iamdk72986

Hi, long time CL reader but not so much of a poster. So my 01 RX300 AWD with 100K miles is having this interesting problem that I can't seem to figure out, especially since it's not throwing any CELs. The light definitely works so a burnt bulb, etc is out of the picture. Under slight load, let's say accelerating on a level highway from 50-60mph, the car hesitates here and there. Let's say I hold my foot at 20% throttle, I can feel it accelerating to my desired MPH albeit very slowly. As it's doing so, every once or twice a second, at random intervals, the car hesitates as if I've tapped the brakes for an instant. I've noticed that every time it has that sensation, I hear knocking from the motor just for that very instant, like in the case of a faulty knock sensor. But due to the fact that it doesn't throw a CEL, I can't begin to figure out the problem. The car in general seems very sluggish.

I've always run the car with 87 octane from Mobil, Sunoco, or Shell. Recently, when I was low on gas, I put in 89 octane to see if that improves my performance and the car knocked like crazy for the first couple of miles. It totally didn't make sense to me as I put in higher octane, not lower.

I've replaced the knock sensor and the harness after I had that fail on me about 10K miles ago, the front air/fuel sensor replaced when that failed, and the post cat oxygen sensor with a Bosch sensor when I had to change the cat from the piping rusting through.

My guess is that it's a fuel delivery issue. This is from my observation of an excessively high LTFT(long term fuel trim), which is at about 20-25%, throughout the entire rev range. I've tried checking for intake leaks but haven't found any. The MAF, idle control valve and throttle body have all been cleaned. The PCV valve has been replaced as well. I'm guessing it's either the in-tank fuel filter or the pump itself.

Sorry for the long thread. Any insight? Thanks in advance.

Daniel

Dadikins

There have been a few reports of Bosch sensors causing erratic behavior. This may be your problem. For the sensors, stick with the DENSO OEM.

iamdk72986

Thanks DanDevoe for the quick reply. Could that really cause my long term fuel trim to be so high? I'm pretty good with a wrench and I know my way around a car but this issue has me stumped.

Daniel

Dadikins

I'm just going with the posts that I read - and many of them state the Bosch may work well in many vehicles, but not in Toyota/Lexus. It is possible that the problem isn't caused by the Bosch products, but where they have been problematic to some here, its best to rule that out.

Did the problem exist prior to you swapping out your knock sensors?

Do you have a code reader? Even if you don't have a CEL, there may be a "pending" code - which won't actually throw the CEL until it happens x number of times in a certain time duration.

BTW - much like you, long time reader, but don't post too often. Please keep us posted. :-)

iamdk72986

I have a Snap-On Modis and there were no pending codes. I've checked the live data and saw that my long term fuel trim was very high, which suggests that the motor is not getting enough fuel, whether it be from an intake leak, clogged/faulty injectors, etc...

I don't recall having this problem when I first got the car at 76K miles back in 2007 if i'm not mistaken. When the CEL came on the for the knock sensor, the car would knock like crazy whenever I reset the ECU until the CEL came back on and put the car in safe mode, which restricted the car from going into OD. After the replacement of the knock sensor, all was well.

I'll probably end up changing out the oxygen sensor with a Denso one and see if that works. That doesn't seem so far fetched after all given that the motor seems to run MUCH better when it is in open loop as opposed to closed loop.

Daniel

Baetke

Check to see if your lean condition exists at low rpm (idle). Then raise the rpm gradually up to about 3000 rpm. If the fuel trim numbers improve, you probably have a vacuum leak. If they are normal at low rpm and get worse as rpms increase, its' likely a fuel delivery probelm. Did you reset your ECU memory after replacing your A/F & O2 sensors. Your LTFT numbers are stored in the ECU memory over time and will gradually update as you drive the vehicle but you can speed up the process by zeroing them out. If you remove the EFI fuse for about 10 minutes the LTFT, spark advance and idle correction memories will all be erased.

iamdk72986

Hey Baetke, my LTFT is at about +20% at idle on bank 2 while bank 1 is within single digits. As I rev the motor, bank 2 stays at about 20-24% and bank 1 goes up go about 20% as well.

Do I need to pull the fuse for 10 min? Whenever I worked on the car, I pulled the fuse for about a minute. Whenever I did that, the motor would initially run terribly. When free revving, the motor would hesitate for a while and not rev smoothly. When I would drive the car, it would knock terribly as if the knock sensor went bad. It'd do that for a couple of miles until the ECU sorted things out.

