Troubleshooting and Diagnosis of Toyota Tacoma Pickup Stumble and Driveability Problems

Our 1998 Toyota Tacoma 4x4 pickup runs very well, except for a stumble and occasional backfire through the intake manifold when under full throttle at 1800-2000 RPM. The stumble could be ignition, emission control, sensor or computer failure, or even a possible mechanical failure. Here are the steps we took to diagnose the cause of the stumbling and eventually cure the problem. Note that your own diagnosis may be quite different based on the age and condition of your vehicle. The process of elimination is useful in any case.

  1. The first thing we did was replace the spark plugs. The old plugs were sooty, indicating possible rich mixture.
  2. We then checked all spark plug wires for continuity and proper resistance, which is supposed to be about 10k Ohms per foot. They were all fine.
  3. Next we ran the engine with the hood open at night, looking for any arcing between the spark plug wires and ground: Nothing.
  4. We checked the air cleaner: It looked fine. Note that a later test (see below) convinced us to replace it anyway.
  5. We then checked the MAF (mass air flow) sensor and the engine coolant temperature sensor.
  6. The engine ran horribly with the MAF sensor unplugged, indicating functionality. We measured base voltage to the sensor and it was 5 volts. As it should be. We disassembled the MAF sensor and carefully sprayed carburetor cleaner all over it. The condenser attached to the MAF sensor is brand new.
  7. Resistance tests done to the coolant sensor indicated normal function: As temp increased, resistance steadily decreased.
  8. We then checked all vacuum hoses for leaks. The hoses were not leaking.
  9. We checked vacuum pressure with a gauge. Everything seems fine.
  10. We checked the vacuum to the fuel pressure regulator. There was none until the engine was revved pretty hard. Albeit weak, the vacuum is there. The fuel regulator gets its vacuum from the resonator on the intake hose. The vacuum in this spot will be weak, just based on where it's located (upstream of the throttle plate and venturi).
  11. We checked the PCV (positive crankcase ventilation) valve: fine.
  12. We checked (vacuum tested) the EGR (exhaust gas recirculation) system and cleaned the little vacuum modulator filter: fine. The test is to open the EGR valve at idle and see if the engine dies. If the valve is stuck closed or plugged there would be no effect. We use a "syringe" to create a vacuum in the hose, and the engine died as the EGR valve opened.
  13. We checked the entire vacuum system. All valves were functioning according to specs: Manifold vacuum was strong.
  14. We replaced the fuel filter. Made no difference.
  15. Ran injector cleaner through the fuel system: Certainly didn't hurt, but no noticeable change in engine performance.
  16. We then received in the mail the OBD II (onboard diagnostics version 2) interface. We downloaded the software onto a laptop and hooked it up to the truck. The first thing that we noticed was that the down stream Oxygen sensor was reading .1 volts with no fluctuation with increase in engine RPM (revolutions per minute).
  17. After several driving cycles no trouble codes are stored in the ecu.
  18. We then replaced the downstream oxygen sensor. Engine ran much more smoothly, but still hesitates, stumbles and occasionally backfires under full throttle at 1800-2000 rpm.
  19. Tests ran on the TPS (throttle position sensor) showed that the ECU (engine control unit, or "computer") thinks the throttle is at 11.5% open when it is actually fully closed.
  20. We removed the TPS and measured conductivity tests across the outer terminals. As we torqued the sensor, simulating acceleration, we discovered a spot where resistance sharply dropped, then picked back up again. This indicated a faulty sensor, so we replaced the sucker.

    *It should be known that I paid $58+tax for the sensor at the Toyota dealer. I called up Kragen auto supply and they had the same part for $160. I then checked the Kragen web site for the same part, to make sure there wasn't any sort of communication breakdown, and sure enough the price was $160.

