How to check fuel trims on OBD1 cars?
12-31-18, 07:45 AM
#1
Intermediate
Thread Starter
How to check fuel trims on OBD1 cars?
My 91 Lexus LS400 is driving me nuts. It's drinking fuel instead of burning it. Some times I wonder if it has a fuel leak. I have done all the maintenance that anyone can think of but no change in fuel consumption. I always idle it for atleat 15 minutes twice a week or whenever I am about to drive it. It's subzero temperature here in Alaska and I don't really like sitting in a cold car before driving it.
I have changed all the ignition related parts. Even recapped the ECU. Replaced the O2 sensor with genuine Denso not Lexus Denso, just Denso. Even replaced the coolant temperature sensor but no difference. I recently tried re-manufactured CARDON AFM and fuel consumption went way high with that AFM, so I returned it.
Engine of the car is running strong and not burning oil. Now the only option left to find the culprit is to check the fuel trims. I took it to Lexus dealer and to my amazement they said they don't have any OBD1 scanner.
Any suggestions?
My car's mpg is 7 to 10 miles per gallon. I did replace the fuel filter and I am suspecting maybe the fuel light are not tight but I don't see any fuel drips.
I have changed all the ignition related parts. Even recapped the ECU. Replaced the O2 sensor with genuine Denso not Lexus Denso, just Denso. Even replaced the coolant temperature sensor but no difference. I recently tried re-manufactured CARDON AFM and fuel consumption went way high with that AFM, so I returned it.
Engine of the car is running strong and not burning oil. Now the only option left to find the culprit is to check the fuel trims. I took it to Lexus dealer and to my amazement they said they don't have any OBD1 scanner.
Any suggestions?
My car's mpg is 7 to 10 miles per gallon. I did replace the fuel filter and I am suspecting maybe the fuel light are not tight but I don't see any fuel drips.
12-31-18, 08:31 AM
#2
Pole Position
My 91 Lexus LS400 is driving me nuts. It's drinking fuel instead of burning it. Some times I wonder if it has a fuel leak. I have done all the maintenance that anyone can think of but no change in fuel consumption. I always idle it for atleat 15 minutes twice a week or whenever I am about to drive it. It's subzero temperature here in Alaska and I don't really like sitting in a cold car before driving it.
I have changed all the ignition related parts. Even recapped the ECU. Replaced the O2 sensor with genuine Denso not Lexus Denso, just Denso. Even replaced the coolant temperature sensor but no difference. I recently tried re-manufactured CARDON AFM and fuel consumption went way high with that AFM, so I returned it.
Engine of the car is running strong and not burning oil. Now the only option left to find the culprit is to check the fuel trims. I took it to Lexus dealer and to my amazement they said they don't have any OBD1 scanner.
Any suggestions?
My car's mpg is 7 to 10 miles per gallon. I did replace the fuel filter and I am suspecting maybe the fuel light are not tight but I don't see any fuel drips.
I have changed all the ignition related parts. Even recapped the ECU. Replaced the O2 sensor with genuine Denso not Lexus Denso, just Denso. Even replaced the coolant temperature sensor but no difference. I recently tried re-manufactured CARDON AFM and fuel consumption went way high with that AFM, so I returned it.
Engine of the car is running strong and not burning oil. Now the only option left to find the culprit is to check the fuel trims. I took it to Lexus dealer and to my amazement they said they don't have any OBD1 scanner.
Any suggestions?
My car's mpg is 7 to 10 miles per gallon. I did replace the fuel filter and I am suspecting maybe the fuel light are not tight but I don't see any fuel drips.
PCV valve and right-left bank hoses?
Spark plugs?
Replaced my Iridium spark plugs with new NGK Iridium..
Much improved could start idle and overall driveability/performance.
See:
https://www.clublexus.com/forums/ls-1st-and-2nd-gen-1990-2000/904773-ngk-ruthenium-hx-spark-plugs.html
What viscosity and type (synthetic?) oil do you use?
Using Mobil 1 Synthetic OW-30...excellent for cold start. (Do not use 0W-20)
See::
https://www.bobistheoilguy.com/motor-oil-101/
If original, then replace your thermostat...or if aftermarket then replace it anyway.
See:
https://www.clublexus.com/forums/ls-1st-and-2nd-gen-1990-2000/871532-replace-your-original-thermostat.html
Automakers do a better job of preventing overheating than underheating engines...
Perhaps in extreme subzero weather it makes sense to cover or partially cover radiator grille.
Last edited by YODAONE; 12-31-18 at 08:34 AM.
12-31-18, 08:44 AM
#3
Intermediate
Thread Starter
Engine vacuum hoses original?
