2001 Toyota Avalon PO171 Bank 1 Lean and PO300, PO301, PO303, PO305 Misfires FIXED!!!!!

I just thought I would share what I have been working on over the past couple weeks that had me stumped and beyond frustrated.
I recently purchased a 2001 Toyota Avalon 3.0 v6 with the 1 MZFE engine from a very nice Elderly family friend because it wasn’t running right at all, and they thought it wouldn’t be worth paying a mechanic to fix it.
The car wouldn’t accelerate up a hill, and had a major loss of power even when driving on a flat road. It would also occasionally backfire when snapping the throttle.

The Check Engine Light was on, so I checked the codes with my code reader and it had PO300 (random misfire) PO171 (Bank 1 lean), PO301, PO303, PO305, cylinder misfire codes for the bank of cylinders closest to the firewall (bank 1 on this vehicle).
After reading a lot of forums and posts, I thought I had narrowed it down to a problem with the Camshaft Solenoid Valve, a.k.a. the Oil Control Valve.
And because I learned that these engines are known to have a problem with oil sludge build up, I figured that there might be a problem with the Oil Control Valve for bank 1. I thought that maybe it was malfunctioning or just having a hard time moving because of the sludge, so I ordered this one on Amazon.com:
Update: this one is for the left side or front bank. Amazon didn’t have the link to the rear bank that I purchased…

I put it in and it made absolutely no difference in the way the car was running. I also cleaned the other Oil Control Valve, and the two Oil Control Valve filter screens to make sure I could rule out the oil control valves as being the problem. I still felt like the engine timing was off a little, and searched and read some more.

Based on what I read, I decided to take off the timing belt cover to verify that the timing belt wasn’t one tooth off on the rear camshaft, but everything lined up correctly.

Next, I decided to clean the MAF Sensor. Many forums and a few YouTube videos suggested to clean the MAF first before doing anything if you had the PO171 code. I cleaned it with CRC Mass Air Flow Cleaner:

The car seemed to run a little smoother, but it just wasn’t right, and it still had a misfire. The power loss was most noticeable under a load like trying to accelerate up a hill, so I decided that the MAF sensor might just need to be replaced. I read many posts about making sure to use Denso brand replacement parts, especially the MAF sensor, so I bought this MAF Sensor and put it in:

Sadly, the new MAF wasn’t any better than the cleaned MAF that was already in it.

I spent many more days reading online forums and talking to friends about the problems I was having this this Avalon
I looked for vacuum leaks by spraying carb cleaner all around the intake and throttle body. I even sprayed around the fuel injector seals and the intake manifold gasket. I inspected all of the vacuum hoses twice. I removed and cleaned the PCV Valve and inspected the hose, but nothing looked cracked or like it would be causing a leak. The PCV Valve was pretty dirty, but after a good cleaning, it seemed to be working fine.

Now, I know it’s not right to just throw parts at the car and cross your fingers that you might get it right (a.k.a. ‘Parts Darts’), but keep in mind, this was after reading a lot of forum posts, and after finding a TSB for the Avalon: T-SB-0114-08. Now, this is for a different year of Toyota Avalon, but the symptoms were very similar to the symptoms I was experiencing.

I did an engine compression test to make sure the motor wasn’t bad, and every cylinder had good compression (around 150psi). When pulling the spark plugs, I noted that the rear spark plugs were very black and fouled as if they had been running extremely rich. I previously thought that the PO171 ‘bank 1 lean’ code meant that it wasn’t getting enough fuel, but it was now clear that the rear bank cylinders were getting way too much fuel. I realized that the OBD PO171 was a ‘false lean’ condition in which the computer was thinking that it wasn’t getting enough fuel on that bank, so it was telling those injectors to stay open longer. I replaced the spark plugs with these DENSO # 3297 Iridium LONG LIFE Spark Plugs SK20R11:

The new spark plugs didn’t cause it to run any better, but I knew it was time to replace them after seeing how bad the rear bank plugs were.

That brought me to my next investment:
Air/Fuel Sensors

This also happened to be the next thing that was recommended on the Toyota Technical Service Bulletin that I found. It said to replace the Air/Fuel sensors in each bank. These air/fuel sensors are similar to O2 sensors, but work on a wider band than a traditional o2 sensor, and many forum posts mentioned that it is recommended to change them after 100k miles (my Avalon has 130k miles).
There are also many posts that warn of using cheaper after-market sensors, and that the Toyota ECM (Engine Control Module) is very picky and/or sensitive to the readings from those sensors. Toyota refers to the upstream sensors in the exhaust manifolds Air/Fuel Sensors but most product descriptions still refer to them an O2 sensor. They weren’t cheap, but I decided to go with the OEM Denso sensors. This is the cheapest I was able to find them:

At the same time, I figured I should replace the downstream o2 sensor since it was just as old and might be giving false readings to the ECM. This one requires removing the passenger seat (not kidding), and the side kick panel so you can pull up the carpet just enough to get your hand in to undo the connector. Here’s the oem Denso sensor that I put in:

Unfortunately, replacing all of these sensors didn’t seem to change anything.