With my LTFT's that high, the motor seems to run the "smoothest" although not as smooth and responsive as when the car is running open loop (low water temp). Just for the record, I don't rev the car like crazy when in open loop. It's just that I definitely feel the motor running much smoother and is much more responsive to any throttle pedal inputs.

I've checked my exhaust system for any leaks and my system is completely leak free. What in the world can this be? Oh boy, this is going to drive me insane haha.

One thing that may or may not be of matter is that I hear a sound under the car by the gas tank that sounds just like a vacuum leak. I haven't been able to pinpoint it but it's constant whenever the car is on. None of the other RX's I've encountered make this sound. Out of curiosity, how loud is the fuel pump? Mine SEEMS to be on the loud side, ALMOST as loud as an aftermarket electric fuel pump like a Walbro 255lph. Either that or I'm being overly paranoid lol. Thanks for all your responses.

Daniel

iamdk72986

Oh and I forgot to mention that the car seems to take like an extra half second or so of cranking to start. It cranks very strong and without hesitation. But just takes a tad longer for the car to start. My friend's 99 RX300 with much higher mileage starts right up.

Daniel

Baetke

The LTFT numbers you have plus the "hissing" sound near the gas tank make me think you have a leak in your EVAP system. Pull the cover off the evap system and look for a leak there. If your not familiar with this system I can send you some pix.

iamdk72986

Baetke, thanks for your input. I'll take a look at my Alldata and try to find pics there. Out of curiosity, wouldn't that cause a CEL? Not doubting you at all, just wondering if it's possible that something like that can somehow not cause a CEL to go off. I miss the good ol' days when cars were so much easier to diagnose.

Daniel

Baetke

Daniel,
I had the same thought. I was looking at my notes on the EVAP system and wondering why you weren't getting a P0440,1,2,or 3 code. Your LTFT numbers indicate a lean fuel condition at all rpms so when you mentioned the hissing sound at the rear of the car I thought since the EVAP components are located there, a leak there could explain both symptoms. BTW did you check your STFT numbers too? The actual fuel trim is the sum of the STFT and LTFT numbers. If you replaced the AF sensor fairly recently they might be reading large numbers in opposite directions, indicating the ECU is running normally but needs re-training. Thats why I suggested pulling the EFI fuse.

iamdk72986

My STFT is at single digits when my LTFT is that high. It may fluctuate a little bit but never more than say 4 or 5%. Usually it's less than 1%. I'm going to try to get the car on the lift at my friend's shop sometime next week and take a look at the EVAP system. I initially singled out the EVAP system because of the fact that no CEL was showing, or pending codes for that matter, but I suppose it's entirely possible that it wouldn't throw a code. Me and my laziness haha. Baetke, thanks a bunch for chiming in and I'll be sure to report on it. At the very least, I'll be learning something new

Daniel

iamdk72986

Baetke, I had the time to put my car on the lift last night and this is what I have discovered. The hissing sound is not coming from where the EVAP things are (under the black cover). It is coming from inside the fuel tank. I first thought it might be the vacuum hose that went around and to the top of the fuel tank, but after fiddling with that hose and not having an effect on the hissing sound, I figured that hose was not the problem. So I have narrowed it down and think it is the fuel pump. Perhaps it isn't throwing out enough pressure or volume, causing the LTFT to be so high.

Daniel

Baetke

Daniel,
First of all, Merry Christmas. I would have responded earlier but my wife wouldn't let me until after we were done opening presents.
I agree with your idea that the fuel pump may be failing. If you have high LTFT numbers at high rpm, that usually points to a fuel delivery problem or a very large vacuum leak which I think you have already explored and ruled out. Could be the pump, fuel filter or the regulator, which is also in the tank, but the noise you are hearing points to the pump. I'm pretty sure there is a photo DIY on this forum for replacing the fuel pump.
If you don't have any vacuum leaks, but there is a fuel delivery problems, you should still see normal fuel trim numbers at low rpm and they should get worse as the rpms go up.
Baetke

iamdk72986

Baetke, Merry Christmas! When I have the time, I'm going to use my power probe to turn on the fuel pump itself and see if I can replicate the noise while the engine is off. Hopefully, that'll suffice. Enjoy your holidays!

Daniel