Recent drop in gas milage - No CEL - Toyota Nation Forum : Toyota Car and Truck Forums
Avalon 1st Generation (1995-1999) Specific discussion of the first generation Toyota Avalon

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post #1 of 26 Old 12-18-2018, 12:47 PM Thread Starter
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Recent drop in gas milage - No CEL

My 98 Avalon recently dropped to about 16 MPG city. I used to get about 23 MPG in the summer and around 20 in the winter. I have read just about every post in the forum on gas mileage and still am at a loss. The car has 250,000 on it but is well maintained and to starts easy and runs great. Things done within the last year.


Plugs and wires
Cleaned MAF
new air filter
PCV
Cleaned throttle body


The mileage just dropped without any recent work. I have eliminated the obvious like fuel leak, low tires, dragging brakes, bearings.
I have a code reader that I can connect to my phone and watch live stats but not sure what to look for. I can see the fuel trims abut not sure what they should be. On thing that I suspect is wrong is that the thermostat could be stuck open. Seems like the engine warms up slow with top temp in cold weather in the 170's.


I have read about possible weak O2 sensors "getting in a loop" (?) as a possible cause but not sure what that means. Also read about EGR possibly being clogged.



Before I start tearing things apart and spending money on sensors, I'd like to hear what others have to say.



What should fuel trims be, cold and hot? Is that something that I should even look at?


This is the one time that I am hoping for a CEL to send me in the right direction


Thanks in advance.

.
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post #2 of 26 Old 12-18-2018, 01:50 PM
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That's a pretty substantial drop in mpg. When is the last time you changed your oil and oil do you use? Did you recently switch oils or do any oil treatments? The 1mz is a sludge monster engine, and if you switch oils or use a treatment at that mileage without doing it as a regular thing sludge deposits can break free and find their way into places that will cause serious reduction in lubricity and increase drag and friction, which can result in lower mpg. That said, I'm not sure just how much it would effect it.

The EGR is always a good place to look, it does nothing good for your engine, which is why many people delete it, and see a bump in MPG & power (both are pretty subtle). When you cleaned your throttle body, did you take it off and clean the backside of the butterfly valve too? The EGR & PCV join together and cause carbon deposits on the back of the valve and all the way down the air plenum, manifold, intake ports on the heads, valves and pistons.

What kind of gas do you use, also? Curious because I've never in my life gotten anywhere near 20mpg in my Avy

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post #3 of 26 Old 12-18-2018, 01:55 PM
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Ditto, where are you buying your gas? I've seen off-season blends take as much as a 20% hit on mileage, although based on experience and observation the hit is usually around 10%.



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post #4 of 26 Old 12-18-2018, 02:02 PM
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Quote:
Originally Posted by gaitch32 View Post
What should fuel trims be, cold and hot? Is that something that I should even look at?
Yes, that's one of the key things to check. If you see warmed-up LTFT (Long Term Fuel Trim) greater than positive 10 or 15%, something's fishy. A "walking wounded" O2 sensor can be giving bad info to the ECU, causing it to raise the fuel trims in error. IIRC, when they get to +25%, the ECU trips the CEL.


I've adopted a maintenance schedule for O2 sensors, replacing them every ~100k miles or thereabouts. The ones my Avy takes are not expensive; around $32-45 each, depending on if I get ones with the connector or not. I might not do that if it took the pricier A/F sensors at $100 a pop. If you wind up replacing them, get Denso sensors. Look up the Denso part number for your Avy here in their online catalog, then price shop Amazon and Rockauto. Beware of cheap counterfeits on eBay and even Amazon. If you find it substantially cheaper than Rockauto, chances are good it's a counterfeit.

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post #5 of 26 Old 12-18-2018, 08:25 PM Thread Starter
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These are replies to a few things that were suggested. First of all, I mostly use BP gas in Wisconsin.



As far as oil, I have been using Quaker State synthetic since I got the car. (200,000 miles and the best $1200 I ever spent) I change it regularly and don't have to add oil between changes. The oil stays remarkably clean for a car with this many miles. It really just gets a dark beige.

I pulled the throttle body when I first got the car, cleaned everything and replaced the TPS because it had a dead spot. During the summer, I did a cleaning through the intake and scrubbed out with a toothbrush and cleaner including the back of the butterfly. That was a few months ago and there was no change at that time. I might pull the EGR when the weather gets warmer to see how clogged it is.
I am taking a 3 hour trip tomorrow and will hook up the reader to watch the trims now that I know what to look for and get back to everybody. I am aware of cheap O2 sensors because my daughter went through 3 of them with her "mechanic" until I put an OEM one in 2 years ago.


