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Another car puzzle
Old 12-13-2019, 11:59 PM   #1
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Another car puzzle

Hi all.

1993 Lexus GS300 with 180K miles.

The only symptom is a sudden loss in fuel efficiency. I'm still working on the measurements, but it's gone from getting ~20-22MPG to getting about ~10MPG.

There are no other symptoms. No rough idle, no loss of power on acceleration, no check engine light, no other dashboard lights, no fuel smell any time.

It does not seem to lose fuel when parked in my garage.

I believe the tire pressure is good, the brakes are fine (no pulling to one side), the air filter was changed recently. Spark plugs and wires were done 2 years / 30,000 miles ago.

The idle RPM is normal. There are no noises and no fluid leaks.

The main suggestion that makes some sense that I've heard is O2 sensors going bad. The puzzling thing is that everyone thinks this should throw a code and trip the check engine light.

Oh, the check engine light itself is working. It comes on with the key in accessory and goes off when the engine is started, just like it is supposed to.

No fluids leaking. Power steering, coolant, and oil all changed recently.

Engine isn't overheating.

The problem cropped up suddenly, and it also happened almost exactly at 180K miles. I wondered if Lexus has some sort of computer thing that trips at that mileage to force you to take it in, but I kinda don't believe they would do that.

I tried taking it to Autozone to read the codes, and they weren't able to read the codes at my nearby store. The code reader couldn't find the vehicle, whatever that means. Maybe try another store?

I have an appointment to take it in next week, but I'm hoping you fantastic diagnosticians can help with suggestions or ways to narrow it down. I'm a tad concerned that if I bring it in with the above information that they'll just have to shotgun-replace parts that could be bad, since it seems like this might be a "process of elimination" kind of issue.
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Old 12-14-2019, 12:12 AM   #2
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Originally Posted by SecondCor521 View Post
1993 Lexus GS300 with 180K miles.
The only symptom is a sudden loss in fuel efficiency.
Try to have the codes read. Not all conditions trip the engine light, and that might turn up what the issue is.

Have you noticed a lack of power also? Maybe a clogged catalytic converter or restricted exhaust system.

Are you actually fueling up more often? If not, maybe the speed sensor is bad (intermittently) and is not logging all the miles you're traveling. Fewer miles logged on the same amount of fuel would look like lower fuel efficiency.

Dirty air filter or dirty/faulty mass air flow sensor?
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Old 12-14-2019, 12:31 AM   #3
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........... Maybe a clogged catalytic converter or restricted exhaust system.........

+1
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Old 12-14-2019, 03:03 AM   #4
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Originally Posted by mountainsoft View Post
Try to have the codes read. Not all conditions trip the engine light, and that might turn up what the issue is.

Have you noticed a lack of power also? Maybe a clogged catalytic converter or restricted exhaust system.

Are you actually fueling up more often? If not, maybe the speed sensor is bad (intermittently) and is not logging all the miles you're traveling. Fewer miles logged on the same amount of fuel would look like lower fuel efficiency.

Dirty air filter or dirty/faulty mass air flow sensor?
Thanks. Your first thought is probably the closest to what's going on.

No lack of power at all.

Yes, I'm actually fueling up more often. I filled up on 11/22, then put in $38 on 12/6, then another $18 on 12/10, and I've used almost a third of a tank to go about 60 miles. My driving is all in town, and it does vary from day to day, but the change was so noticeable that I went from going a couple of weeks between fill ups to almost being able to see the fuel gauge dropping while I'm driving.

As noted, the air filter is only a couple months old. I haven't actually checked it, but how could an air filter getting dirty cause such a sudden drop?

I've heard of the MAF sensor, that's another one on my list of suspects.

How do exhaust systems get clogged all of a sudden?
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Old 12-14-2019, 06:33 AM   #5
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They get clogged all of a sudden after degrading slowly to the point where a baffle inside the muffler or converter breaks loose. I think you would’ve notice a dramatic loss of power. Sometimes it’s fine when cold and the clog is apparent when it warms up or going g up a steep hill.
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Old 12-14-2019, 07:00 AM   #6
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A clogged converter is easily diagnosed by measuring exhaust back pressure at the primary oxygen sensor bung. A good shop should be able to do that.

