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Discussion Starter · #1 ·
I have a lot of engine hesitation, or a bog-down feeling when accelerating, as if it is not getting sufficient fuel. I was almost convinced to change out the filter/pump, which is in the tank on my model ('05 Tacoma). That job is no picnic (plus expensive!), and I do not want to undertake it unless I am convinced it is, in fact, a clogged filter. The reason I am doubting this particular analysis of the problem is because it will only hesitate when I am not kicking it into a lower gear. The "passing gear" will never cause the engine to hesitate at all, and I seem to get all the power I need. Since there is often more fuel required for such a demand, then a clogged filter would simply not allow it. And engine light has appeared three or four times. My OBD code reader indicated a lean fuel situation. Probable causes were vacuum leak or clogged filter. I reset the CEL, first time, but it reset itself after that, indicating to me that the lean fuel error occurs only when a lot of demand is placed on the engine (like when I vacationed in the mountains). Any ideas? I have changed the plugs, and visually inspected all such things as vacuum lines. All other scheduled maintenance has been maintained properly. Would 10% ethanol cause this problem? Forgot to mention.....cleaned the MAF more than a couple of times.
 

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I would think an O2 sensor maybe? You are right about it not being the fuel filter/pump because more fuel is required at the higher RPMs. The ethanol might cause the problem but I doubt it. Sorry I'm not of more help.
 

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E10 fuel won't cause it. Toyota's short term and long term fuel trims adjust the AFRs as needed, but that's assuming you have a working AFR sensor to let the ECU know. Normally, though, the ECU will throw a code for the AFR sensor because they have a programmed response time and are monitored actively.

AFR sensors are basically wideband O2 sensors, and are considerably more expensive, so don't go out and get one if you don't need it.

I'd do a fuel pressure test to single out the fuel pump or fuel pump flow. For a return system, it should be about 40-45 psi, and for returnless about 48-50 psi. I think both engines in the Tacoma use a return type fuel system. If you can, test before and after the filter. If the pressure is low after the filter, you have your culprit. If it's low before the filter, ouch, your fuel pump is going out or the fuel pressure regulator is toast and sticking open. Full unregulated pressure should be over 60psi (likely around 80psi or more for a healthy pump). If the kit has it, use the supplied FPR block off bolt to test full pressure or just clamp the lines.

Higher elevation will definitely lean the AFRs a bit, but it shouldn't throw a code unless you have a fuel supply issue to begin with.

Short the FP and +B connectors under the "diagnosis" cap to supply power to the fuel pump while the ignition is switched ON.
 

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Discussion Starter · #5 ·
Just returned from Thanksgiving holiday, and was continually annoyed with the lack of fuel/power throughout the trip. Your suggestions, Jason, will be very helpful, and I will analyze the fuel pressure as soon as I can get to it. One question, though, is where to attach a pressure gauge? Is there a fuel line tap, in both lines, for such a test? Is there any special gauge that's recommended? Answer to Gregory....I changed the plugs less than 3,000 miles ago. The skipping took place prior to changing plugs, anyway. O2 sensor should not be a problem, as no error code shows up regarding it, and gas mileage remains a fairly constant 20.5-21 mpg. I did receive another lean fuel error code during my 1000-mile trip, but it extinguished itself after a few startups. Am I correct in assuming that it would take more fuel for a kickdown into "passing gear"? Maybe, maybe not, depending on how much demand is placed on the engine, I'm sure. By the way, I experienced the frightening ordeal of the pedal-being-lodged-into-the floormat, during this trip. Even though I was quickly aware of the cause, it occurred at a very inopportune time and was difficult to resolve the problem until I passed about 90 mph in some rather unpredictable traffic! It's odd that the situation never occurred until a week after receiving the recall notice. I am glad that I understood the problem beforehand.
 

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I ran into lean conditions right after buying the TRD blower....running the trd reflash only, no tune.

Anyways, this lean issue melted my cats and clogged the exhaust, causing very strange hesitation's and in some instances, no throttle at all. Cut those cats off and everything was fine again and later added a tune to correct this.

Im not saying its your cats, but that is a possibility.


As for the floormats issue. What brand floormat do you have? Ive checked my stock trd offroad mats and they are far enough away from that mat.
 

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Discussion Starter · #7 ·
Don't know the brand of floormats.....I assumed them to come with the truck (I bought it from stepson when it was a year old). I ordered the clips that hold it in place. For some reason, no clips were ever there. It now seems to clear the accelerator enough that sticking shouldn't be a problem again.
 