  21. We replaced the old TPS and the problem persists.
  22. It should also be noted that due to the design of the TPS, once you put the sensor on the throttle body, it gets advanced from its 0% state, to about 11.5%, because the throttle shaft itself is not aligned with the sensor when there is no pressure on the throttle. This seems rather ridiculous. However, the tang on the throttle body which the TPS hooks onto is perfectly parallel to ground. The slot on the TPS, is already at an angle before it is put on. So, when it is put on, it is torqued, causing the 11.5% reading. The ECU apparently does not "normalize" this to 0%.
  23. We checked the TPS on a 98 Saturn, and noticed that it was 0% at idle, contrasting to the Toyota's 11.5%. The Saturn also showed 90% at full throttle, compared to the Toyota at 78%.
  24. We ran an additional vacuum test the fuel pressor regulator, but there was nothing showing on the vacuum gauge when it was attached to the hose leading to the regulator. We sucked on the hose and it seemed restricted, so we cleared out the port on the resonator with a bit of copper wire. It seems to have no effect. Still concerned about the rumored vacuum leak, we unscrewed the MAF sensor and pulled it out of the aluminum manifold slightly while the engine was idling. Air rushed in, and the motor died. If significant air had been entering through a different path to the engine, the MAF would not be detecting much airflow and it should not have made much difference whether it was "cracked" out of it's casing or not.
  25. Another test we did was simply to remove the air cleaner and drive a short distance. This test resulted in less stumble; almost all of it was gone. However, as the engine warmed it returned but it was not as consistent. The most disappointing aspect of the performance was that occasionally there would be a backfire through the intake manifold when we "punched it" at 1800 RPM in the higher gears. The air cleaner was replaced anyway to ensure that it was not a problem.
  26. The final test that was run was run at a repair shop. Before replacing any more parts, we decided to have the engine "scoped" or monitored with an oscilloscope. The problem was located within 5 minutes. Take a look at the inside of this cutaway of a spark plug boot and you will see the problem. The spark was not going through the plug at all times. In high-pressure conditions (under full throttle) it was arcing from the spark plug wire down through the burned "carbon track" (looks like a black "river channel" on the orange silicone rubber boot) to the base of the spark plug. No spark inside the engine at the business end of the spark plug meant no power, and a bad misfire plus backfiring as the spark intermittently jumped off the plug and then back through it, igniting unburned fuel from the previous injector cycle. With the ignition system designed to spark on every revolution as it is in these distributorless designs, a misfire on compression can easily become a backfire on the exhaust stroke - and if the intake valve has started to open, the backfire can enter both the intake and exhaust manifold.

We had tested the plug wires for continuity, but we had not carefully inspected them, or we would have caught this problem. The problem is completely solved, and the truck has smooth acceleration from low rpm's, much better torque, and the backfire is gone.

If you are troubleshooting your own vehicle, don't be in a hurry. Once you have parts off (as we did several times while replacing spark plugs and testing spark plug wire continuity) - inspect them very, very carefully. Of course, we could have just replaced the wires and we would have been successful, but the limited testing we did (looking for visual arcing, which showed nothing because the arc was deep inside the cylinder head where the spark plugs live, and also testing for continuity) did not indicate a problem.

We suspect the problem started in the past when the truck was driven past the time when the spark plugs should have been replaced. If the resistance of the plug increases to a high enough level (defective plug, worn electrodes) the spark will simply find a different path to ground. Once the path has been established, the spark will continue to use it whenever if offers lower resistance than the plug - even a new plug - and you will have a misfire. By the way, our plug wires have the year of manufacture printed on them and the bad wire was manufactured in 2000 - the wires had already been replaced once on this truck! If you are reading this and you have a Toyota truck, and similar problems, pull the wires and have a really good look at them. If you can find a wire from a junked vehicle and "rotate" it through your cylinders, you may catch a similar problem that even a visual inspection could not find. I am not impressed with the lifetime of the factory wires. The next time the wires fail, we will use premium aftermarket wires instead of the stock Toyota wires. If you have the same problem we had, you may wish to consider the same thing. It may help to use premium spark plugs, such as platinum plugs or iridium spark plugs as well, to help conduct the spark through the plug under high compression.

In some related news on the 98 Saturn SW2, lately the engine had been running rough, the gas mileage was dropping slowly, and the power was dropping as well. No check engine lights or other notices were showing. Then the at stopsigns fluctuating idle appeared. The car would almost die on deceleration. After a few hours of troubleshooting, I noticed that the plug wires for the last two cylinders had rubbed against each other until the insulation was almost completely gone down to the conductor. The factory does not install spark plug wire separators on the wires running over the top of the engine in my model (1.9 liter DOHC 4 cylinder engine), and crossfire was killing the spark. I replaced the wires, added some $4 "wiring looms" and the car runs perfectly again, and the power and gas mileage are back. The stalling on idle disappeared too. Saturn Owners, Check Your Wires!!! It's a shame the Factory and the Dealers don't do the right thing and install wiring separators the first time you come in for service.

Good luck! We solved our problems, and we hope you solve your problems as well.



Inexpensive OBD II hardware for using your computer to monitor your vehicle, and free software to go with it.

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