Problem with my 2002 Tacoma 4cyl. please help!!!!!!!!!!

I have a 2002 Tacoma 2wd reg cab with a 2.4L 4cyl. One day while driving I came to a stop when all of the sudden the engine stumbled then stalled on me. When I tried to start it again it would just run for a second then stall. I had to keep my foot on the gas to keep it from stalling again. After I got the truck home I began to investigate what went wrong. The check engine light never came on.

When I can get the truck to idle the engine runs really rough and I can see black smoke coming out of the exhaust telling me its running very rich. I changed the sparkplugs and checked the compression of the engine and its within spec. I also changed out the coils packs that connects to each spark plug. At this point I assumed it was the MAF sensor so I replaced it but the problem still remains. I also replaced the air/fuel & rear oxygen sensors but that didn't seem to help.

I hooked up an OBD2 scan tool to monitor the various sensors while the truck is idling and noticed the ecu is retarding timing which I'm assuming is causing the engine to run rich & rough idle. The check engine light never came on so I don't know why the ECU is pulling back timing.I notice when I unplug the MAF sensor the truck runs normal. I'm guessing the ECU is running in "limp mode" at this point but when I reconnect the MAF sensor the problem comes back. Eveything is telling me the problem lies with the MAF sensor but I've replaced it with a brand new one. I even tested out the old MAF sensor on a friends Tacoma and it runs fine.

Does anybody have any ideas to what the problem could be? I'm at a loss................

Reply to
gmiyahira
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Did you verify in the data list that the coolant temp sensor is telling the computer the proper temperature? Usually when they go bad they report absurdly low temps (-44F), causing the ECU to go back into open loop mode and richen the crap out of the mixture. This, of course, is not good when the engine is already warm.

Reply to
qslim

The coolant temp looked normal. At full operating temp the scan tool showed between 170-180 deg. I also checked the TPS and it's within spec. Could the problem be with the knock sensor? Remember the truck isn't throwing any codes.

Thanks

Reply to
gmiyahira

Here are the results from the scan tool. This is with the truck idling

TPS (%) - 13.3%

RPM - 507

Eng. Load (%) - 100.0%

MAF - 3.71

Coolant Temp (F) - 163F

Intake Air Temp (F) - 66F (same as ambient temp.)

Ignition Timing - -9.5 deg

Long Term Fuel Trim (%) - 0

Short Term Fuel Trim (%) - -20.3

Fuel System Status - Closed

Heated Exhaust Gas Oxygen Sensor (bank 1, sensor 2) (V) - 0

Heated Exhaust Gas Oxygen Sensor (bank 1, sensor 1) (V) - 3.0

Note the Ign. timing is at -9.5 deg. and the MAF sensor voltage is at

3.71. Also note the fuel trims.
Reply to
gmiyahira

Why is the calculated engine load at 100% at idle? Have you tested the throttle position sensor?

Reply to
qslim

I don't know why it's like that. According to the scan tool the TPS is within spec.

Reply to
gmiyahira

100% engine load at idle is not within spec !
Reply to
Mike

Exactly. The fuel management is percieving that the engine is fully loaded (or requesting full load), and is matching fuel delivery accordingly. Does that figure change at all during run, or is it always pegged at 100%? Also, your MAF data line, is your scanner providing that format in grams/sec? And when you say the TPS is fine according to the scantool, are you going by the dataline change compared to your foot on the throttle/ fact that no codes exist/ or have you checked the TPS with a meter to verify voltage readings?

Reply to
qslim

It's usually pegged at 100% when I have the MAF sensor connected. When I run the truck with the MAF sensor disconnected "limp mode" the load is around 20% (within spec)

I'm not sure about that. I think its just measuring the voltage coming from the MAF sensor signal wire.I know with the trucking running normal the voltage is supposed to be like 1.3-1.6v at idle give to take but now is at 3-4v which is not right. I did replace the MAF sensor so its not the problem.

The way I checked the TPS was according to the FSM with the throttle fully closed it should be between 7-11% and fully opened it should be between 65-75%. I hooked up the scan tool and while the truck is running I depress the throttle and check the % by the scan tool. With the throttle closed it measures 11% and when the throttle is fully opened it measures 75%.