Thanks for the help so far
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post #6 of 26 Old 12-19-2018, 10:04 PM
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same here.. all these sudden drops in mpg posts are ironic, though my 97 xls only has 179 k, there are good suggestions on this thread, like the sludge, and i did just change the oil with high mileage synthetic blend and Toyota oem oil filter, also i put seafoam in the crankcase a month ago before the oil change last week, this thing has always had a bad egr code before i bought it from a friend who left town a few months ago, now i could have a plugged cat, i need to know how to clean the throttle body and deal with the pcv vaslve as well. also as soon as the codes come back after i reset the ecu after disconnected the battery for half hour.
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post #7 of 26 Old 12-20-2018, 11:23 AM Thread Starter
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Here is what I found with the fuel trims. Drove about 50 miles yesterday, some start and stop and a little highway.
The LTFT seems to stay around 3-5+ with drops to 0 or -1. The absolute highest is saw was 9+ but that was just briefly and seemed random. Bank 2 seems to run +2 higher than bank one so bank one was 1-3+ range.

Based upon what was told to me by BMR above, they don't seem out of line.
I am going to run this tank out and try another gas station. I have limited stations in my area and always go to the same one. I will get back to this after I get through another tank. Now I am hoping that the gas is really the problem.
If anybody has any other ideas, let me know.
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post #8 of 26 Old 12-20-2018, 12:13 PM
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Those LTFT's sound great!


Just caught your comment about coolant temp in your first post... You can use your OBD2 reader to double check it. Should be ~180F when fully warmed up. This is what the ECU sees and uses to adjust fuel trims when it's cold. The gauge in the cluster gets its signal from a different sensor.


FYI: The O2 sensors used in these are heated, which is done to get the fuel injection system into closed loop quicker. You can usually use your ODB2 reader to watch when it goes from open to closed loop operation. On my Avy, it takes less than 30 seconds to go into closed loop. I'd say the most common O2 sensor failure mode is a blown heater. You can do a resistance check on those across the two black wires. I don't recall the spec, but it's around 10-20 ohms. Usually when they go, the resistance goes to infinity (open circuit).

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post #9 of 26 Old 12-20-2018, 01:36 PM
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Quote:
Originally Posted by gaitch32 View Post
Here is what I found with the fuel trims. Drove about 50 miles yesterday, some start and stop and a little highway.
The LTFT seems to stay around 3-5+ with drops to 0 or -1. The absolute highest is saw was 9+ but that was just briefly and seemed random. Bank 2 seems to run +2 higher than bank one so bank one was 1-3+ range.

Based upon what was told to me by BMR above, they don't seem out of line.
I am going to run this tank out and try another gas station. I have limited stations in my area and always go to the same one. I will get back to this after I get through another tank. Now I am hoping that the gas is really the problem.
If anybody has any other ideas, let me know.
Not that I'm target fixated, but based on what you've written I agree I still think it's the fuel. Even when a fuel source is top-tier, individual retailers can still purchase the fuel they sell to the public from various tank farms, and since the cheapest option is most often off-season blends, then low mileage is often a result. I'd try a different station for sure--even a different top-tier supplier as the change in detergents, etc., included in the mix can only help--and see what happens.

For some background where my information comes from, I have more than one friend or family member who works/ed in the refining industry, including a best friend who worked over ten years at the Shell refinery in L.A. as the lead chemical engineer. You learn a lot about how that industry functions.



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post #10 of 26 Old 12-21-2018, 06:02 AM Thread Starter
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@BMR. I am watching the temp trough the OBD2 and it does hit 180 once in a while. When it's really cold outside, stays in the 170s. I think all of the gauges on this car respond slow or are not quite accurate. Fuel takes 5 minutes to get to full after fill up and speedometer is is + - 2MPH when compared to the OBD2, so assuming the temp is a "best guess".
Can you recommend an Android app that would give me the O2 sensor readings? The one that I currently use doesn't get them. Also, if you could explain open loop- closed loop a little more, that would help. What should the app say or am I just looking for voltage readings?

@TrailDust. I am going to run this tank out and try another station. I kinda suspected that what you said about gas suppliers was happening. With luck, that will be the answer but going to try to test the O2 sensors anyway.


Thanks for everybody's help so far.
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Last edited by gaitch32; 12-21-2018 at 06:16 AM.
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post #11 of 26 Old 12-21-2018, 07:16 AM
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Thermostat:
How long does it take to get to operating temperature? If it drops below the recommended operating temperature the thermostat is bad and can cause your mileage drop. Has it ever been replaced? If not now is the time. Normal operating temperature should be achieved within a little more than a mile from initial start up. My bet would be operating temperature is closer to 190 degrees and should never drop below the thermostat opening temperature, (probably 82 centigrade=just below 190) Your heater is probably making it go lower with the thermostat not closing completely when necessary. Very common for OE Japanese stats to fail partially open.

