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Running lean/poor performance

I am a college student with a 1999 Toyota Tacoma. Couple problems. First: check engine light is on with a code of running lean. Second: when accelerating, it feels as if there is little power, or something is being restrained (I liken it to not breathing fully). Thirdly: On cold start up, RPM’s bounce at about 500 - 650, and sounds as if its about to stall. When it warms up, the RPM’s go back to normal at ~1000. I’ve changed plugs, wires, and cleaned MAF. My next thought is a bad O2 sensor. Before I drop 90 of my hard earned and very scarce dollars, I would like some confirmation. Thanks

Catalytic converter failure comes to mind, fuel pump/filter failure comes to mind. Oxygen sensor should be cheaper than these and easier to replace. If you are going to do it yourself, get a Denso or NGK brand oxygen sensor (factory recommended brands and part numbers).

Have you ever used Lucas fuel treatment?

I can confirm for you…that unless you do some troubleshooting you’re likely to keep putting new parts in fruitlessly.

My money is on the fuel pump or fuel pressure regulator. Sounds like th epoor thing is starving for fuel. Try checking the line pressure for starters.

Thanks for the replies. A mechanic told me it was the upstream O2 sensor- which I replaced. 20 miles later no check engine light, and shifting feels “tighter/smoother”, so it may have fixed one problem-so far. The problem is that now the hesitation more pronounced and the acceleration is non - existent.

To describe a little more. If I hammer the accelerator rpm’s increase, but seemingly at random rpms hold as if I let up slightly on the gas, then it might continue accelerating, then doing this again. On a throttle blip, it hesitates or bogs for a second before picking up.

Fuel filter?
I did run techron fuel injector cleaner through, not sure how helpful it was.

CHANGE the fuel filter. then have fuel pressure checked if problem persists…

Thanks for the reply. I’ll update after its changed.

Alright- its been awhile but I finally changed my fuel filter. Acceleration is still terrible. The fuel filter did have dirt-y fuel coming out of it when shaken so I’m sure it has helped somewhat - but not the cause of my problem.

Note the CE light is off because I pulled the neg battery terminal. I am expecting this to improve somewhat after its driven for a bit and the Running lean code is thrown again (I’m sure it’ll happen).

I did also check the voltage of the MAF compared to what the FSM says it should be and it checks out.

Also- gas mileage has improved about 1 mpg since O2 sensor change.

Any thoughts as to what it could be now? I’m baffled.

Any number of issues can cause a lean condition from MAF sensor, O2 sensor, engine coolant sensor, etc. Anything that tells the computer what the conditions are and is out of whack will cause the engine to run lean. Also could be fuel pump but I’m not sure how a converter would cause a lean condition. When they hook the diagnostic computer up to it, and test for fuel pressure, they will be able to tell what the sensors are reading and if they are correct or not. Outside of replacing a multitude of parts just to see, its hard to avoid a computer scan. I had a lean condition traced to a new MAF and only way I proved it was to substitute one out of another one of my cars.

HOW did you clean the MAF? One fellow said he used a cotton swab. That, of course, was futile. Use an MAF Sensor Spray Cleaner, available from auto parts stores.
The MAF sensor tells the engine computer how much air is going into the engine. At different engine speeds, the output voltage of the MAF Sensor will change as the air flow changes. You can measure this voltage by back-probing the electrical connector terminals with straight pins and the voltmeter.
There are tables of air flow values for different engine speeds. Your measurements should be compared to those expected voltage values.
The throttle position sensor (tps) tells the engine computer where the throttle is and how much it is moved. The engine computer uses this data (and others) to control fuel flow. You can do an ohms check on the tps with the engine OFF. Watch the ohms as you s-l-o-w-l-y move the throttle. There shouldn’t be any spikes or dips in the ohms indications. The ohm values should vary smoothly.
An EGR valve which sticks open can cause a lean condition. The checks are in the repair manual.
You can check most of the sensors, and valves, with a digital multimeter. When you check it, and it checks good, you know you won’t have to replace it to find out if it is “good”. Which is cheaper, and surer? You DO have a repair manual, don’t you? A Chilton’s, or Haynes, or something? Knowledge and instructions are much cheaper than a handfull of parts.
Go to this site, register, and check on the menu for information and instructions of the various components:

Cleaned the MAF with MAF cleaner, just by spraying it. Yes I have a Chiltons. TPS checked out good. Next step is to check EGR valve, and after some reading I thought maybe the PCV?

Still has the tendency to stall occasionally on a cold startup because the RPM’s go so low. Given a couple minutes, revs up like nothing happened. Although the acceleration is better onced warmed up, it is still nothing like it should be. Thanks for all the replies.

By all means check the EGR valve, leave no stone unturned, however if the EGR were the problem it would accelerate properly.

I’ve gone back ovet the thread and didn’t see anywhere that you had checked the pressure in the fuel line…did I miss something? Until you know you have good fuel pressure IMHO you’re wastng your time playing with everything else. I still suspect the pump or pressure regulator. And right now I’m leaning toward the pressure regulator.

A fuel pressure test gauge is one of the easiest things to connect. The one problem may be tapping into the fuel line, or connecting to the fuel rail. You may need an adapter. Still, $40 for a fuel pressure test gauge is a lot less than an MAF which is changed experimentally, “to see what happens”.
Here are some instructions, and a video! to help you do the fuel pressure tests:

Thanks for the replies and the link. I’ll try to get this checked out in the next couple of days and post the results.


Well I did what I should have in the first place and let a toyota mechanic drive it. First thing he said was the MAF sensor, and even though I had cleaned it, evidently not good enough. There are two more wires not immediately visible in the plastic housing of the sensor that I missed. Five minutes and some Brakleen later, the truck runs perfectly.