Right, temp sensor was more a preventative maintenance item. MAF sensor is most likely.

 

If your car was buried up to the doors in snow, as I suspect it was since you stated the exhaust was blocked, you have probably overheated your exhaust system in the insulated pocket of heat created under the car while you warmed it up while shoveling it out of snow, most likely melting the element in one or more catalytic converters leading to higher-than-normal back pressure exhaust restriction. It will get worse the longer it runs as the pressure builds. It will have noticeably decreased power, poor fuel economy, and eventually cause engine damage in increasing severity.

You can test with an inexpensive vacuum gauge. Hook it up to a port on the intake manifold via an unused port or unplug a suitably-sized hose, hook up the gauge, and plug the hose you disconnected. Start the car and see if the gauge reads 16-22 in. Hg at idle. "On most engines, intake vacuum should be steady between 16 and 22 inches Hg (Mercury). A lower reading usually indicates a vacuum leak, or one of the other problems just mentioned. A reading that gradually drops while the engine is idling almost always points to an exhaust restriction. An oscillating vacuum reading usually indicates a leaky valve or badly worn valve guides that leak vacuum" (Engine Vacuum Leaks. (n.d.). Retrieved February 17, 2016, from AA1Car.com, http://www.aa1car.com/library/vacleak.htm).

I always start with free diagnostics before parts-swapping. I know you said you were going to freshen-up the car's sensors anyway, but when you said you'd replace the catalytic converters if the sensors didn't fix the problem I had to chime in because there are three (yes, three cat's and two resonators) and they are over $1K each for OEM (if you go that route) so your costs are exponentially rising throwing parts at it. I'd recommend a vacuum test, taking down the front exhaust at the cat/manifold and looking at the condition of the catalyst, and doing a compression test before spending any more time or money taking shots-in-the-dark, and for sure before taking it in to the stealer! (Ouch!)

 

Thanks for that break down. This is one of my theories on what is wrong with my ride although im leaning towards a sensor. I replaced the middle cat last year due to the infamous leak thats associated with our cars. That one is also the easiest to replace in case of damage. RockAuto has all three for $150 each so thats a plus if this is the case. Labor on the front two will be the kicker.

 

You're welcome; I'm happy to give back to all the members on this forum who've helped me over the years!

I know what you mean about the infamous leaky center cat; I also replaced mine a few months back with a Borla unit from RockAuto.com, as well. I love that site, awesome deals and fast shipping. Many members on here like to keep their cars all OEM, but many do not mind aftermarket parts. I am a mixture of both in most respects but exhaust is exhaust, and in the case of the Borla Y-pipe cat, it is a better design with higher flow with a true Y junction instead of the crimp-and-join-into-one design of the OEM pipe. I imagine the aftermarket cats flow better than stock, too, especially if you go with something like Magnaflow, so it sounds like you have done your homework on them.

They really are not difficult to replace yourself as they're only torqued to ~40 ft. lbs. if I recall correctly. I can send you the manual pages if you like. One tip though, if they truly are bad and you do replace them: get all new nuts, bolts, and gaskets from Lexus before you start the repair. The cat O-ring gaskets are crush-washer type and are coated with a green high-temp sealant on both sides, and the bolts are treated with anti-seize.

Still, it wouldn't hurt to test engine vacuum for signs of exhaust restriction, first. If the car isn't throwing any codes, and based on your car's symptoms and the circumstances of the issue, I'd strongly suspect the catalyst has melted and collapsed internally, or you cooked one or more oxygen sensors bad enough to cause problems but not enough to throw a code. Techstream would be ideal to check them out if you have it, otherwise any good scan tool would do the trick. Let me know if I can help, and good luck on your repairs!

 

Based on what you said, ill grab a vacuum gauge from the parts store and give it a try. Another thing i noticed is the condition gets worse the longer the car runs. Starting out cold its not as bad but once warm/hot, power really takes a dive.

 

OK so I replaced the MAF sensor yesterday. Wasn't the culprit. Now although my problem still exist, I did notice a slight increase in power even tho she's still sick. Also a lot less NVH from the engine. Like you I'm also leaning towards a restricted exhaust. When revving the engine while parked, I hear a metallic clanging underneath the car and as I said before, when cold, she acts sorta normal but once driven for a while, it really gets bad. If it is a bad cat, RockAuto has front Davico ones for around $150. Might as well replace both. Middle one new and I'm hoping that hasn't been damaged. But....there's still the threat of a bad TPS. It will be replaced anyways. Keep you posted.