Another possibility is that you might have one or more brakes dragging. Drive a few miles and see if the wheels are getting hot. I doubt that that would produce a 50% drop in fuel economy, but it's easy to check.

A bad fuel pressure regulator can reduce fuel economy. Again, easily detected by testing pressure. You might observe some black smoke or soot on the bumper near the exhaust pipe if the engine is running rich.
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Old 12-14-2019, 07:03 AM   #7
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You need to get a really good independent mechanic that's current on technology to troubleshoot your vehicle. Stay away from a Toyota or especially Lexus dealer as they'll really get into your pocketbook.

I agree about you maybe having a catalytic converter, MAP sensor or other relatively minor problem. Spark plugs may also need replacement, as they're now rarely changed.

You cannot just throw parts at your car because such parts are very expensive. They cannot be returned if they're not the problem. A 100% accurate diagnosis is required.

The nice thing is that a Lexus with 180,000 miles still have a lot of miles left in them. I've got an IS, and have had RX and ES models in the past.
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Old 12-14-2019, 07:40 AM   #8
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A number of valid comments above, but don't rule out a fuel leak. I have seen (more than once) fuel leaking on a hot engine block that will evaporate before dripping to the ground. I had one leak on an older car that would not show up at idle but would start leaking at speed when the fuel pressure increased. (Yes scary and dangerous, but possible)

Have you ever seen a burned out car on the side of the road... I know I have a number of times...
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Old 12-14-2019, 08:34 AM   #9
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Don't know if a bad O2 sensor necessarily results in a CEL.

20 years ago I had a bad sensor result in 33% reduction in fuel economy. There was no CEL.
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Old 12-14-2019, 08:39 AM   #10
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Is it possible someone is siphoning your gas?
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Old 12-14-2019, 09:21 AM   #11
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Originally Posted by SecondCor521 View Post
.... I tried taking it to Autozone to read the codes, and they weren't able to read the codes at my nearby store. The code reader couldn't find the vehicle, whatever that means. Maybe try another store?....
Their code reader couldn't read a 1993?
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Old 12-14-2019, 10:13 AM   #12
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Their code reader couldn't read a 1993?
The current code system (ODBII) came out in 1996. For 1996 + owners, I recommend everyone buy a Bluetooth ODBII adapter, they are ~ $10 ~ $20 on Amazon, and they connect to free apps on your phone/tablet - cheaper and more information than the hardware scanners.

https://www.amazon.com/s?k=Bluetooth...f=nb_sb_noss_2

To read the codes (kind of a Morse Code system) for the OP's 1993 Lexus:

https://www.2carpros.com/articles/le...eval-procedure

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Old 12-14-2019, 10:43 AM   #13
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I was being somewhat facetious... I think technically a 1993 is an antique car.
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Old 12-14-2019, 11:38 AM   #14
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don't rule out a fuel leak. I have seen (more than once) fuel leaking on a hot engine block that will evaporate before dripping to the ground. I had one leak on an older car that would not show up at idle but would start leaking at speed when the fuel pressure increased. (Yes scary and dangerous, but possible)
Agreed, my last car had a leak like that. I literally drove it years with a strong gas smell. There were never any drips below the car, and I couldn't see any leaks when I checked the hoses while running. It always took longer to start when it had been sitting for a while, so I assumed gas was just evaporating out of the carburetor somehow.

One day I decided to replace all of the rubber fuel lines and discovered the hose between the carburetor and fuel pump had a big split running down the back side. When the fuel pressure got high enough it allowed gas to drip out the back side of the hose. Replaced the hoses and all issues went away. I was really lucky it never caused a fire.
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Old 12-14-2019, 11:47 AM   #15
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the air filter is only a couple months old. I haven't actually checked it, but how could an air filter getting dirty cause such a sudden drop?
I've seen mice build nests inside air filter boxes, or in the intake tubes. It would be fairly easy to pop the cover and look, but you would probably notice a reduction in power too.