PCV valve and right-left bank hoses?
Spark plugs?
Replaced my Iridium spark plugs with new NGK Iridium..
Much improved could start idle and overall driveability/performance.
See:
https://www.clublexus.com/forums/ls-...ark-plugs.html
What viscosity and type (synthetic?) oil do you use?
Using Mobil 1 Synthetic OW-30...excellent for cold start. (Do not use 0W-20)
See::
https://www.bobistheoilguy.com/motor-oil-101/
If original, then replace your thermostat...or if aftermarket then replace it anyway.
See:
https://www.clublexus.com/forums/ls-...hermostat.html
Automakers do a better job of preventing overheating than underheating engines...
Perhaps in extreme subzero weather it makes sense to cover or partially cover radiator grille.
PCV valve and right-left bank hoses?
Spark plugs?
Replaced my Iridium spark plugs with new NGK Iridium..
Much improved could start idle and overall driveability/performance.
See:
https://www.clublexus.com/forums/ls-...ark-plugs.html
What viscosity and type (synthetic?) oil do you use?
Using Mobil 1 Synthetic OW-30...excellent for cold start. (Do not use 0W-20)
See::
https://www.bobistheoilguy.com/motor-oil-101/
If original, then replace your thermostat...or if aftermarket then replace it anyway.
See:
https://www.clublexus.com/forums/ls-...hermostat.html
Automakers do a better job of preventing overheating than underheating engines...
Perhaps in extreme subzero weather it makes sense to cover or partially cover radiator grille.
Denso (5304) IK20 Iridium
About PCV Valve I believe there is one PCV Valve on the left bank that I replaced it with genuine OEM PCV Valve. I didn't know if there is another on the right side.Type of oil I use is Mobil 1 5W 30 fully synthetic .
I replaced the thermostat with the timing belt 4 months ago. I used OEM Lexus thermostat.
I even cleaned the EGR and replaced the vacuum modulator with OEM.
12-31-18, 09:53 AM
#4
Pole Position
These are the spark plugs that I am using:
Type of oil I use is Mobil 1 5W 30 fully synthetic .
I replaced the thermostat with the timing belt 4 months ago. I used OEM Lexus thermostat.
I even cleaned the EGR and replaced the vacuum modulator with OEM.
Denso (5304) IK20 Iridium
About PCV Valve I believe there is one PCV Valve on the left bank that I replaced it with genuine OEM PCV Valve. I didn't know if there is another on the right side.Type of oil I use is Mobil 1 5W 30 fully synthetic .
I replaced the thermostat with the timing belt 4 months ago. I used OEM Lexus thermostat.
I even cleaned the EGR and replaced the vacuum modulator with OEM.
Use 0W-30 oil for easier cold start and reduced engine wear.
The PCV system uses two hoses; one each on right and left bank valve covers. Replace them along with clamps if original. PCV valve grommet too
Your ECU capacitors have been replaced??
Typical problem.area that can be repaired with new capacitors. This forum offers advice.
12-31-18, 10:53 AM
#5
Intermediate
Thread Starter
I have recapped ECU. I have mentioned that in my first post. I have done it myself though and car is running fine, hopefully I didn't do it wrong that is causing massive fuel consumption, but it was getting the same mpg even before recapping the ECU.
I have read the FSM and looked at the troubleshooting page for poor fuel consumption and tried most of the solution mentioned by FSM but no luck. There are 4 more things according to FSM to look into but those are pain in the *** to reach. One of which is cold start injector. The other three things are TPS, neutral start switch and injectors.
I have read the FSM and looked at the troubleshooting page for poor fuel consumption and tried most of the solution mentioned by FSM but no luck. There are 4 more things according to FSM to look into but those are pain in the *** to reach. One of which is cold start injector. The other three things are TPS, neutral start switch and injectors.
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12-31-18, 05:48 PM
#8
Driver School Candidate
Check the vacuum line on the fuel pressure regulator for the presence of fuel. If the internal diaphragm has ruptured; it could be aspirating fuel right into the plenum.
If that doesn't turn up anything abnormal, check the oxygen sensor feedback with a digital meter. You should have a rapid swing from near zero to one volt DC as the sensor goes from rich to lean. Slow response time can negatively impact the fuel mileage, and a consistently high reading would confirm a rich mixture. Use the OX1 and OX2 terminals in the diagnosis port to check this value.
Have you checked for codes?
If that doesn't turn up anything abnormal, check the oxygen sensor feedback with a digital meter. You should have a rapid swing from near zero to one volt DC as the sensor goes from rich to lean. Slow response time can negatively impact the fuel mileage, and a consistently high reading would confirm a rich mixture. Use the OX1 and OX2 terminals in the diagnosis port to check this value.