 

The cats are pretty easy to remove. I just used a 1/2" drive 14mm deep socket and a few extensions. Spray the nuts with WD40 the day before as that make them much easier to remove. If you just want to check them you can remove the y pipe side only and shine a flashlight up into them. It will be pretty obvious if one is bad. If you do end up removing both cats or replacing them just know that the front and rear gaskets are different so you need 2 of each. They are about $10 a piece.

 

The readings through the OBD2 scanner should give you a lot of clues regarding what your problem is. If the car is running smooth when cold, and not so when the engine is warm enough maybe related to change from the open loop condition to closed loop condition (when the car starts processing signals from the O2 sensors and act on them). The short and long term fuel trim levels available through the OBD2 should give you some clues if the engine is running rich or lean. Also if the output of your post cat O2 sensors are not at a steady voltage but following the up-down waveform of the pre-cat sensors that would indicate defective cats. You may not have the CEL, however you may have some pending codes when you check with an OBD2 scanner. When the car is warm, does the engine run rough at highway speeds, when idling, or both times ?

 

[QUOTE=mrboca;9374497]The readings through the OBD2 scanner should give you a lot of clues regarding what your problem is. If the car is running smooth when cold, and not so when the engine is warm enough maybe related to change from the open loop condition to closed loop condition (when the car starts processing signals from the O2 sensors and act on them). The short and long term fuel trim levels available through the OBD2 should give you some clues if the engine is running rich or lean. Also if the output of your post cat O2 sensors are not at a steady voltage but following the up-down waveform of the pre-cat sensors that would indicate defective cats. You may not have the CEL, however you may have some pending codes when you check with an OBD2 scanner. When the car is warm, does the engine run rough at highway speeds, when idling, or both times ?[/QUOTE

Well after i changed the MAF, it idlings, running and sounding even better than before. Now when its time to step on it, thats when i have the biggest issue. When its cold, power is about 70% better than after its been running for maybe 20mins or so.

 

Silly question, but have you peeked down underneath and viewed the exhaust system after she's been warmed up? You might see something obvious like a cat glowing or one of the cats a different color than the rest? What are the chances you have one of those infra-red laser temp measuring gizmos? That would be interesting to shoot the temp of the cats, but again, you might SEE something obvious if you just get your belly down on the ground!

 

And maybe invest in a $15 bluetooth OBDscanner and $5 Torque Pro app for the android...With that you should be able to check out the O2 sensors, pending faults, fuel trim levels, temperatures etc. If you have problems with the cats you would get some indication like a pending code regarding cat efficiency. The question is why is your car performing great when warming up and sluggish when fully warmed up ? What changes when it it warmed up (if the car actually "knows" that it has warmed up) ? Is the problem air/fuel related or is it some sort of restriction in the exhaust side that comes up when the car warmed up) or both? Any unusual smells from the exhausts ?

You seem to have some of the symptoms, now you should get some data from the ECU to pinpoint the problem.

 

I'm surprised it was not the MAF. Going back and reading through the post I did not realize you warmed the car up with the exhaust system plugged up. We had 27 inches of snow during that storm and I made sure to dig out my rear end and intentionally cleared out the muffler hole.

Anyway, hope you find out the issue. I replaced cats on my old ls400 and of course went aftermarket as OEM were $1200 a pop. Aftermarket was about $450 for the set. To my knowledge, there are only 2 cats and the other 3 are resonators. Make sure you gets cats rated for the vehicle otherwise you will get codes 420 & 430 in about 2-4 months. Surprising you are not getting them now if the cats are the problem. 420 & 430 are catalytic efficiency errors bank 1 and 2.

 

I'm very suprised I'm not getting any codes popping up although there may be some hidden ones. I did replace the middle cat last year due to a whole. Rock Auto has pretty good ones that are reasonable. They tend to deal with a lot of OEM manufacturers. I will be going to my exhaust shop tomorrow. Also will get some data pulled.

 

You will not get any codes if your car does not switch from open loop operation to closed loop operation. One of the things your car's ECU checks whether the car has reached proper operating temperature. You replaced the coolant temperature sensor, however a thermostat stuck open condition will prevent the car to reach proper operating temperature. When was the last time you replaced the thermostat ? When you get the car scanned the closed loop operation should be confirmed.. If the thermostat is stuck open, especially in cold weather, your engine will run rich wasting gas, EGR will not turn on, and your will have poor idle and drive performance. Do you see fluctuating engine temperature needle on the dashboard with differing driving conditions?????

 

I havent changed the thermostat since owned the car(7 years) and the needle, once up to temp, stays at the same position. Car does idle fine before and after warm. Only problem i have is the acceleration especially after running for a while. The longer car runs the worst it gets.