into reverse, it would go, but felt like the brake was engaged while trying to reverse. After about 20 seconds, it would clunk and then reverse normally. Replacing the fluid and filter resolved the problem (the previous fluid had about 50K miles on it and was not black). It still works fine now, with 409K miles on the clock. Pitch black fluid is a problem.
 

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Well, lets break it down. Fuel is just a form of energy storage. Economy is how efficiently you can use that power to get to speed and maintain speed. So a light and aerodynamic vehicle will ne better than ol' boxy on both accounts. Displacement doesnt factor for that.

So you had a catalytic issue and a poor economy. That means bad combustion. Using lots more fuel to get the power to have speed. So this is incorrect fuel mixture, or the combustion isnt delivering the power to the wheels.

I got a car from the eighties that does over 40mpg with a carburetor and three speed. Yes, transmission does assist in economy, especially in city, but that much MPG wouldnt be a trans issue. Poorly maintrained transmission manifest in poor shifting long before noticable economy variation.

So poor combustion: Spark, timing, fuel delivery. Symptom, fouled catalytics and happened again after addressing. So incomplete combustion. O2 sensors would also be contaminated and need at least to be cleaned to ensure the correct fuel metering is going on.

Why is it fouled in first place? Changes to intake system (Cold air intake) issue with/dirty/ damaged mass air flow sensor? Are you getting a good spark with proper plugs and good wires/coils. Not having any form of misfire code. Perhaps injectors are worn. They run fuel through them and do erode over time resulting in too much fuel. Because ignition timing is all done with sensors vs mechanical distributor, that cannot be factored in.

Last could be engine is just worn out. If a cylinder has bad leakage, then that is a probable cause. A compression test helps, but a leakdown test is more informative. Will help identify what and how engine is leaking. Valves or rings? Cause that means air fuel is off, oil can get into combustion, not all power is going to the wheels and so on.

The cats would be a symptom, not the cause.
 

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Checking the condition of the spark plugs should help you determine a cylinders condition. If they are black, your fuel mixture is rich for some reason. If they are oily, you have piston ring problems.
Things like brakes binding like 75aces mentioned, are often overlooked. Jack up the car and spin the tires.
If the spark plugs look good, then I would suspect the transmission. Not shifting properly could add to your fuel consumption.
Let us know what you find.
 

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Discussion Starter · #13 · (Edited)
Sorry for the delay, no mechanic shops open on Saturdays/Sundays, and Monday was tax day. I was also trying to decide if I should have the nearest dealership do a diagnostic, but decided not to (as you'll find out). And last, but not least, that thing called work. Anyway, I've decided that I'll start with cleaning the maf sensor and throttle plate. Then replace the fuel injectors, because I have absolutely no idea how old they actually are.

John Anthony- 100% agreed that the trans was very neglected. No idea what fluid was actually used, but hopefully they used T-IV compatible fluid. Yes, it was a drain-pan drop-filter replacement-refill. I asked the shop I took it to- about a flush, and they didn't recommend doing a flush (because of possible damage- which I was 100% ok with). Don't need to worry about a cooler line flush, the radiator had an air leak, so away it went. I COULD be wrong- but I don't think the trans filter is dirty, because the last time I checked the trans dipstick, it was still pretty pink. But I'll check it again. Here's hoping I can get 400K miles out of my Corolla!

Vaporisor- Actually, I was getting better mileage, with the old (code throwing) cat. As I said earlier (and thanks to you), I'm going to replace the injectors. Worn out at almost 116k miles? Man, THAT would be a terrible day!
 

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Sorry for the delay, no mechanic shops open on Saturdays/Sundays, and Monday was tax day. I was also trying to decide if I should have the nearest dealership do a diagnostic, but decided not to (as you'll find out). And last, but not least, that thing called work. Anyway, I've decided that I'll start with cleaning the maf sensor and throttle plate. Then replace the fuel injectors, because I have absolutely no idea how old they actually are.

John Anthony- 100% agreed that the trans was very neglected. No idea what fluid was actually used, but hopefully they used T-IV compatible fluid. Yes, it was a drain-pan drop-filter replacement-refill. I asked the shop I took it to- about a flush, and they didn't recommend doing a flush (because of possible damage- which I was 100% ok with). Don't need to worry about a cooler line flush, the radiator had an air leak, so away it went. I COULD be wrong- but I don't think the trans filter is dirty, because the last time I checked the trans dipstick, it was still pretty pink. But I'll check it again. Here's hoping I can get 400K miles out of my Corolla!

Vaporisor- Actually, I was getting better mileage, with the old (code throwing) cat. As I said earlier (and thanks to you), I'm going to replace the injectors. Worn out at almost 116k miles? Man, THAT would be a terrible day!
Oh. Didnt mean that to come off as an absolute, just as a potential depending on stuff like grades of fuel hours of operation etc.

What I was meaning is that there could be many causes because bad fuel economy means that you are not getting the energy from the fuel to the ground, or vehicle is just having to work far to hard to maintain speed/accelerate.

Question. Is your exhaust sooty at all? IE, if you wipe finger in the tailpipe, is it all carboned up?
 

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Do you drive with a loaded cabin and trunk? Mpg is related to additional weight of stuff.
 

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I would struggle to believe your fuel injectors are bad. Strong doses of seafoam or techron should rule them out. Most cars still have the same injectors at 300,000 miles. Ive got 280,000 kms and the engine idles strong and works well.
What do your plugs look like?
 

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If you replaced something, then suddenly a big change happened, go back to that change first...i.e. your catalytic converter, and work your way from there. Troubleshooting one step at the time. I agree with the poster above that I doubt your injectors are bad, unless someone replaced them with bigger injectors for whatever reason
 

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If you replaced something, then suddenly a big change happened, go back to that change first...i.e. your catalytic converter, and work your way from there. Troubleshooting one step at the time. I agree with the poster above that I doubt your injectors are bad, unless someone replaced them with bigger injectors for whatever reason
I have had leaky injectors before, but part of that was a previous owner using cheap gas and it was an older 90s car plus it would run higher rpms on highway. I see it a lot more with throttle body injectors, not so much with multi port, I only meant when I said as a potential.

Before replacing more, definitely want to do trouble shooting to try and diagnose where fuel is going.
 

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Don't be the parts cannon operator!! Diagnose Diagnose Diagnose!!!
 
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Discussion Starter · #20 · (Edited)
Vaporisor- No worries. So if I've been using 85 grade fuel, and if I'm understanding correctly, I should switch to 87 grade? If it helps, its hours of operation aren't that much.

75aces- No. 90% of the time, it's just me in the car.

Kevcules46- Thanks for the info! I was NOT looking forward to spending the $$ on new injectors!

Get ready, everybody... Here's what you've been waiting for...
 
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