Ls400 Running Rough At Idle

[rain1341]

I have a 1991 LS400 with 210,000 miles that recently started running bad. It came on suddenly during a commute to work when the engine started idling rough at stops. The problem has been very consistent since then for the past few months. The symptoms are:

• The car starts and runs fine when cold

• After the engine warms up, it idles rough with the engine surging

• The care lacks power when accelerating with a narrow range on the gas peddle where it has decent acceleration

• Check engine light comes on – code 71, EGR failure

o Trac/Off light is coming on at same time

• The exhaust smells like there’s too much fuel

The steps I’ve taken to correct the problem are:

• Replaced spark plugs, wires, distributors, and rotors

• Replaced Mass Airflow Sensor

• Replaced EGR Valve

• Replaced water coolant temperature sensor

None of these steps affected fixing the problem. Does anyone have experience with this type of problem? What actions were taken to correct the problem?

[Al.]

When was the last time you replaced your fuel filter?...if recently then I would also look at the fuel pump...either one of those could be the culprit.

[rain1341]

The fuel filter was changed less than 6-months ago. If it was a fuel problem, wouldn't run rough when cold?

[AndyLowe]

I have a 1991 LS400 with 210,000 miles that recently started running bad. It came on suddenly during a commute to work when the engine started idling rough at stops. The problem has been very consistent since then for the past few months. The symptoms are:

• The car starts and runs fine when cold

• After the engine warms up, it idles rough with the engine surging

• The care lacks power when accelerating with a narrow range on the gas peddle where it has decent acceleration

• Check engine light comes on – code 71, EGR failure

o Trac/Off light is coming on at same time

• The exhaust smells like there’s too much fuel

The steps I’ve taken to correct the problem are:

• Replaced spark plugs, wires, distributors, and rotors

• Replaced Mass Airflow Sensor

• Replaced EGR Valve

• Replaced water coolant temperature sensor

None of these steps affected fixing the problem. Does anyone have experience with this type of problem? What actions were taken to correct the problem?

I have not had these symptoms, but wanted to weigh in on some ideas for you:

- If you purchase a subscription to www.alldatadiy.com, you can learn from the ECM wiring Diagrams which small pins to monitor on the rectangular Diagnostic Connector, mounted on the top of your engine. I would recommend, have the engine at idle, normal temp and with a volt meter, set on DC 1 volt range, sample the two or four O2 Sensor readings. Connect your volt meter red lead to these pins, one at a time, and the black lead to ground on your engine. You want to see each O2 Sensor voltage porpoising slightly above and slightly below half of one volt, while engine is warm and idling. If you have O2 sensor readings that are always above .5vdc, they are indicating RICH mixture in the exhaust, back to your ECM. (Electronic Control Module, Brain, Computer)

Lean exhaust is Low Voltage; below .5vdc. (.2, .3, .1, .4vdc) Lean would indicate a Vacuum Leak.

Rich might indicate a ruptured Fuel Pressure Regulator Diaphragm or blocked air passage at the throttle plate, or Idle Air Control Valve, Bypass air route.

- Because you lack power and have an EGR code, I would also recommend a nice tool, called a Back Pressure Gauge. I bought mine and rarely need it, but it is cost effective, when compared to an innacurrate diagnosis at a local shop. If you buy a Snap On EPV311A Vacuum/Pressure Gauge Set, you remove one O2 Sensor and install this tool. Now you start your engine and simply don't EVER want to see over 3 PSI when at idle or driving on the highway. (Simulate this with pressing the accelerator pedal and raise engine RPM to a similar highway speed.

This will quickly eliminate or condemn the mufflers and catalytic converters. These are suspect for your low power, based on the parts you've recently replaced.

Have you really opened up and inspected your Air Filter? I've seen one car a long time ago, that sucked up a plastic bread bag and reduced airflow across a semi clean air filter to approximately 30% airflow.

I hope this helps~

Andy

[rain1341]

Thanks for the great suggestions. It will take some time to work through this list. I'll post later to let you know what I discover.

[VBdenny]

Go to your local auto parts store and buy a bottle or two of Complete Fuel System cleaner. Spend at least $10 a bottle or you aren't getting the good stuff. I have used both Slick 50 and Valvoline with excellent results. I know you have probably heard people say it doesn't work but it always has for me. After 100K miles my truck was running slightly rough and the CE light even came on. After a bottle of the Complete Fuel System cleaner and a little Interstate driving, it has never run better.

It is a very cheap effort which might yield big results.

[rain1341]

Go to your local auto parts store and buy a bottle or two of Complete Fuel System cleaner. Spend at least $10 a bottle or you aren't getting the good stuff. I have used both Slick 50 and Valvoline with excellent results. I know you have probably heard people say it doesn't work but it always has for me. After 100K miles my truck was running slightly rough and the CE light even came on. After a bottle of the Complete Fuel System cleaner and a little Interstate driving, it has never run better.

