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Chris (aka-Moose)
Chris (aka-Moose), Auto Technician
Category: Toyota
Satisfied Customers: 947
Experience:  16 years of experience
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I have a well-maintained 2004 Toyota Tacoma (6-cyl, 4dr)

Customer Question

I have a well-maintained 2004 Toyota Tacoma (6-cyl, 4dr) with 150,000 miles.
Recently, about half the time I accelerate (from a dead-stop or even on the freeway), the acceleration bogs down, rpms drop, and it feels like my truck is about to stall. Then the acceleration picks up, and the car drives normally.
I took it to a Toyota mechanic, who said he found reduced compression and coolant leakage in two cylinders. He did a top-end engine rebuild (including new gaskets, resurfacing valves, new cylinder head bolts, new air filter, fuel filter, new belts including power-steering alternator, waterpump, AC, and idler drive belt, new spark plugs and spark plug wires, ), and the same problem happened again as soon as I drove 10 miles.
I sure don't want to bring it back to that mechanic, but would like some ideas about what else might be the problem, before I take it to a different mechanic.
Submitted: 1 year ago.
Category: Toyota
Expert:  Genchi Genbutsu replied 1 year ago.

Thank you for allowing me to help you with this concern today and am sorry to hear that this is happening. I will have to ask you a few questions so that I can get a better understanding as to what may be going on with your Tacoma.

That sure is a lot of work for not fixing your problem. Are there are warning lights on or codes that were stored?

Customer: replied 1 year ago.
No, unfortunately. No codes. At first we were suspecting a problem with the catalytic converter, but there is an engine code for that sort of trouble. Again, no error codes pointed to catalytic converter or anything else.
Customer: replied 1 year ago.
No 'check engine' light either.
Expert:  Genchi Genbutsu replied 1 year ago.

The catalytic converter is a good idea but as you said there are/were no codes. The two areas that I would suspect and look towards are the MAF sensor and the Fuel Pump. This type scenario doesn't really lend itself to a mechanical or an ignition related fault.

Customer: replied 1 year ago.
MAF sensor is error code P0101, which is not occurring on my truck.
Expert:  Genchi Genbutsu replied 1 year ago.

Yes, but what about codes P0100, P0102 and P0103? :)

So you think that if a code isn't showing up then that component is good?

Just trying to pick your brain and see where you stand as a Diagnostician.

Customer: replied 1 year ago.
I am actually asking you for your advice. From the Toyota forums, it's not likely the MAF would go bad without a code being shown, and I would expect to see the 'check engine' light.
Expert:  Genchi Genbutsu replied 1 year ago.

Ok, I can see that you have already spent considerable money with a Toyota mechanic and it wasn't fixed. Now you have read some forums and are going to compare my thought process to Toyota Forums. I am not going to be able to compete with that. You have my thoughts as to where I would go with this.

First, check the fuel pressure and then if needed, look towards the MAF. Those are two places to start with. Also if you have access to a Toyota TechStream Scantool, put the Taco in "Check Mode" and see if you can get any codes to set under the trip one lowered criteria.

I will not submit this as a rated question but will opt out, leaving you with my ideas and the advice from your Toyota Forums.

Expert:  Dj replied 1 year ago.

Hi, I'm DJ.

It sounds like there's a misunderstanding about how the failure codes work. If a sensor, like the MAF (Mass Air Flow) sensor is out of range it the ECU (computer) will detect it and set a code in memory and turn on the check engine light.

But if the sensor is putting out an incorrect reading but the reading is still in range, the ECU will not set a code because it doesn't know that something is wrong.

Let's say that the ambient air temperature is 70' and the MAF sends a signal to the ECU that the air temperature is 35' the ECU doesn't know that the information is wrong. So it makes adjustments based on wrong information.

There is also no failure code for fuel pressure. So if the fuel pump or the fuel pressure regulator were failing, the ECU wouldn't know it and therefore wouldn't set a code.

So the fact that there are no failure code narrows the items that can be causing the problem.

The first step would be to monitor the fuel pressure while the vehicle is bogging down. If the pressure stays steady and in range, the fuel pump and everything related to it is good, move on to monitoring the live inputs from the sensors.

With one person driving and another looking at the live inputs you can see what the computer sees. If the live data shows that the outside temperature just dropped 50 degrees or it shows that you took your foot off the throttle even when you didn't, you'll see the problem.

Don't allow any mechanic to replace parts without a clear and specific reason for doing so.

Please let me know if you have any additional questions on this or if there is anything more I can do to help you.


Customer: replied 1 year ago.
This was a much more helpful and less argumentative answer, thank you.Please close this case - my first experience with JustAnswer was unpleasant, and I don't wish to get any more answers.