Mystery Problem.

[AdamCue]

Ok so here is my situation. My 1996 lexus ls 400 starts fine. Sometimes when I start the car it will drive fine but then out of nowhere it starts to hesitate and I loose power. I am getting codes p0172 and p0301. Have changed spark plugs and wires, Fuel filter, fuel injectors, misc vacuum hoses and the egr valve. And still having problems. 3 spark plugs keep getting black. Coil packs are good. Cats are good. O2 sensors good. The mechanic gave up on me today. Please help.

[RX350lover]

P0172 Bank 1 - System is too Rich:

The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters can cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.

There could be a vacuum leak.

There could be a fuel pressure or delivery problem

http://www.obd-codes.com/p0172

P0301 Misfire Detected - Cylinder 1:

Faulty spark plug or wire

Faulty coil (pack)

Faulty oxygen sensor(s)

Faulty fuel injector

Burned exhaust valve

Faulty catalytic converter(s)

Running out of fuel

Poor compression

Defective computer

http://www.obd-codes.com/p0301

[AdamCue]

Thanks for the quick response. But regarding p0172 my MAF sensor seems to be fine. When I disconnect it and drive it still runs the same which means the MAF sensor is fine. Correct? Also the mechanic fixed all possible vacuum leaks. He put in new fuel injectors and checked the fuel delivery system over and over... Everything seems fine... Until you drive it for a while at certain times... And regarding the misfire.. Everything has been inspected and seems fine.

[RX350lover]

Thanks for the quick response. But regarding p0172 my MAF sensor seems to be fine. When I disconnect it and drive it still runs the same which means the MAF sensor is fine. Correct? Also the mechanic fixed all possible vacuum leaks. He put in new fuel injectors and checked the fuel delivery system over and over... Everything seems fine... Until you drive it for a while at certain times... And regarding the misfire.. Everything has been inspected and seems fine.

If the MAF sensor malfunctions, the computer cannot determine how much air is going into the engine so it cannot determine how much fuel to put in there. The fact the car runs the same with or without the MAF cannot be used as an assumption that the MAF is fine.

I once had a defective MAF, so I took a can of contact cleaner (Radio Shack) and sprayed the inside of the MAF. It then worked fine for a few weeks after that. After a few weeks when it stopped working properly, I sprayed again and it worked ok again. But only lasted about a week. So I bought a new one. I'm not suggesting you do that. Just sharing my experience.

[steve2006]

Hi,

A very common problem is the driver's side coil pack and although you say it is OK it may be failing when it gets warmed up as they break down internally.For what they cost it may be worth replacing it.

[curiousB]

Ok so here is my situation. My 1996 lexus ls 400 starts fine. Sometimes when I start the car it will drive fine but then out of nowhere it starts to hesitate and I loose power. I am getting codes p0172 and p0301. Have changed spark plugs and wires, Fuel filter, fuel injectors, misc vacuum hoses and the egr valve. And still having problems. 3 spark plugs keep getting black. Coil packs are good. Cats are good. O2 sensors good. The mechanic gave up on me today. Please help.

A misfire will result in a rich reading as the fuel in unspent. The O2 sensors will sniff a problem. I'd focus on the mis fire in hopes it will solve both. Since it is one cylinder check wire plug and cap. Maybe a hairline crack on cap creating a breakdown path to ground. Watch plug one wire in darkness and see if there is any arcing to ground. Swap cap to other side and see if problem moves over. Same for plug wire...

[SRK]

A misfire will result in a rich reading as the fuel in unspent. The O2 sensors will sniff a problem.

O2 sensors don't measure fuel. They measure oxygen, which is why they are called "O2" sensors. So a dead misfire produces a huge amount of oxygen, and the computer sees that as lean, and adds fuel to the operating cylinders causing THEM to be rich.

[landar]

This from a wiki on oxygen sensors:

"The sensor does not actually measure oxygen concentration, but rather the amount of oxygen needed to completely oxidize any remaining combustibles in the exhaust gas. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess."

Interesting read -> http://en.wikipedia.org/wiki/Oxygen_sensor

[SRK]

The sensor compares reference air and it's oxygen on the outside of the sensor (usually admitted along the wires) to the oxygen level present in the exhaust stream. Voltage is generated depending on the difference between the two.

So it's oxygen, and nothing else, that drives the sensor to produce a voltage, interpreted by the computer as fuel/air ratio, which is adjustable over a fairly narrow band.

I like Wikipedia a lot, but it's not perfect, and sometimes downright misleading.

[curiousB]

A misfire will result in a rich reading as the fuel in unspent. The O2 sensors will sniff a problem.

O2 sensors don't measure fuel. They measure oxygen, which is why they are called "O2" sensors. So a dead misfire produces a huge amount of oxygen, and the computer sees that as lean, and adds fuel to the operating cylinders causing THEM to be rich.

Yes very helpful. So we agree a misfire results in a rich mixture which is what the ECU is reporting.

To OP: As I said address the misfire problem in hope of it resolving both. If there is a secondary issue you surely have to get the misfire resolved anyway. Divide and conquer.

[SRK]

We are agreed on the misfire issue. The OP reports three black plugs. It should be four if a coil was acting up, but three is close enough to warrant a careful check of the coils. As we know these engines do have coil problems. The OP refers to "coil packs", which isn't a proper description of these coils. Coil packs mean things like GM or Mazda to me.

So I'd put it on an oscilloscope and check coil function, primary and secondary. OP may not be around anymore though...

[AdamCue]

Ok so here is what the mechanic said: vehicle needs to go to dealer or a specialty shop. Stopped at stop sign and started running rough, check engine light came on. Codes 300 301 302 308 and 0172. He did a smoke test. He said he checked the coils..... Not sure with what. Its going to the dealer tomorrow morning.... Ill let you guys know what they say. Thanks for all the help

[SRK]

Whatever it is, I hope it's inexpensive and fixed quickly.

[RX350lover]

Ok so here is what the mechanic said: vehicle needs to go to dealer or a specialty shop. Stopped at stop sign and started running rough, check engine light came on. Codes 300 301 302 308 and 0172. He did a smoke test. He said he checked the coils..... Not sure with what. Its going to the dealer tomorrow morning.... Ill let you guys know what they say. Thanks for all the help

Thanks for keeping us in the loop. Please let us know the details of the problems. Thanks.