It is a very cheap effort which might yield big results.

Bad fuel is a good guess, but I've ran through several tanks of gas with fuel additives to address water in the gas or other fuel related problems. I've also switched to different gas stations to make sure. There was no improvement for these efforts pretty much eliminating the fuel as the problem.

[rain1341]

I have a 1991 LS400 with 210,000 miles that recently started running bad. It came on suddenly during a commute to work when the engine started idling rough at stops. The problem has been very consistent since then for the past few months. The symptoms are:

• The car starts and runs fine when cold

• After the engine warms up, it idles rough with the engine surging

• The care lacks power when accelerating with a narrow range on the gas peddle where it has decent acceleration

• Check engine light comes on – code 71, EGR failure

o Trac/Off light is coming on at same time

• The exhaust smells like there’s too much fuel

The steps I’ve taken to correct the problem are:

• Replaced spark plugs, wires, distributors, and rotors

• Replaced Mass Airflow Sensor

• Replaced EGR Valve

• Replaced water coolant temperature sensor

None of these steps affected fixing the problem. Does anyone have experience with this type of problem? What actions were taken to correct the problem?

I have not had these symptoms, but wanted to weigh in on some ideas for you:

- If you purchase a subscription to www.alldatadiy.com, you can learn from the ECM wiring Diagrams which small pins to monitor on the rectangular Diagnostic Connector, mounted on the top of your engine. I would recommend, have the engine at idle, normal temp and with a volt meter, set on DC 1 volt range, sample the two or four O2 Sensor readings. Connect your volt meter red lead to these pins, one at a time, and the black lead to ground on your engine. You want to see each O2 Sensor voltage porpoising slightly above and slightly below half of one volt, while engine is warm and idling. If you have O2 sensor readings that are always above .5vdc, they are indicating RICH mixture in the exhaust, back to your ECM. (Electronic Control Module, Brain, Computer)

Lean exhaust is Low Voltage; below .5vdc. (.2, .3, .1, .4vdc) Lean would indicate a Vacuum Leak.

Rich might indicate a ruptured Fuel Pressure Regulator Diaphragm or blocked air passage at the throttle plate, or Idle Air Control Valve, Bypass air route.

- Because you lack power and have an EGR code, I would also recommend a nice tool, called a Back Pressure Gauge. I bought mine and rarely need it, but it is cost effective, when compared to an innacurrate diagnosis at a local shop. If you buy a Snap On EPV311A Vacuum/Pressure Gauge Set, you remove one O2 Sensor and install this tool. Now you start your engine and simply don't EVER want to see over 3 PSI when at idle or driving on the highway. (Simulate this with pressing the accelerator pedal and raise engine RPM to a similar highway speed.

This will quickly eliminate or condemn the mufflers and catalytic converters. These are suspect for your low power, based on the parts you've recently replaced.

Have you really opened up and inspected your Air Filter? I've seen one car a long time ago, that sucked up a plastic bread bag and reduced airflow across a semi clean air filter to approximately 30% airflow.

I hope this helps~

Andy

The O2 sensors are indicating ~2.25 volts or higher when the engine is warm and running rough - indicating that the fuel is running rich. I put a vacuum gage on the Fuel Modulator inlet and can see that the inlet vacuum is fluctuating up and down in rythm with the engine surging while running rough.

A question is what would cause the engine to run rich and the inlet vacuum to fluctuate?

[SRK]

So if it runs well in open loop and poorly in closed, it's probably one of the O2 sensors (pre-cat). Check the code again - you may have interpreted incorrectly.

[AndyLowe]

I have a 1991 LS400 with 210,000 miles that recently started running bad. It came on suddenly during a commute to work when the engine started idling rough at stops. The problem has been very consistent since then for the past few months. The symptoms are:

• The car starts and runs fine when cold

• After the engine warms up, it idles rough with the engine surging

• The care lacks power when accelerating with a narrow range on the gas peddle where it has decent acceleration

• Check engine light comes on – code 71, EGR failure

o Trac/Off light is coming on at same time

• The exhaust smells like there’s too much fuel

The steps I’ve taken to correct the problem are:

• Replaced spark plugs, wires, distributors, and rotors

• Replaced Mass Airflow Sensor

• Replaced EGR Valve

• Replaced water coolant temperature sensor

None of these steps affected fixing the problem. Does anyone have experience with this type of problem? What actions were taken to correct the problem?

