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Toyotas

Toyota Fuel Injection System Diagnosis & Repair

Edited from an article by Bob Dowie, ImportCar, July 2001

The most difficult part of diagnosing and repairing a Toyota fuel injection system is that we have so little experience with it. The system is so reliable that it gets little attention. That’s not to say that there aren’t good, profitable repair opportunities available. Fuel injection has been on these cars and trucks for years and the customers keep them in the family for a long time. So, it’s not unusual to see more than 200,000 miles on these vehicles, even in the northeast where rust usually takes its toll before wear. Eventually, even the most reliable cars will need service in this area.

There are two common fuel injection systems in use by Toyota. Both are similar in how fuel is controlled, but differ in how the ECU gets its load or air flow signal.

One system uses an air flow meter mounted to the air filter housing that is connected to the intake manifold by a hose. As the air is drawn into the engine, the volume of air is metered mechanically by swinging a door open that acts on a variable resistor, that changes the voltage at the signal wire from the ECU. One wire on the air flow meter is for an air temperature sensor that is located in the unit, while the others usually providing a 5-volt reference signal and a ground.

The other variation on this system uses a more modern mass air sensor. Sometimes called a hot-wire system, this sensor has no mechanical parts. Instead, there is a strand of wire that is heated to a certain temperature by the ECU. As the air flows through the sensor trying to cool the wire, more current is needed to keep the wire at the proper temperature. Based on this current, the ECU can deliver the proper volume of fuel to mix with the air. The mass air sensor is considered to be the better of the two versions. Since the density of the air is monitored, not only does the hot-wire system control fuel more precisely, but there is no need for the intake air temperature sensor and there are no mechanical parts to fail or stick.

INPUTS TO THE ECU
The other system that Toyota uses (mostly on the Corolla and Tercel) doesn’t use an air flow meter at all. Instead, it relies on a rheostat-type throttle position switch (TPS) and a MAP sensor to monitor engine load. By knowing the throttle position and the manifold pressure (vacuum), the ECU can deliver the proper amount of fuel for the conditions. Both systems work equally well, with no difference being felt behind the wheel. Although the air flow signal is one of the important signals the ECU needs to make good decisions, both systems need some more information to keep the engine supplied with the proper amount of fuel.

I mentioned the TPS when describing the second system, but both systems use one. They look similar but are not tested in the same way. As always, refer to your repair information source for more details. As the name implies, this switch tells the control unit what position the throttle is in. If the system doesn’t know that the car is idling, it can’t very well adjust the mixture when you tip in the throttle to pull away. The same applies for full throttle. The only way that the ECU knows what you’re asking the car to do is through this switch. You can solve many driveability problems by simply adjusting this switch.

While we’re talking about the throttle switch, it’s a good time to bring up throttle plate binding in the throttle housing bore. The customer will call it a "sticking throttle." Your first thought would be that the throttle isn’t returning. Take a few minutes and question the customer. I’m sure you will find that the problem is that the throttle is hard to push open, usually after the car sits overnight. You have to push hard, almost snapping the throttle open. Sometimes just cleaning the throttle body will seem like it fixed this problem. But before you put the intake hose back on, double-check it by letting the throttle snap closed to be sure it doesn’t stick. If the vehicle is equipped with a dash pot, be sure it is retracted so the throttle can return to its stop. If the throttle plate continues to stick, you’ll have to adjust the stop screw. Open the plate just far enough to prevent sticking. When you’re done, be sure to check that the TPS is adjusted properly.

Another variable that has to be monitored by all fuel injection systems is the engine coolant temperature. This is handled by the engine coolant temperature sensor, or ECT. The ECU sends a reference voltage to this sensor that changes as the coolant warms up, telling the control unit to modify the fuel curve based on temperature. When you’re testing the sensor, look for signs of coolant leakage by the connectors. If any exists, replace the sensor. Dried up coolant will cause a bad connection resulting in an intermittent problem.

The other critical input to the ECU is the oxygen (O2) sensor. It’s located in the exhaust system, before the catalytic converter. Its job is to trim the mixture based on the oxygen content of the exhaust. This fast-acting sensor works to keep the mixture at a 14.7:1 ratio – Just what the converter needs to work efficiently.