Quote:
Originally Posted by SecondCor521 View Post
How do exhaust systems get clogged all of a sudden?
The catalytic converter can break internally and the pieces can clog up the exhaust. I actually think I'm experiencing that myself. I replaced my catalytic converter last year and the old one was nearly empty inside. The pieces had to go somewhere, so I'm guessing they're stuck in the mufflers somewhere. My car still drives fine, but doesn't seem to have the power it used to. I keep meaning to replace the exhaust from the cat back, but haven't had the time or motivation, since the car is still driveable.
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Old 12-14-2019, 11:54 AM   #16
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I don't know if Boise requires winter fuel formulations or not, but that can account for 1 or 2 mpg. Not all of it, of course.
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Old 12-14-2019, 12:17 PM   #17
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I was being somewhat facetious... I think technically a 1993 is an antique car.
Anything over 25 years old qualifies for collector status insurance in NY.
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Old 12-14-2019, 01:10 PM   #18
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Lots of good replies, thank you. I won't quote them all but will try to respond to comments in order as needed.

No noticeable loss in power.

Pretty sure no problem with the brakes dragging. But will double check today. Also will check for black soot on the bumper. Come to think of it, there has been some black deposits that could be described as soot around the radio antenna, which is on the back of the car. I noticed these deposits right before the drop in MPG.

I plan on taking it to an independent mechanic who has high marks and has treated me right as far as I can tell. Agree I should stay away from the Lexus/Toyota dealership. They too, are nice people and treat you right, but they do assume you can afford a lot if you have a Lexus.

Spark plugs and wires were replaced 2 years / 30K miles ago.

I doubt anyone is siphoning my gas. First, the car is garaged in a residential neighborhood, and the only other person around is my son, who wouldn't do something like that (he doesn't drive and doesn't drink alcohol ;-P).

@pb4uski, they had to find the old fashioned adapter. Basically we got to the part in the process where the reader said "turn the car key in the ignition" (presumably to provide electrical power to the OBD port), and that's when the whole thing stopped. We were able to pick my car's model and year from the reader's menus, so it seems like it should have worked. Could be a problem with my car's port, or with their reader, or their old fashioned adapter. Who knows.

I'll have to check and try that OBD Morse code thing. The Autozone guy found a round port under the steering wheel which accepted his connector, so they assumed that was the right way to do it. The port was white plastic just under an inch in diameter.

No gas smell. No carburetor.

Will check the air filter just to be sure. Easy enough to do.

I think the 25 years of age does qualify it for collector plates, but (a) it costs extra in plate registration, so no thanks, (b) it's a hassle to switch plates, and (c) kinda silly for my car.
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Old 12-14-2019, 02:11 PM   #19
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Many pre-1996 cars record trouble codes and have means to read them, but they can differ from brand to brand. Here's a thread in a Lexus forum explaining how to read the codes and what problems they denote. https://www.clublexus.com/forums/es-...ght-codes.html
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Old 12-14-2019, 02:39 PM   #20
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Well, the Morse code thing worked. Thanks @ERD50!!

The car is throwing two trouble codes: 24 and 41. The first one is related to the intake air temperature sensor, and the second one is the throttle position sensor.

The intake air temperature sensor is somehow related to the mass airflow sensor. Since I find references to it being associated with air / fuel mixture, I'm going to tentatively conclude that this is the primary source of my problem.

I did also check the rear bumper for soot - none. I did check the air filter and it's fine. The air filter housing is also clean and fine.

No fuel smell in the garage after leaving it overnight in that enclosed space, so I doubt a fuel supply leak or issue. Plus, of course, the codes above.

The code 41 could be old or could be a related problem, or could be an unrelated problem. Will investigate further. So far it seems less likely, as a code 41 has all sorts of associated behavior that my car doesn't exhibit.

Thanks to all who are offering suggestions and thoughts -- I really appreciate it! It's so much better to go into a mechanic with some knowledge and understanding and prime suspects rather than "I dunno. Car broken. Please fix. Here's wallet." Even if the mechanic is honest (I think mine is), I'm sure they'd rather have a lead on what's wrong and a somewhat educated customer to discuss repair options.

Off to google how to replace the IAT sensor...
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