1edventurer

I used to get 18 mpg, but one day after revving on the freeway, it dropped to 16 mpg and the check engine light (CEL) came on. I took it to a mechanic who found a questionable airflow sensor, replaced it, and the CEL turned off. However, the fuel economy was still poor at 15.4 mpg, so I went to another mechanic. He didn’t find anything wrong but said all my spark plug coils need be changed, likely to cover his thorough troubleshooting. I eventually accepted that changes in the fuel might be causing the poor fuel economy because my 2013 Camry had been reading 18 mpg city. It didn’t seem worth spending more money since the car wasn’t showing a CEL and was driving fine with no misfires.

Just recently at 140K miles, my starter went bad and had to be replaced, which required removing the intake manifold. Now, my fuel economy is down to 13 mpg with no CEL on. I’m baffled about what could be causing this. My hunch is that it could be a clogged fuel strainer or possibly misread oxygen sensors. I think only a dealer, with the specific values to compare, can accurately identify the issue. Any thoughts?

alextv

Have you noticed any power decrease and if so the catalytic converter could be clogging up. Also is your transmission shifting into the highest gear. I still get 25 hwy and 18-19 mostly city on my 2001. Regular mechanic with techstream should be able to diagnose the values. Also try disconnecting battery to reset ecu especially after replacing the air flow sensor.

TominPT

Check and see if the engine ECU is receiving accurate coolant temperature readings - if not the engine will burn a lot more fuel if the coolant temp sensor is reporting cold temperatures. There are two coolant temp sensors, one for the guage and another for the ECU if I remember correctly. Use techstream or an engine moonitoring app to see what coolant temperature is being reported when the engine is cold and fully warmed up.

1edventurer

Just a short update on progress and any additional inputs:

I had my OBD readout 2 years ago and had exported a readout recently. I printed just the four pages of data from each year and fed it to AI and asked the question and here is the output:-------------------------------------------------
I reviewed the OBD data from 2022 and 2024 for your 2005 Lexus LS430. Here are some observations and potential causes for the drop in fuel economy:

Key Observations:

1. Long Term Fuel Trim (LTFT):
   
   • 2022: LTFT for both banks was around 14%.
   • 2024: LTFT for both banks has increased to around 20-22%.
2. Short Term Fuel Trim (STFT):
   
   • 2022: STFT values were generally negative, indicating the engine was running rich.
   • 2024: STFT values fluctuate more, with some positive values, indicating the engine is compensating for a lean condition.
3. Engine Load:
   
   • 2022: Calculated engine load values were around 20-25%.
   • 2024: Calculated engine load values have increased to around 35-50%.
4. Engine RPM and Coolant Temperature:
   
   • Both years show similar RPM and coolant temperature values, indicating no significant changes in these parameters.

Potential Causes:

1. Vacuum Leaks:
   
   • An increase in LTFT suggests the engine control unit (ECU) is adding more fuel to compensate for a lean condition, which could be caused by a vacuum leak. Check for any cracked or disconnected vacuum hoses.
2. Fuel System Issues:
   
   • The fuel injectors might be clogged or not functioning optimally, leading to poor fuel atomization and combustion. Consider using a fuel injector cleaner or having the injectors professionally cleaned.
3. Mass Air Flow (MAF) Sensor:
   
   • A dirty or faulty MAF sensor can cause incorrect air-fuel mixture readings, leading to increased fuel trim values. Cleaning or replacing the MAF sensor might help.
4. Oxygen Sensors:
   
   • Although there is no Check Engine Light, aging oxygen sensors can become less responsive, causing the ECU to adjust fuel trims incorrectly. Consider checking the oxygen sensors’ performance.
5. Exhaust Leaks:
   
   • An exhaust leak before the oxygen sensors can cause false lean readings, leading to increased fuel trims. Inspect the exhaust system for any leaks.

Next Steps:

1. Inspect for Vacuum Leaks:
   
   • Use a smoke machine or carburetor cleaner to check for any vacuum leaks around the intake manifold and vacuum hoses.
2. Clean or Replace MAF Sensor:
   
   • Clean the MAF sensor with a dedicated MAF cleaner. If the problem persists, consider replacing it.
3. Check Fuel Injectors:
   
   • Use a fuel injector cleaner additive or have the injectors cleaned professionally.
4. Monitor Oxygen Sensors:
   
   • Use a scan tool to monitor the oxygen sensors’ response times and voltages. Replace if they are slow or not within the expected range.
5. Inspect Exhaust System:
   
   • Check for any exhaust leaks, especially before the catalytic converter and oxygen sensors.

-----------------------------

revved the engine to 5K RPM and was smooth without any hesitation.
coolant temperatures appear operational and in expected range.
I’ve already used fuel injector cleaner additives several times in the tank and even sprayed and poured lacquer thinner through the brake vacuum hose .

I also had a smoke test done to check for vacuum leaks, but nothing obvious was found. However, it made me wonder if the hoses might have fine cracks that could collectively contribute to the issue...
Is there something I can spray on these hoses to seal microcracks? If it works, it would confirm that the old, fraying hoses are the issue and buy some time until an overhaul.
For clarity, I chose to live with 15.4MPG in 2022 but now it's worth fixing it

BigBoomer

This is the big stand-out to me.

Engine Load:
2022: Calculated engine load values were around 20-25%.
2024: Calculated engine load values have increased to around 35-50%.

You either have dragging brakes or else you are driving a lot heavier footed than 2 years ago. :-)
More load on engine = more gas used.

1edventurer

These readings were taken at idle while in park.
One thing is clear: after my mechanic removed the intake manifold, something was disturbed. The 2 MPG drop is evident before and after this work.

TominPT

Were those load readings idling in park taken with the engine fully warmed? Did you check what the ECU is telling you the coolant temperature is when the needle says the the engine is fully warmed up? It sounds to me that the engine is running as if the engine ECU was thinking the engine was still cold even when the coolant is actually hot - in that "cold temp" mode the engine uses a lot more fuel than when the ECU is receiving correct temperature readings from the coolant temp sensor and the engine is fully warmed up. Use techstream or an engine monitoring app on your phone to get coolant temp readings reported to the ECU.

Or you could do a simple driving test looking at the instantaneous mpgs, As the engine warms up, the mpgs should improved steadily as the engine warms up say driving on level ground at a very steady 30-35 mph from and initial value, say 15 or 20 mpg, to closer to 25-30 when fully warmed up engine, At a very steady 45 mpg in 6th gear you should get about 30 mpg or a little more with a proper functioning engine, coolant temperature sensors working great and clean MAF sensor etc.

Also, if you have old, fraying or cracked hoses replace them now - regardless.

I can also say from experience that a change in tires and.or low tire inflation can easily account for a 1-2 mpg difference in overall mileage.

Dubbayoo

100%. Changed tires and lost at least 1mpg, maybe more.