Advice on poor mileage 04 Corolla 5-spd

[Jimigunne]

It has about 200K miles now, I drive about 550 miles/ week average, and almost all hwy miles. Originally it got about 38-39 mpg hwy when I bought it nearly 5 yrs ago. Now I get only 32 mpg hwy. Of course since I got it, Valero has switched to a "up to 10% ethanol blend". Maybe a small part of decrease is due to this. But I hear that other Corollas, and even ones with auto trans, are still getting 38 MPG! I suspect I should still be getting somewhat better MPG than just 32! At the high current cost of unleaded, if I could just do something to get it back up to around 35 MPG, over the course of a year it would be some significant bucks saved. At abt 180K, the check engine light came on, but it is the common cat converter efficiency below threshold" code. I changed both the upstream and downstream O2 sensors with sensors bought from Rockauto, but this did not fix light, or improve MPG. About the same time, I put in new platinum spark plugs, and sprayed out the MAF sensor with MAF sensor cleaner. It was very dirty....but MPG did not improve. The intake filter is new. My understanding is that the "cat converter efficiency down" code is set if the efficiency is just down by as little as 5%, but if its getting major plugged, and there is abnormal back-pressure, would this decrease MPG? How to test for this? (Can't very well drive around with exhaust disconnected). Suggestions as to what to check for decreased gas mileage?

[hardtopte72]

Platinum plugs may be your problems.

Simple hierarchy of plugs.

Copper - Very short life, best spark
Platinum - Long life worst spark
Iridium - Longest life, very good spark.

Your car came with Denso Iridium plugs. Try swapping for some Denso Iridium, Denso Iridium Power, NGK Iridium IX, or NGK Iridium Laser.

The Denso Iridium and NGK Iridium laser are 120K mile plugs. The others are 60K mile plugs but offer better spark as performance as well as fine wire.

On top of that check out the maintenance sticky and DIY sticky sections for relevant maintenance. An injector cleaning could help.

What sensor type did you use to replace your sensors?

Tire type and pressure?

Oil type and weight?

Brake condition and maintenance?

Throttle body, IACV, and injector maintenance?

Commute type include hills?

Valve adjustment ever been done?

[Jimigunne]

Hardtop, thanks for the reply.

I checked what sparkplugs I had installed.... they were not platinum, they are NGK laser iridium, gapped to .044. done at 190K, so they are still fairly new.

The tires are Pirelli P6, kept at 35 PSI
All of my commute distance (which is like 99% of the miles) is on fairly flat ground, only a couple of low hills.

Both upstream and downstream 02 sensors are Denso from Rockauto.... downstream changed 11/2010, and upstream changed 1/2012.
Brakes are excellent, no dragging.

IAC valve and throttle body have not been serviced/cleaned. Don't know much about cleaning/servicing those parts. Have cleaned out only the MAF sensor.
Have used nothing but 5W or 10W-30 Mobil 1 synth. oil
Clutch failed at abt 200 K (very recently) and was replaced, but it did not improve gas mileage at all.
There is one gas station in town that sells no-ethanol gas. I burned a tank of this gas, and it did not improve gas mileage more than 1 or 2 MPG. I was surprised...thought that "up to 10% ethanol blend would hurt mileage more than this.
I have added Seafoam to the tank a while back, no change. I also regularly use Lucas injector cleaner.
There are no codes set except for the "Cat converter efficiency below threshold " code
It has never had any valve work done. Possible culprit?? What to do ?
A valve job cost big bucks! Its probably so costly I would just use this car as a backup and buy a new (used) Corolla or Honda Civic!

[tgferg67]

Other suggestions
*Intake gasket - they fail/leak there is a tsb with a new part no. When they leak it causes increased fuel trim.
*Thermostat
*Drag - check each wheel for brake or wheel bearing drag.

[MD1032]

I would definitely check the intake side first. Do the intake manifold gasket job and get that out of the way, then clean the throttle body, IACV, and MAF while you're in there. Replace the air filter. Put it all back together and reset the ECU by disconnecting negative battery cable for >90 seconds (you'll know it worked when the radio loses everything), then fire it up and let it idle with nothing turned on for at least 5 minutes.

Then drive it for a few days and let a couple tanks go through before passing any judgement. Also, try not putting any injector cleaner in your gas. Those things aren't intended for "regular" use and can decreased mileage.

[Jimigunne]

I cleaned throttle body, IAC, MAF sensor, I also checked for intake leaks very carefully listening for any change in RPM.... the intake is not leaking at all! Also found no vacuum leaks. Changed air filter. Disconnected battery/ reset computer afterward. It still has only the code for "cat. converter efficiency out of limits" set. Still no improvement in MPG. As I think I said, the MAF sensor has not been replaced. There is not the slightest fuel leak anywhere....absolutely no fuel or water coming out tailpipe. It has absolutely
stable and smooth idle, and at same RPM as the day I got it (at abt 80K miles).
I read articles saying that a bad fuel injector or injectors could decrease MPG. I have ability to replace all of them, but am a bit reluctant to do so as it would cost about $300 for four injectors, and rebuilt ones at that. IF one or more were bad, wouldn't this affect the idle adversely in some detectable way....like it would become rough? I have never changed the PCV valve either (looks Okay at least), but I think this screws up the idle also when bad. It has lost about 8 MPG from the time I bought it, that is a lot of money in a year considering the miles I drive it, and currect high cost of fuel! But $300 for new injectors has a fairly long pay-back.
Could the culprit be not the injectors, but the cat. converter is so plugged that it is affecting MPG? What other symptoms would a plugged converter cause. POwer decrease? But (although its hard to say for sure) there is no noticeable decrease in engine power. How can I check for abnormally high pressure ahead of cat. converter? Ideas what should be done/ checked next?