The next thing I thought it could be was a fuel delivery issue, but I had my doubts as to why that would only affect one bank unless all 3 fuel injectors on that bank decided to fail at the same time. Still, I wanted to at least rule out the fuel filter and pump. I rented a fuel pressure test kit from my local O’reilly Auto Parts Store and got a fuel pressure reading of around 50psi. On this car, the fuel pump doesn’t activate until the car is started. The pressure seemed to climb slowly, so I suspected that the fuel filter might have been partially clogged, or that the fuel pump was starting to fail. The fuel filter was cheap enough, and looked like it hadn’t been changed in a while, so I replaced it with this Wix filter which was recommended by my local O’reilly Auto Parts store:

Or Here’s the toyota version of the same filter:

I tested the fuel pressure again after replacing the fuel filter, and the fuel pressure was still around 50psi but climbed much quicker. I blew through the old fuel filter and could feel quite a bit of resistance, so I felt like I was making progress, and that the old filter was definitely bad. I noticed that when revving the engine the fuel pressure seemed to bounce around a lot. I thought that maybe the gauge wasn’t entirely accurate, or that it just wasn’t working correctly. (I later realized that the gauge was trying to tell me the real problem).

I figured with fuel pressure at 50psi and the fact that the car would start up right away that maybe it wasn’t a fuel delivery problem. I still wanted to be absolutely certain that it wasn’t something restricting the flow of fuel in the tank, like a plugged intake filter (a.k.a. the fuel sock) that might be restricting the fuel volume. I took out the fuel pump assembly (under the back seat), and everything looked ok. I even removed the in tank fuel pressure regulator and blew air through it and it seemed to be working okay as well. It’s just a bypass type of regulator set at a certain psi so that if there is too much pressure from the pump, it would just bleed off back into the tank. There is no fuel tank return lines on this car, and the fuel pressure is supposed to be constant. This year of Toyota Avalon does not have a vacuum assisted fuel pressure regulator like some older models, which allows the fuel pressure to increase as the engine vacuum decreases upon acceleration.

After putting everything back together, it still fired right up, but still wouldn’t accelerate up a hill, and the codes would come back as soon as I pushed down on the gas pedal.

So, I actually ended up doing something that I never do. I took my car to a Local Mechanic that was recommended by the guys at O’Reilly Auto.
I drove the car to his shop and explained everything I had done and everything I had already replaced. He seemed confident that he could diagnose the problem, and offered to diagnose it for $100. Now, this seemed like a great idea, because at the time, I only had a basic code reader and didn’t have the ability to see fuel trims or live data. I thought he would be able to diagnose the problem, and I could finally get it fixed and move on with my life.

They called me the next day and said that they wanted to do a back pressure test for $50 more because it was acting like it had a plugged catalytic converter or converters (there are 3 on this car). I agreed, and they they called me and said ‘Yep, that’s the problem, all 3 catalytic converters need to be replaced, or at least the 2 front ones”.
(He was right but also wrong).
I picked up the car, and did a little research on the catalytic converter replacements (I am in CA so it has to be C.A.R.B. compliant) and found that they are very expensive.
I decided to take off the front converter which is a part of the front exhaust manifold to see if I could tell if it was blocked or not. The Catalyst element looked intact and clean. I was puzzled, because it looked fine to me (I’ve seen bad ones before). I even tested it with a shop vac, and it had no blockage at all
So, that brought me to the rear bank catalytic converter next. It kind of zig zags off the rear exhaust manifold, but again, after taking it off, and verifying that it was clear, I was starting to doubt that mechanic’s diagnosis.
I did notice that the rear catalytic converter did appear to have a darker dirtier color on the honeycomb element which was visible with the middle converter and zig zag pipe removed. I put a shop vac on the tail pipe and sure enough the tone of the shop vac motor changed to a higher pitch when compared to the shop vac sucking air disconnected from the exhaust pipe.
I know what you’re thinking, this is not a very scientific test, but it worked for me. I determined that the downstream catalytic converter was plugged.
Before I replaced it, I made my own back pressure tester with an old engine vacuum/pressure gauge connected to a rubber cone style engine compression test nozzle with the schrader valve removed. It worked great. With the downstream catalytic converter disconnected I got zero back pressure from both the front and rear banks by testing in the Air/Fuel Sensor ports, but after reconnecting the downstream catalytic converter the backpressure went all the way to 3 on the vacuum/pressure gauge. I also got a reading of zero pressure from the downstream o2 sensor port (after the plugged catalytic converter).
I decided to purchase this Universal Catalytic Converter from Magna Flow:

I had a friend with a welder help me weld it in place and of course once we had the old cat off, you could see that it was indeed plugged.

I knew that it was plugged, and I knew replacing it might help the car run better because it would be able to breathe better.
I still felt like the plugged cat was more of a symptom of the problem, and not the problem itself.
It did help very little. The misfire was still happening as soon as I tried to drive up any hill, or any time I tried to accelerate quickly. It was weird though, because the engine would rev in park or neutral, but bog down in drive. Of course, I didn’t drive it much to test it, only around the block once or twice because I didn’t want to ruin the brand new Catalytic Converter.

