ednigma

I am working on a my Mom's 02RX300, that is not driven much. It was not driven for at least a year and left outside. Replaced all brakes and drove it to an appointment. On the expressway, the car seemed to run a bit rough. Driving home, the CEL and VSC lights come on. I stop at Autozone and the codes are P0300, P0301,P0303,P0305. The Autozone guy recommends running a can of Seafoam. Since the gas is pretty old, I try it (I usually use Techron, but the Seafoam was on sale. Driving home again, the CEL starts to flash.

I put a new spark plug in cylinder 5 and swap coils with cylinder 6. The old plug was full of black soot (thinking bad coil #5). Cleared codes and started car, get the same codes. Was expecting a misfire on cyl 6, but no P0306. Decided to change all plugs and run a compression test. All cylinders were within 5-10%. All the bank 1 plugs were black, all the bank 2 plugs were tan. I swapped all bank 1 coils with bank 2. Afterwards, reconnected battery and connected a scanner to my laptop. During idle and warmup, no codes. Went for a test drive and within a few miles, I get pending codes of P0171, P0174, and P0125 and very bad stuttering and surging.

Clean the MAF with CRC MAF cleaner, now the pending P0125 is gone, the P0171 and P0174 are now DTCs and the Pending DTCs are now P0136, P1133, and P1153. Doing some research, it seems that the P0171,0174 point to the MAF. Some opinions run strong that it would NOT be the AF sensors. But, I think it may be the AF sensors. The freeze frame shows that the Short Term Fuel Trim for bank 1 is 20.3% and bank 2 is 20.3%. The Long Term Fuel Trim for bank 1 is 52.3 and bank 2 is 46.1, so the ECU is trying to correct a lean condition by enriching the mixture. Could be fuel pressure, but with no schrader valve on the fuel rail, my fuel pressure guage needs an adapter. So I pull the spark plug from cyl 6. It is black with soot. This tells me that the system is actually running rich and the fuel pressure is OK. So I conclude that both my AF sensors went bad and are now telling the ECU that the mixture is too lean.

Am I off base? The car has only 59K miles. Should I also replace the MAF? What about the post cat O2 sensor. Not much experience with Lexus, so I am looking for experienced opinions. Lastly, where is the best place to get OEM AF sensors?

Sorry about the long first post
Thanks.. Ed

ATLs460

Honestly that sounds exactly how I talk in my brain. Same order of operations and logic lol

salimshah

Make sure addition of seafoam is not polluting the observation. Lot of people swear by it, I have never used it. A colleague of mine jokingly mentioned that seafoam should be used at night time as neighbors may call 911.

Salim

ednigma

@ Salim,

The guy at autozone says his mechanic uncle says it's great stuff. Autozone was also out of Techron and just had STP. I figured that at worst, it would be totally ineffective. I've seen youtube vids that showed people pouring it into the brake booster vacuum hose and others that show thick white smoke coming out the tailpipe. I had no thick smoke coming out the tailpipe, just an acrid smelling exhaust that waters the eyes due to unburned fuel. When gunning the engine, black soot shoots out onto the ground.

Since it looks like bank2 looked normal (the spark plugs were tan colored) and now with the System Too Lean DTC tripped for that bank, it sure seems to me that the ECU is being told that the AF mixture is too lean. I think that bank 1 was misfiring due to an overrich mixture - bank1 AF sensor was bad. After changing all the plugs, bank2 AF sensor goes bad (what are the chances?) and now bank2 is running overrich. Both AF sensors erroneously reporting a lean mixture.

The Seafoam website claims that it is EPA approved for all 50 states and is safe for all sensors and cats. There may or may not be any connection, but I think I'll stick to Techron.

If I drop $300+ on new OEM sensors and still have the same problem, do I get a new MAF sensor? When do I throw in the towel and take my car to a professional?

Thanks.. Ed

salimshah

Please take my advice with a grain of salt (I have previously stated limited experience with seafoam). I have used techron/chevron cleaner additive to the gas and on after next full tank, I change my engine oil.

I would try to almost empty out (by using) the current gas in the tank and then fill with fresh gas. After another 50 miles or so, clear all codes (if present) and start the diagnostic from that point onwards.

Once again, advice from other members who have used seafoam would be more appropriate.

Salim

ednigma

Ordered two AF sensors from Amazon for about $300 shipped. Installed the two sensors and connected my laptop scanner and started the engine (Battery had been disconnected overnight). At first the engine was running rough, but soon smoothed out. Watching the live data feed of the short and long term fuel trims, I noticed that the fuel trims were mostly negative at first, but upon warming up and during a drive, they fluctuated around zero. Seems to me that this is the expected behavior where the ECU is constantly adjusting the injector on time to keep the AF ratio around stoich, based on feedback from the now functioning AF sensors. Pulled a couple of plugs and saw that the new plugs which became carbonized are now relatively clean. Several days later, no CEL light.

The lesson I learned here is to consider all evidence and reason the problem out fully. I came across several sources that were adamant that the P0171-0174 codes were cause by the MAF and one website even said the the most misdiagnosed reason was the AF sensors. But thinking about this, I don't get how one could rule out the AF sensors. If the sensors fail like mine did, they would lie to the ECU and say that the system was running lean, so the ECU would increase the fuel trims to compensate (this caused my bad mileage). The telltale signs for me that the AF sensors were bad (even though no codes were thrown) was the unburned fuel smell, the bad MPG, and the carbon fouled plugs were in direct opposition to the failing code that indicated a lean system. I almost spent money on a new MAF, fuel filter/pump, and/or injectors to fix a system that was running too lean.

The moral of the story is that each case is unique and that you must consider as much physical evidence as possible along with the OBD2 codes when making a diagnosis.

Hope this helps someone
Regards.. Ed