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Diagnosis of cars with electronic fuel injection

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of cars with electronic fuel injection

It is often cheaper to simply replace parts which commonly fail, if you do it by yourself, than to take your car to a mechanic, who often will do the same thing but charge a premium price to do it. Most of the parts that fail on Chrysler, Dodge, and Plymouth vehicles are relatively inexpensive and easy to replace. Get a Chilton's or Hayne's guide from your book store or, if you're really serious, a factory service manual. Then you'll be able to locate and replace each part.

Chrysler cars with fuel injection all have electronic ignition (which became standard in the mid-1970s across the line). A number of common problems tend to occur. Some of these are in our stalling / non-starting repair page.

We strongly suggest that the first action you take is to check the computer codes, which can tell you what part is failing - most of the time. Sometimes, especially with the Hall Effect and MAP sensors, it will not notice a failed part.

  1. The MAP sensor fails. This can result in the car not starting, or stalling. The MAP sensor is usually attached to the fender, and you may be able to test it simply by disconnecting the lines going in.
  2. The Hall Effect sensor fails. This is cheap and easy to replace, but if you want to test it, Chriszw wrote that you can do this using a multimeter set to provide an audio continuity signal. Connect a 9V battery to the power and ground pins, the red test lead to the sensor signal pin, and the black test lead to the 9V battery negative terminal. Then slide a fat blade of iron, such as a feeler gauge, between the sensor and a magnet. The multimeter should stop sounding a tone, and the display should overrange. Chris suggested carrying a spare in the trunk since the sensor costs about $30 and tends to go bad. It really is a two minute job to replace it - it is in the distributor underneath the rotor. (Note: if you have distributorless ignition, which was phased in starting in 1991 with the Spirit R/T but really took hold after 1994, the Hall Effect sensor is most likely mounted on the block, since there is no distributor.)
  3. There is a bad connection to one of the sensors, including (frequently on 1991-92 Sundance/Shadows) the speed/distance sensor.
  4. The oxygen sensor needs to be replaced. These normally last about 60,000 to 100,000 miles, and some automakers replace them as part of the scheduled service interval. A bad oxygen sensor can cause the engine to run roughly and to have a rich fuel mixture. Bad gasoline can coat the oxygen sensor, causing it to fail.
  5. As cars get older, the fuel injectors start to drip rather than spray, so cleaning the fuel injectors periodically is not a bad idea - nor is it difficult. Depending on the quality of your local gas, cleaning every 100,000 miles or so might be sensible.
  6. There are a number of other potential issues; click here to return to the repair page (we suggest you read the rest of this page first).

If you fail a smog test, with high oxides of nitrogen (NOx), Mark Swingle advised checking:

  • Car runs too hot or too lean (carburetor adjustment or bad fuel injector or bad fuel pump or possibly bad oxygen sensor)
  • Catalytic converter is bad (only for three-way catalytic converter systems)
  • Carbon buildup
  • Bad EGR valve
  • Timing too far advanced (for cars with distributors)

If you have a 2.2 or 2.5 liter engine (or for that matter, numerous other engines), the following sensors may be mounted on your throttle body; they are present in many cars, whether Chrysler or other brands, too. (Descriptions courtesy Bob Lincoln.)

  1. Throttle Position Sensor (TPS) is a potentiometer (variable resistor) that reports how far open the throttle is, to the computer. It converts electrical resistance (zero to 5,000 ohms scale) to a voltage signal that the computer interprets when setting ignition timing, fuel mixture and idle speed. It's mounted on the end of the throttle arm that's opposite where the throttle cable hooks up, and has 3 wires (5V, signal and ground). Webmaster note: this sensor tends to get "rough spots" in the middle and is tested by measuring its voltage as it is slowly moved up and down; sudden spikes or dropouts show a problem.
  2. Fuel Pressure Regulator is a simple mechanical device that holds constant fuel pressure or relieves excess fuel pressure by way of the pressure compressing an internal spring and opening an orifice back to the fuel pump. It's mounted on top of the TBI throttle bodies, on top of the injector.
  3. AIS (Air Idle Speed) motor is a stepper motor that drives a pintle (pointed shaft) into or out of an air passage inside the throttle body, to supply either more or less air. This varies the fuel mixture and idle speed. It's driven by the computer into either 4 or 6 wires, depending on your car, and essentially acts like both the idle mixture screw and the choke of old carburetors. It's also active during deceleration, because if you snap the throttle shut, emissions can jump, so it opens at the beginning of deceleration to reduce HC emissions and ease the idle down. It also is active during shifts on a manual transmission, when you let up on the gas - it eases the transition of RPMs up and down. Webmaster note: this is a common "wear" item on all brands of cars.
Fundamentals of troubleshooting and fixing EFI cars

Let's go to the fundamentals before troubleshooting. On EVERY EFI engine, the computer expects to see pickup pulses (from crankshaft position sensor if DIS system or distributor if not) if it is to activate the fuel pump beyond the fuel system priming stage and to provide spark. Chrysler is not different. Here's what I'd do:

  1. Pull fault codes and see what's causing the problems. You may not have any codes stored...
  2. Disconnect battery for about 30 seconds and then crank the engine.
  3. Pull codes; is code 12 (working from memory here...) present? This code is an "informational" code which states that the engine was not cranked since the battery was last disconnected. No distributor pickup pulses, no cranking evident to the processor ==> code 12.
  4. Alternative procedure to see whether distributor pickup signals are present: listen for fuel injector cycling while cranking. Lack of pickup signals will result in no injector activity.
  5. Suppose your diagnosis suggests that the ECM is not seeing pickup signals. In *my* case, I'd stick an oscilloscope across the pickup and see whether I have a pulse which is not riding on a DC pedestal; i.e., it is switching cleanly between *power* ground and the voltage which is supplied to the Hall Effect pickup in the distributor. If it's riding on a pedestal, I'd locate the poor ground connection and fix it.
  6. If a signal is not present, then I'd look at the pickup supply (my recollection is that is 8 volts...could be 5) WITH the pickup disconnected, I'd do a resistance test between the signal ground and power ground. If pickup power and ground are ok, then either the pickup has failed (most likely) or there's a bad electrical connection in the pickup's connector (less likely).

- Bohdan Bodnar

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