[Jimigunne]

Should have also mentioned that I have not had valves adjusted. This again is big bucks, no doubt. At 215K miles are they going to be out of adjustment? This would cause loss of engine power in addition to MPG decrease? Right?

[hardtopte72]

It depends. From what I gather when the valves go out they get too tight.

Adjustment requires a special tool and/or a tedious swap procedure.

Check out the DIY section for injector cleaning DIY you can do yourself before cranking out for new injectors.

I would replace the PCV valve as preventative maintenance also. There is a DIY for that as well as there are 2 very different PCV valves.

Start with the small maintenance first. I clean my injectors every 30K with a Toyota flush kit.

A plugged converter is noticeable. It feels like you are towing a ship and most cars won't go over 30-40 MPH that I have ever had plug converters on.

[Jimigunne]

I put in a bottle of Lucas injector cleaner abt every 10K miles. Is this not adequate or as good as this cleaning procedure you mentioned? So your asessment is that its not that the valves need adjustment? Can a pro mechanic test this? would this be a compression test on each cylinder? Gee, valve adjustment might be even more costly than replacing all the injectors....I'm sure it takes a lot of time. The only cheap thing I haven't done is to replace PCV valve, I think . I could spend hundreds and hundreds of dollars on an old car and still not hit on the right thing actually causing the decreased MPG. Don't like this guesswork.

[MD1032]

It requires a lot more effort with this car because there's no adjusters in the head. The only way you can adjust the clearance is by swapping the lifters themselves out for ones of different dimensions, which as you can imagine is a lot less fun than turning a screw.

At 150k the valves on my car still sound well within normal limits...personally, I'd look elsewhere first because, come on, when was the last time you heard about someone having gas mileage problems due to valve clearance issues?

[hardtopte72]

You have an issue that is caused by lots of factors. Since you haven't gotten it professionally diagnosed (through things like bench testing injectors, computer diagnostics for fuel trim, compression tests, valve checks) then it can be assumed you are trying to save money.

Given this we are merely suggesting alternative solutions without diagnosing which is a fancy way of saying Internet guess work. Without actually seeing the car its hard to say what is your problem. It could be the gas, weather, commute, engine, brakes, computer, etc.

My advice would be to buy a scan tool that has a read out for parameters like an UltraGauge, Scan Gauge, or cable supported mobile phone app to read your fuel trim and MAF numbers. These will give a good indication of where your problem lies.

Also it looks like you haven't done the intake gasket. This can cause a vacuum leak, misfires, over lean conditions, and decrease fuel economy slightly.

[Jimigunne]

Mentioned in earlier reply that I carefully checked the intake manifold for leaks....there were none.

Well, I only have a cheap code scan tool, it can't read out any parameters. Would a local Toyota dealer have the equipment like this Ultra Scan to be able to determine if fuel mixture is off and why? (Thing is, I cannot TRUST them. THey make huge $$$$ by selling parts with an absurd markup.) An advanced analyzer like that is what I need...something to indicate if the engine is running rich in real time. Thanks!!!

[MD1032]

If you have an Android or iPhone smartphone, you can buy a very reasonably priced Bluetooth ELM327 adapter that can read live data. It is not a true diagnostics scan tool. It cannot perform tests on your car or send any commands, however, it's probably the highest usefulness-to-price ratio you will find.

Even if you bought that and plugged it in, though, I seriously doubt your car will tell you that it is running lean or rich. The Corolla's ECU is programmed to give the AFR sensor's equivalence ratio first priority when balancing parameters as far as I know. The problem is likely more complicated than turning a screw like on older cars that have an air-to-fuel ratio adjustment. Or simpler, of course...like the weather, and driving conditions...

[hardtopte72]

If the car is is running rich the A/F sensor will tell you. The ECU adjusts based on the sensor. If it keeps adjusting with no fix then you know you have a problem. Either the A/F sensor is bad, the injectors are dumping fuel, the MAF sensor is bad (reading too much air which causes a reading of too much fuel).

The intake gasket is a part guaranteed to fail. It's not if. It's when. At your mileage I would bet money it was bad years ago. If its the orange gasket then its likely been replaced but it sounds like you have owned it a while.

I don't think any bad gasket I have replaced leaked enough to cause an idle change. Yet they all caused over lean conditions (over 30 lean) as well as misfires, rough idle, and reduced economy.

It's only 7-10 bucks and super easy to replace.

[Jimigunne]

Don't have a smart phone so that option is out. I did however buy a USB scan tool that reads out many parameters, through the interface to my laptop. 44.00 on eBay. Will see if that tells me something useful. Yea, I understand the basics of modern computer controlled feedback control systems from reading up on it...and working on a few OBD cars I have had.... although my trade is actually avionics. Thanks again. Have a good President's day holiday.