1992 Plymouth Laser fuel delivery

  • 4 CYL
  • AWD
  • 170,000 MILES
I have a 1992 plymouth laser awd turbo that seems to flod itself out when the fuel pump is plugged in, but will idle relatively smoothly for more than 20 minutes with the fuel pump unplugged. Could this be a problem with the throttle position sensor?
Do you
have the same problem?
Monday, March 1st, 2010 AT 10:47 PM

1 Reply

There has to be a mixup with what you are unplugging. The engine can't run more than a couple of seconds with the fuel pump not running. There is no means of storing pressurized fuel.

In the event of a crash that ruptures a fuel line, with no fuel pressure, the engine will immediately stall due to the fuel's inability to force its way through the injector nozzles. The engine computer knows the engine stalled by the absence of pulses coming from the crankshaft position sensor. Within one second, the computer turns off the Automatic Shutdown (ASD) relay. That relay is what powers up the ignition coil, alternator field, oxygen sensor heaters, injector(s), and fuel pump or pump relay. The pump stops instantly so it will not pump raw fuel onto the ground creating a fire hazard. If the pump were to keep running for any length of time, or if a significant amount of fuel was stored in the system and could leak out, that would render the entire safety design useless.

The throttle position sensor actually has relatively little say in how much fuel is delivered to the engine. It is used more for fine tuning the amount dictated by the MAP sensor. He has the greatest affect on fuel metering because he looks at engine load. Open throttle blade equals decreased manifold vacuum which equals increased load which commands increased fuel. The throttle position sensor reports idle, wide-open-throttle, any point in between, direction of movement, and speed of movement. It is supplied with 5.0 volts on one pin, ground on the other, and the center pin finds the voltage between them based on its position. Mechanical stops in the throttle body causes that signal voltage to vary from roughly.5 at idle to 4.5 volts at wide-open-throttle. You can actually measure that voltage with a digital voltmeter while you move the throttle. A broken wire, (typically a corroded connector pin in the wiring harness), will cause the signal voltage to go to 0.0 volts or 5.0 volts. The engine computer recognizes those as unacceptable values, sets a diagnostic fault code in memory, turns on the Check Engine light, disregards the sensor, and injects pre-programmed approximate values to run on. There will be a little hesitation or sputtering, but the engine will run. Once you have your original problem solved, unplug the TPS and drive the car to see what affect it has. Once you plug the sensor back in, the engine will run fine although you might have to stop and restart it. I think the Check Engine light will turn off then. Whether or not it turns off, the fault code will erase automatically after approximately 50 engine starts, (about one week).

The MAP sensor can incorrectly ask for the wrong amount of fuel when it starts to fail. Very often the engine will run better if you unplug it because the computer will substitute values it thinks are appropriate based on the other sensors' readings. Driving the car is difficult and usually the only way to keep the engine running at all is to keep the gas pedal in motion. It doesn't matter where the pedal is, which way it's moving, or how fast it's moving. If it's normally a no-start condition, suspect the MAP sensor if moving the pedal keeps the engine running.

The two-wire coolant temperature sensor voltage is used to modify fuel delivery under various temperatures since gas doesn't vaporize as well when it hits cold engine surfaces. They give very little trouble on Chrysler products, but if the voltage is too high, again, typically due to corroded, high resistance connector pins, the computer will think the engine is colder than it actually is. This could result in the computer commanding too large of a priming squirt from the injector(s) for engine startup.

To get back to the original problem, can you describe exactly what you are unplugging, where it is, or the colors of the wires in the connector? What are the symptoms that you believe indicate flooding? If you don't unplug the connector, can you get the engine to run?

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Wednesday, March 3rd, 2010 AT 6:20 AM

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