Reply to
gmiyahira

We have a late model Ford SUV in the driveway that did a very similar thing. In our case, it was the MAF sensor. We proved it by creating the same symptoms by disconnecting the new one while the motor was running. I wish I could tell you where the wires go, but since you've swapped the MAF with another, and the "bad" one works in the other car and the good one fails in your car, then I'd be looking to the wire harness for the problem.

Reply to
Jeff Strickland

Looks to me that the Bank 1, Sensor 2 02 sensor (or the cable) has gone south.

0V is a absurdly lean reading, so the ECM is trying to compensate by enriching the mixture.
Reply to
Andy Hill

I have replaced both the Air/Fuel & O2 sensors and checked the wiring from the sensor plugs all the way back to the ECU. The problem still remains.

When the truck idles in limp mode the scan tool shows that both the Long Term & Short Term fuel trims are at 0. I assume at this point the ecu isn't reading any feedback from the A/F & O2 sensors. But when I run the truck with the MAF sensor plugged in the Long Term fuel trim stays at 0 while the Short Term fuel trim is at -20. I'm guessing thats why the ECU is pulling back timing and dumping fuel into the engine. Can someone explain to me what Long Term fuel trim exactly is and what could cause the Long Term fuel trim to stay at 0 and not change?

The most fustrating part of all this is the ecu is not throwing a CEL so I don't know where to look for a problem. The only time when the CEL light comes on is when I disconnect the MAF sensor so the truck would run on limp mode.

Reply to
gmiyahira

I considered that there might be a short somewhere so what I ended up doing was remove the entire engine harness from the truck and do a continuity test from each sensor plug to the ecu harness and every single wire checked out fine.

Reply to
gmiyahira

The bank 1, sensor 2 does not affect the fuel mixture. It's only purpose is to monitor the efficiency of the catalytic converter.

Reply to
Mike

You need to check the harness while it is installed in the vehicle. You would be looking for a wire that is shorted to power or ground.

Reply to
Mike

Fuel Trims

These two figures show fuel trends in two different ways. 0% is the theoretical stoichiometric fuel mixture as programmed by the engineers. Any change represents the amount of fuel that is being added to or taken away from this "perfect" value. A + reading means fuel is being added to the mixture (this would be because of a lean condition), and a - reading means fuel is being taken away. In your case the engine is running rich, so the computer is saying that it has reduced the injector duty cycle by

20% because there is too much fuel. So + is lean, - is rich. I've been told by product engineers that a deviation either way under 10% is normal. Since these engines run in the real world, the computer is constantly adjusting the trim based on feedback from other sensors.

Long vs Short.

Short - short trim is a representation of how the engine is running right this very second. In your case of -20% short trim, this means the engine is trying to take away as much fuel as possible (the max is around %25) and is actually not getting enough air. Long - Long is a representation of how much the trim program has had to adjust from its base programming in order to get the short trim back to zero over a period of operation. For example a short trim of around 0% but long trim of +20% would tell us that the engine at the moment is running fine, but in order to get there the computer has had to compensate by changing the fuel program dramatically. Usually what you see when you have a problem is the short fuel trim will peg in one direction first. As the computer slowly adjusts, you will see long trim creep in the same direction in order to pull the short back to zero. In this case, I would expect that if you left the truck running for a while the long trim would eventually reach the neighborhood of -20% as well. I would also expect to see codes po170 (fuel trim malfunction) and p0172 (system rich). Anyway, I hope this all makes sense.

Reply to
qslim

No, the negative fuel trim shows that the computer is leaning out the mixture, not enrichening it.

Reply to
qslim

What is the best way to check for a short?

Reply to
gmiyahira

Use an ohm meter. Disconnect wiring harness at both ends. Put one probe on a good ground, use other probe on each end of harness and look for continuity to ground. To test for a short to power use above test but move the probe from ground to positive battery post.

Reply to
Mike

With that information have you checked you air intake, beyound the filter element for cloggs? If the engine is not getting enough air you could have a leak.

Reply to
Tony

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