Oxygen sensor:
Definitely can cause the mileage drop you describe, and it can do that without a CEL light code.

Your fuel gauge is dampened intentionally by Toyota, so you are not looking at every splash of fuel in the tank making the gauge bounce around like crazy.

Significant inclement weather could mean warming up and defrosting windows?
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post #12 of 26 Old 12-21-2018, 07:58 AM Thread Starter
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You have me convinced that a new thermostat is needed. It takes a good 5 miles or so to get to around half way up on the gauge and like I said, colder weather keeps the temp in the 170's or even 160's. I will have to look for a tutorial on how to replace because I think it is buried on the drivers side of the engine. Last thermostat I did was on an AMC 232....2 bolts and 5 minutes. Guessing this one is a little more complicated.
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post #13 of 26 Old 12-21-2018, 08:25 AM
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Thermostat is pretty annoying to change on these cars. Driver side, under a bunch of vacuum lines and other fun stuff that makes it a tight squeeze of a job. If I were you I'd keep it OEM in the coolant department, and head to a Toyo dealer for the thermostat and some red coolant. It's ~$20 a jug, and you'll have to mix it yourself with distilled, but trust me Toyo makes very good coolant. Also a really good oppertunity to just change the coolant while you're there, two jugs will do it. Also recommend changing the o-ring gasket on the metal tube that runs into the thermostat housing, they can get a little leaky after awhile. I believe it's called a water inlet pipe in the toyo catalog.

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post #14 of 26 Old 12-21-2018, 11:26 AM
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Quote:
Originally Posted by gaitch32 View Post
@BMR. I am watching the temp trough the OBD2 and it does hit 180 once in a while. When it's really cold outside, stays in the 170s. I think all of the gauges on this car respond slow or are not quite accurate. Fuel takes 5 minutes to get to full after fill up and speedometer is is + - 2MPH when compared to the OBD2, so assuming the temp is a "best guess".
Those temps make me think like the others; it needs a new thermostat. No matter how cold it is outside, the temp should always get in the 180's and stay there.

Quote:
Can you recommend an Android app that would give me the O2 sensor readings? The one that I currently use doesn't get them. Also, if you could explain open loop- closed loop a little more, that would help. What should the app say or am I just looking for voltage readings?
Sorry, but I know nothing about Android apps. The instructions for whatever you're using should explain how to find that, if it's capable. It should be under "system status" or something like that.

Regarding open/closed loop:

The O2 sensors need to be very hot to function, like 800įF. Until they get there, the ECU ignores the O2 sensors and uses default fueling curves, which are intentionally rich to avoid damaging the engine. This is called "open loop" operation. Once it goes into closed loop, the ECU relies on O2 sensor output to adjust fueling. Curious note: Most systems exit closed loop operation if the throttle is wide open. Prior to the advent of heated O2 sensors, it took a few minutes for them to get hot enough, sitting in the hot exhaust stream. By adding a heater, it takes 15 seconds or less, in my experience. This reduces emissions during that warm up period. Adding heaters happened somewhere in the early-90's, I think.

The ECU stores LTFT (Long Term Fuel Trim) values indefinitely, or until power is removed or it's reset using an OBD2 reader. Clearing the error codes, disconnecting the battery, or pulling the EFI fuse = a reset. It uses these values when it enters closed loop operation. If they ever get too high (>25%, as I mentioned earlier) the ECU sets an error code for "Bank X too lean", and turns on the check engine light.

There are also STFT's (Short Term Fuel Trim). These indicate what the ECU is doing in the very short term; like the last second, I think.

V6's have separate controls for each bank, so you'll see LTFT1 and LTFT2, for banks 1 & 2, respectively. Bank1 = rear, bank2 = front.

A malfunctioning sensor may give the ECU such a flaky signal that the ECU never goes into closed loop. This can be checked by monitoring the ECU with an OBD2 reader. A sensor like this, "walking wounded" as I like to say, doesn't always trip an error code. But if you see your engine never goes into closed loop, or goes in/out while you're driving, it could be a bad sensor.

Hope that helps.

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Last edited by BMR; 12-21-2018 at 01:42 PM.
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post #15 of 26 Old 12-21-2018, 12:21 PM
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Check for a dragging caliper or handbrake. Also, if you change the thermostat make sure the "jiggle valve" is at 12 o"clock.
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