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.

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?

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.

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.

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

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?