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Discussion Starter · #1 ·
So I work on cars and trucks for a living, and I have for 13 years or so professionally, but I'm not perfect and I run into diagnostic BS all the time. When I bought this baby, she had 338k miles on her, I did the usual stuff, new pads and rotors in front, drums and shoes in back with wheel cylinders, bled and flushed, had to replace 3 idle pulleys for whining noise, the U-joints we're awful and causing vibration at freeway speed when you let your foot off the throttle so I replaced them, did the spark plugs, valve cover gaskets, put a new front clip on and head lamp due to a dear being hit by previous owner, and an O2 sensor and a cam sensor to pass inspection which I narrowly did, 5 minutes later the catalyst system below efficiency MIL came on. Put a battery in a year later after the Texas ice storm of February 2021. Almost 2 years later now, my first symptom was bogging while going over hills on the freeway, so I went ahead and put new cats on it, bank 1 and 2. The problem got a bit worse, now what it does it basically starts ok and runs ok going across town, say 30 or 40 minute drive across Houston. But then as now it is warmed up, when I go to restart it, it has a flashing check engine light and feels like it's misfiring once I get it running, at first it may die the first time I start it. Then the throttle is largely or completely unresponsive, unless I turn it off then back on a few times and give it a bunch of gas, once I start it 3rd or 4th time and the check engine light is solid again it will pretty much get me back home, but it still struggles to accelerate smoothly past 70, unless I dump the pedal pretty far down, and then it accelerates pretty quickly up to or last 80 or 90, but it should do that smoothly and naturally without downshifting. It's like turning it off and back on a couple times resets it or maybe it just takes that long to overcome whatever imbalance is going on. The codes are PO171 and PO174 which are lean codes bank 1 and 2, then it also has the PO300 multiple random misfires, that's pretty much it. I replaced the MAF 2 days ago, nothing. Does anybody know if this could be the fuel pressure regulator or maybe some kind of 2 speed fuel pump issue? I've also cleaned the throttle body, took it out to clean it. I don't want to throw any parts at it, it's got 385,000 miles now, any experience with this 4.0 and this issue please don't hesitate to offer an idea. Thanks in advance.
 

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Good old diagnostics are going to help you here. Being that you have lean codes and it appears that you are having random misfires at startup when an engine generally uses more fuel when cold, I would zero in on the thing that would affect all cylinders and see if you are getting proper fuel pressure AND volume. More than likely that would be an original fuel pump as they last a long time in toyotas but it could simply be worn out. That's were I would start. I don't have specs so you would have to look this up.

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Discussion Starter · #3 ·
Thank you, and yes usually I'd have a fuel pressure test kit and would have done this already. Probably going to buy a new set and just add it to my box. The Matco truck guy has started visiting me lately, I'll get a set from him. Also planning on replacing the throttle body along with the fuel pump, I'm about 95% sure it's one of the 2, or else the ECM is glitching which would literally be the last thing in my line of process of elimination.
 
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