ECU OUTPUTS
We’ve talked a lot about inputs to the ECU, but not much about the outputs. Like most systems in the modern car, the ECU serves more than one function. For this article, we are concerned with the fuel control portion. The ECU regulates the fuel flow by changing injector pulse width, varying how long it grounds the injector. Naturally, the longer the injector is open, the more fuel will be delivered. Of course, the ECU controls other functions like ignition timing, EGR and fuel pump operation. All of these functions are controlled by the same inputs we just discussed, so it’s easy to see how important it is that we have proper inputs.

We talked about how the ECU serves the function of the choke with information from the coolant temp sensor, but we still have to get fuel to a cold engine for it to start. Most of the Toyotas we see will have a cold start valve and a start injector time switch. The injector time switch, like the ECT, is screwed into a water jacket, as the cold start valve is not needed when the engine is warm. Its operation is simple. When the starter is engaged, battery voltage is sent to one terminal of the cold start valve and to the timed terminal of the injector time switch.

The other terminal on the cold start valve goes to the injector time switch where it gets grounded, opening the cold start valve. At the same time, current is flowing through a heater inside the time switch, keeping track of how long the cold start valve has been open. At a certain time, the switch will open the ground circuit for the valve, stopping fuel flow and preventing flooding.

Be careful when diagnosing a cold hard-start problem. On some models, Toyota has an alternate ground going to the ECU that provides a 13-second pulse of the cold start valve anytime the coolant is under 113 degrees. Don’t mistake this short pulse for one that may want to be longer. If that short pulse were enough, the car wouldn’t be in the shop. We have seen some plugged cold start valves. If everything checks out electrically, suspect a plugged valve.

SELF-DIAGNOSTIC SYSTEM
Not only did Toyota build a good system, but the carmaker made it easy to work with. Through the diagnosis box under the hood, all it takes is a simple jumper wire to access a reliable self-diagnostic system. Install a jumper as if you were going to check base timing. You might as well check it while you’re at it. With the jumper installed, the check engine lamp will blink out codes. Long flashes are tens with the shorts being ones. Four long flashes followed by two short flashes is a 42. The most common codes we see are for 02 sensors and a switch code that sends you looking at all of the switches.

One of them is the idle switch. The trick here is that if the ECU doesn’t see an idle signal, it won’t allow the timing to go to the base setting. So when you install the jumper, you should hear a change in idle speed and quality. If the base timing is off, remove the jumper wire. The timing should have changed. If this is not the case, check the idle switch. The diagnosis box also gives you a place to monitor the 02 sensor operation (you will find some bad 02 sensors). And, of course, a cracked air intake hose between the air flow meter and the manifold is, by far, the most frequent failure. Always check the air intake hose first.

HIDDEN CULPRITS
One of the most common ailments on the Camry is also one of the trickiest to diagnose. I’ve seen a lot of fuel injection parts being installed on cars by technicians who are trying to repair a problem that is not fuel-injected related. If you have a customer complaining of an occasional stall out on acceleration or a bucking under load, suspect a coil that is cross-firing to the distributor shaft. What makes this tricky to diagnose is that the coil is mounted under the distributor cap. To diagnose, remove the cap and look closely at the coil, at the point where it is closest to the distributor shaft. If the coil case has a bluish tint, you’ve found a bad coil. I suspect that a lot of these bad coils are a result of high resistance in the ignition secondary.

Take a good look at the condition of the ignition rotor, as well as the distributor cap and ignition wire assembly. Don’t forget to also take a look at the spark plugs. A word of caution here – it is false economy to try to save money on Toyota ignition parts. Anything but the highest quality parts will come back to bite you.

It may be the lowest tech part of the system, but the best operating fuel injection system can’t operate without fuel. Don’t try to diagnose a fuel problem without a good fuel pressure gauge. We’ve seen everything from "no starts" to "die out at high-speed" complaints that end up being fuel pressure related. The high-speed problems were pumps with low pressure. The no starts were either bad pumps or wiring problems at the fuel pump brackets. Again, you can use your diagnose box under the hood to jump the fuel pump to check for operation. If the pump doesn’t run, check the wiring at the tank, as it’s not unusual for the wire connectors to corrode badly enough that they come off. The brackets are available without the pump. Of course, test the pump before reinstalling with a new plate.

That gives you an overview of the Toyota fuel injection system. If you have worked on any injected vehicle, the Toyotas shouldn’t be a problem for you. If you’re a young tech who’s just getting your feet wet with fuel injection, Toyota is good car to learn on. With the popularity of these vehicles, it’s a safe bet that what you learn today will serve you well into the future.

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