I have not had these symptoms, but wanted to weigh in on some ideas for you:

- If you purchase a subscription to www.alldatadiy.com, you can learn from the ECM wiring Diagrams which small pins to monitor on the rectangular Diagnostic Connector, mounted on the top of your engine. I would recommend, have the engine at idle, normal temp and with a volt meter, set on DC 1 volt range, sample the two or four O2 Sensor readings. Connect your volt meter red lead to these pins, one at a time, and the black lead to ground on your engine. You want to see each O2 Sensor voltage porpoising slightly above and slightly below half of one volt, while engine is warm and idling. If you have O2 sensor readings that are always above .5vdc, they are indicating RICH mixture in the exhaust, back to your ECM. (Electronic Control Module, Brain, Computer)

Lean exhaust is Low Voltage; below .5vdc. (.2, .3, .1, .4vdc) Lean would indicate a Vacuum Leak.

Rich might indicate a ruptured Fuel Pressure Regulator Diaphragm or blocked air passage at the throttle plate, or Idle Air Control Valve, Bypass air route.

- Because you lack power and have an EGR code, I would also recommend a nice tool, called a Back Pressure Gauge. I bought mine and rarely need it, but it is cost effective, when compared to an innacurrate diagnosis at a local shop. If you buy a Snap On EPV311A Vacuum/Pressure Gauge Set, you remove one O2 Sensor and install this tool. Now you start your engine and simply don't EVER want to see over 3 PSI when at idle or driving on the highway. (Simulate this with pressing the accelerator pedal and raise engine RPM to a similar highway speed.

This will quickly eliminate or condemn the mufflers and catalytic converters. These are suspect for your low power, based on the parts you've recently replaced.

Have you really opened up and inspected your Air Filter? I've seen one car a long time ago, that sucked up a plastic bread bag and reduced airflow across a semi clean air filter to approximately 30% airflow.

I hope this helps~

Andy

The O2 sensors are indicating ~2.25 volts or higher when the engine is warm and running rough - indicating that the fuel is running rich. I put a vacuum gage on the Fuel Modulator inlet and can see that the inlet vacuum is fluctuating up and down in rythm with the engine surging while running rough.

A question is what would cause the engine to run rich and the inlet vacuum to fluctuate?

O2 Sensors, are Thermo Couples.

They are incapable, by themselves, of creating 2.25 volts from my experience. ( I could be wrong). Today's Modern O2 Sensors, have Heater Elements to accelerate the ability of the 02 Sensor to begin reporting useful data (voltage) back to the ECM. These Heater Elements live within the O2 Sensor, and warm them up, even when the exhaust has not risen to normal temperature. A properly operating, Heated 02 Sensor, will warm up quickly, and begin giving reliable/useful voltage inputs to the ECM for proper fuel mixture calculations. Heated 02 Sensors reduce bad emissions, overall, as they get the mixture correct quickly, from first start of the day.

When a Heating Element partially shorts to the 02 Signal Line, which normally porpoises around .5vdc, it caused this normally low voltage level to be above 1 volt.

In your case, you have a partially shorted heater element, on one of your 02 sensors, partially shorted to the 0s Signal, back to the ECM.

Others may correct me, but I have never seen an 02 signal above .9vdc, ever. The ThermoCouple (02 Sensor)is incapable of a voltage above .9vdc.

Your Rectangular diagnostic connector, on the top of your engine has a flip open lid/cover. Within this Lid is a legend for pin assignment and purpose. Find the two or four different 02 Sensor pin assignments, start and run the engine, let it get warm, and compare the four.

I think you will see only one of them is above .9vdc; your defective, heater element shorted 02 Sensor.

Disconnect one 02 sensor at a time, to determine which one is THE one, when the volt meter goes to zero.

You will fix this!

My email is andylowe@sbcglobal.net

Andy

[curiousB]

Not sure O2 sensors are the culprit here. You didn't mention if the CEL light was on or any codes in the log. Also under heavy acceleration (WOT) the O2 sensors have little to do with engine performance (ECU reverts to open loop for WOT). So maybe try flooring the car on an open road with a warm engine. If it sputters and doesn't run smooth then I'd check spark or fuel.

A dirty fuel filter would cause fuel starvation that would be more noticeable under heavy engine load. Same goes for a worn out fuel pump. These cars are also (in)famous for ignition problems and intermittents which can kill 1 bank of cylinders.

[VBdenny]

nice information Andy, I (and probably many others) appreciate you sharing your knowledge.

Regarding the issue at hand, you said you replaced the EGR valve. Isn't that the pipe behind the engine that is very difficult to access? Maybe the tube is not connected well.

[rain1341]

Thanks to all of you who providing replies on this posting. I finally gave up on diagnosing the problem myself and took the car to my local Lexus dealer to have the professionals look at it. It took them a while to figure it out, but the problem turned out to be one of the ignition coils that was going bad. The car is running fine again after replacing the coil.