Have you checked for codes?
01-01-19, 12:58 AM
#9
Intermediate
Thread Starter
Can you please enlighten me some more on how to test O2 sensors in diagnosis port? I do have multimeter. What are those ports labeled on diagnosis terminals?
01-01-19, 05:01 AM
#10
Driver School Candidate
Set DMM to VDC. Ground the black probe and test voltage at ox1 and then ox2 terminal in diagnostic plug, with engine idling. Voltage should fluctuate from 0.1 to about 0.9v at a decent pace. Frequency of switching should increase with engine RPM. Both banks' sensors should operate at the same speed. A lazy response time or fixed voltage indicates need for replacement.
Code 25 stored in memory can indicate either a failed or lazy sensor.
The O2 sensor feedback can also be used to diagnose vacuum leaks, with an external fuel source.
Code 25 stored in memory can indicate either a failed or lazy sensor.
The O2 sensor feedback can also be used to diagnose vacuum leaks, with an external fuel source.
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AlaskanLS (01-01-19)
01-01-19, 10:54 AM
#11
When you're idling it for 15 minutes to warm up each time before you drive it, you are getting ZERO MPG. Burning gas, getting zero miles, so miles/gallons = 0.
That will be averaged in with whatever MPG you get while driving. So depending on how long your drives are, the long idling time will negatively impact the total MPG you're getting.
BTW, good to hear Alaska has a Lexus dealer.
That will be averaged in with whatever MPG you get while driving. So depending on how long your drives are, the long idling time will negatively impact the total MPG you're getting.
BTW, good to hear Alaska has a Lexus dealer.
01-01-19, 02:45 PM
#12
Intermediate
Thread Starter
Check the vacuum line on the fuel pressure regulator for the presence of fuel. If the internal diaphragm has ruptured; it could be aspirating fuel right into the plenum.
If that doesn't turn up anything abnormal, check the oxygen sensor feedback with a digital meter. You should have a rapid swing from near zero to one volt DC as the sensor goes from rich to lean. Slow response time can negatively impact the fuel mileage, and a consistently high reading would confirm a rich mixture. Use the OX1 and OX2 terminals in the diagnosis port to check this value.
Have you checked for codes?
If that doesn't turn up anything abnormal, check the oxygen sensor feedback with a digital meter. You should have a rapid swing from near zero to one volt DC as the sensor goes from rich to lean. Slow response time can negatively impact the fuel mileage, and a consistently high reading would confirm a rich mixture. Use the OX1 and OX2 terminals in the diagnosis port to check this value.
Have you checked for codes?
91 LS400 has two vacuum switching solenoids. One is attached to the EGR system and the other is attached to fuel regulator that has diaphragm.
The part number written on the VSV for fuel system isn't readable. Any one knows the part number?
Regards!
Thanks a lot again DrC10!
01-01-19, 02:49 PM
#13
Intermediate
Thread Starter
When you're idling it for 15 minutes to warm up each time before you drive it, you are getting ZERO MPG. Burning gas, getting zero miles, so miles/gallons = 0.
That will be averaged in with whatever MPG you get while driving. So depending on how long your drives are, the long idling time will negatively impact the total MPG you're getting.
BTW, good to hear Alaska has a Lexus dealer.
That will be averaged in with whatever MPG you get while driving. So depending on how long your drives are, the long idling time will negatively impact the total MPG you're getting.
BTW, good to hear Alaska has a Lexus dealer.
01-01-19, 05:00 PM
#14
Intermediate
Thread Starter
I found the part number. Please check the location circled in red in top diagram, this VSV is completely dead and this is the one that provides vacuum to fuel pressure regulator. I hope this will fix my car's MPG. Here is the part number 90910-12076
01-03-19, 05:21 AM
#15
Intermediate
Thread Starter
Set DMM to VDC. Ground the black probe and test voltage at ox1 and then ox2 terminal in diagnostic plug, with engine idling. Voltage should fluctuate from 0.1 to about 0.9v at a decent pace. Frequency of switching should increase with engine RPM. Both banks' sensors should operate at the same speed. A lazy response time or fixed voltage indicates need for replacement.
Code 25 stored in memory can indicate either a failed or lazy sensor.
The O2 sensor feedback can also be used to diagnose vacuum leaks, with an external fuel source.
Code 25 stored in memory can indicate either a failed or lazy sensor.
The O2 sensor feedback can also be used to diagnose vacuum leaks, with an external fuel source.