I ordered this cheap but awesome OBD2 Dongle on Amazon:

I then downloaded the OBD2 Fusion App for my iPhone so I could see the fuel trims for each bank, and verify certain sensor operation etc. I wasn’t sure about all the readings, but I knew that the short term fuel trims were very high – especially on bank 1. Everything else seemed to be working properly, and I was glad to finally have the ability to read the fuel trims and get MAF sensor data.

But, this is when I decided I might not get it figured out. I was beginning to think that it had beaten me, and that I was going to have to kick this car to the curb, or sell it for parts. I wanted to cut my losses and maybe try to recover some of the money I had spent on parts.

Feeling desperate, I posted my problem on a Toyota Forum in the Avalon section, along with the PID from the OBD Fusion App.
I got a couple replies about the timing and ignition, and I thought that I might need to replace all of the coil packs. I had previously decided that I wasn’t going to replace the ignition coils because it was unlikely to have all 3 coil packs fail on one bank. I knew it wasn’t impossible, just unlikely.
In order to be sure there wasn’t a problem with the ignition coils, I swapped all 3 coil packs on the rear bank with the 3 on the front bank. This didn’t cause a change at all, so I felt like I was able to rule out the Ignition coils.

One person from the message forums suggested that it might just be a very hard to find vacuum leak (although I knew it would have to be a pretty large leak if that’s what was causing this misfire). They even suggested a video on how to make a DIY smoke machine to test vacuum leaks.
I went ahead and made the DIY smoke machine – drilled two holes in a jar lid and put two pieces of hose in the holes and sealed them with silicone. Then, light a piece of charcoal and put some cardboard in the jar to make smoke. Then connect one hose to the intake (I used the brake booster inlet) and blow compressed air through the other hose. It worked great, but there were absolutely no vacuum leaks at all. I even propped open the throttle plate with a screwdriver and no leaks at all. I did this test twice just to make sure I was getting enough smoke inside the intake to get any leaks to show up. Still no vacuum leaks at all.

One afternoon while a friend was helping me try to figure out what was wrong, I noticed when he would put the car in drive or reverse that the whole engine would shift forward or backwards much more than normal. I discovered that the ‘Dogbone’ motor mount was completely broken. I thought that maybe that is causing the vacuum leak or short in a wire somewhere causing the misfire when it’s in gear.

Another forum member said he remembered reading somewhere that a bad motor mount can cause a faulty ground wire on the firewall.

So, I spent a bit of time checking and inspecting every ground cable I could find under the engine. Everything looked nice and secure.

I figured that I might as well replace the upper motor mount since I had already spent so much money on other parts, so I ordered this one here:


Again, I was stumped. Standing in my driveway, trying to figure out how to get rid of this car that had been consuming every ounce of my free time, and keeping me awake at night.
I was standing there staring under the hood at the engine for a good amount of time. I kept staring at the Fuel Damper on the fuel rail. I had previously suspected that maybe it had gone bad, or was plugged, but after all the research and reading I did, the information I found on the Fuel Damper said that it would leak when it went bad, and it wasn’t leaking at all.
Randomly, I still decided to take it out and see if there was a way to inspect it, and/or see if something in it was plugging the fuel rail. (In the back of my mind, I kept going back to the fuel pressure test when the pressure seemed to jump when accelerating).
I took it out, and as soon as I pulled it out of the fuel rail, I noticed that the small phillips screw in the center of the diaphragm was sticking out. I hadn’t unscrewed it, and it wasn’t sticking out when the Fuel Damper was sitting there on the fuel rail.
Here’s what I think happened, and why that screw was sticking out:

When I bought this car, the previous owner didn’t hardly drive it, and couldn’t find any of the maintenance records, but I could tell that the Idle Air Control Valve had been replaced recently. I think that before replacing the IACV, the ‘Mechanic’ thought they could fix the low idle by ‘adjusting’ the fuel pressure by unscrewing the center screw on the fuel damper.
According to Injector-Rehab.com “A fuel pulsation damper is a device used to regulate the oscillation of fuel caused by the injectors opening and closing and smooth this out.”
This isn’t something that you need to adjust, and the screw is just supposed to hold the diaphragm in place so it can do it’s job. The screw was completely backed out and wasn’t even attached to the diaphragm (or whatever is inside this thing). I was able to grab the small screw with my fingers and it came right out.

I decided to not replace it, I just screwed the small phillips screw back into place and put the fuel damper back on the fuel rail

Started it up and VROOM VROOM! Runs just like a new car!

The fuel damper diaphragm was just fluttering around or moving in a way that was blocking the fuel rail for the rear cylinders which is what was causing the fuel pressure gauge to bounce up and down.

All said and done, I am very happy that I didn’t give up, and I am very pleased to have a great reliable car for my kids to learn how to drive.

I hope this post will help someone avoid the anxiety and stress I went through, or at least help diagnose a similar problem.

Thanks for Reading my Post.

Please subscribe to my YouTube Channel by clicking the link below, and look for some new videos on this vehicle in the coming months!


Also, Here’s a few other things that I purchased for this car:

Dan the Fix it Man