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Just returned from Thanksgiving holiday, and was continually annoyed with the lack of fuel/power throughout the trip. Your suggestions, Jason, will be very helpful, and I will analyze the fuel pressure as soon as I can get to it. One question, though, is where to attach a pressure gauge? Is there a fuel line tap, in both lines, for such a test? Is there any special gauge that's recommended? Answer to Gregory....I changed the plugs less than 3,000 miles ago. The skipping took place prior to changing plugs, anyway. O2 sensor should not be a problem, as no error code shows up regarding it, and gas mileage remains a fairly constant 20.5-21 mpg. I did receive another lean fuel error code during my 1000-mile trip, but it extinguished itself after a few startups. Am I correct in assuming that it would take more fuel for a kickdown into "passing gear"? Maybe, maybe not, depending on how much demand is placed on the engine, I'm sure. By the way, I experienced the frightening ordeal of the pedal-being-lodged-into-the floormat, during this trip. Even though I was quickly aware of the cause, it occurred at a very inopportune time and was difficult to resolve the problem until I passed about 90 mph in some rather unpredictable traffic! It's odd that the situation never occurred until a week after receiving the recall notice. I am glad that I understood the problem beforehand.
I'm not sure what type of connector the Tacoma uses from the fuel filter to the fuel rail. It's probably a quick connect like my 1MZ, but I'm not sure. If it is, you'll have to either use a special adapter for the Toyota quick connect line, or you can just tee into the line from the pump to the filter (AN connector, IIRC) instead of teeing into the rail line with the quick connect.

You assume correctly about fuel enrichment during kickdown/acceleration. Toyota programs its ECUs to asynchronously inject fuel during acceleration to ensure adequate fuel supply. If you have the 1GR V6, then all 6 injectors fire regardless of crank angle during that time. After that, the ECU resumes fuel corrections from all sensors. So, that's likely why the lean code cleared.

The fact that the lean code clears on its own is an indication that the AFR sensors are functioning normally.

If the fuel pressure is good, then I'd look into the possibility of clogged injectors, a poorly functioning MAF, clogged cats (as said above), or an ECU issue.

For clogged injectors, I highly recommend removing the injectors and sending them to either RC Engineering or WitchHunter. RC is based on the west coast, while WitchHunter is on the east. I had my 1MZ injectors cleaned by RC Engineering, and while they only had 72k miles on them, they still registered a 10cc increase in supply, a better spray pattern, and had better balance across the board (0.8% variation; awesome). Worth every penny, IMO.
 

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Discussion Starter · #9 ·
It looks as if a large part of my problem was with the K&N filter that replaced the standard paper filter inside the breather. I have kept the K&N clean, probably at intervals shorter than recommended. But it made a LOT of difference to pull it out and install a common Fram unit in its place. I still have a slight hesitation, but so seldom that I hardly notice it without paying close attention. It isn't a skip, as in misfiring plugs, but it feels as if it starves for fuel or air. As I drove home, the other day, I began to wonder about that filter. I pulled over as soon as I could and removed the K&N filter, leaving no filter at all for the half hour drive home. It showed little, if any, improvement right away. But as time went on, my acceleration went through a vast improvement. Long term fuel AFR fuel trim and compensation by the computer, perhaps? The next day, I installed the Fram filter, and so far, everything seems to be working a lot smoother. I will make another long trip during the upcoming weekend, and will pay close attention. If there is still minor hesitation, my suspicion will be directed more toward the injectors, as you recommend, rather than system pressure. At 137k miles, I am sure the injectors could undergo a cleaning job. Thanks for all the replies, so far......Bob
 

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Toyota MAF sensors don't like oiled filters. I used a Purolator replacement filter. It's the exact same construction as the OEM filter, so it's a better deal as well. Purolator is one of Toyota's tier-one suppliers.
 

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Discussion Starter · #11 ·
Interesting. Next filter buy will have to be Purolator. For now, it appears that my jubilation was a bit premature. The problem still persists. Initially, the hesitation nearly went away soon after removing the K&N filter. Though it's back to the sluggish acceleration, it doesn't seem nearly as bad as before. I'll clean the MAF again, now that the oiled filter is out of there. Perhaps there is a film of oil on the little sensor bulb. If so, I could see how it might send false readings to the computer. A trouble code indicating a MAF error popped up a few months ago, but it never came back after cleaning it with MAF sensor cleaner. I have a few suggestions from this forum, that I will follow up whenever things slow down a little. All suggestions have been helpful, even if they didn't directly fix the problem. It is good to learn of real life experience about all that goes on within a system. Thanks.
 

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Finally!!!!!!!!!!!!!!!!!!!!! Someone has the same problem as me. I too feel the hesitation at exactly 2500 rpm in my 6spd manual 4.0L v6 2005 Tacoma (91,250 miles). I have gone over everything and even taken it to 3 Toyota techs just to find out that they have no idea what is causing the problem. My problem started approx 6 months ago and has since increased in severity from where only I could notice the hesitation through the accelerator to where any passenger w/o prior knowledge of the problem can detect it from the passenger seat during acceleration (head bob).

Have you taken note of your RPMs when the hesitation occurs?
 

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Posting because I'm having the EXACT same problem with an '07 Tundra. The dealer has had it for three weeks, and still doesn't know what's wrong. No error codes show up, and they don't know where to start. I'll post if I remember and if they manage to fix this!
 

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Lean fuel condition + mid-acceleration issues + no hard-acceleration issues + no trouble codes... why haven't you check for vacuum leaks as your scanner suggested?
In most cases if you have a faulty component, a trouble code would set indicating some type of "out of parameter" reading for the component (or related components) Short of that, most issues are basic mechanical failures of one type or another..
Ported vacuum (open throttle) leaks can be difficult to locate, but are relatively easy to detect/diagnose..
 
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