Engine dies while driving

Tiny
DAVID COVINGTON
  • MEMBER
  • 2005 DODGE MAGNUM
  • 5.7L
  • V8
  • RWD
  • AUTOMATIC
  • 150,000 MILES
Engine sputters at versions speeds. Usually when off throttle then start back on. Like going down a hill then starting up next. It might sputter. But if I'm at a red light or maybe turning into a parking lot, when the rpm's are low, it might die. When it does, I can put in neutral and it restarts. I have cleaned the throttle body, and the map sensor. New plugs and Egr valve. I am currently running premium gas and some fuel system cleaner. Can you help?
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Sunday, September 4th, 2016 AT 8:11 PM

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Tiny
CARADIODOC
  • EXPERT
The first thing is to stop using a higher grade of gasoline than what is called for. Higher octane fuels are designed to be harder to ignite. Use of them allows engineers to design an engine to produce more power from a higher compression ratio, but higher compression ratios lead to spark knock and pre-ignition. The higher octane is needed to reduce that spark knock by making the gas less likely to self-ignite, (like we want diesel fuel to do). We need the gas to wait for the spark to show up.

That higher octane rating means the fuel is harder to get it to start burning. If you have an existing misfire condition, it is only going to get much worse with gas that's harder to ignite.

There's two things you might consider. This first one happened to my '88 Grand Caravan twice. It ran fine at highway speeds but would die when slowing down. It would always restart, but it ran poorly at idle. It actually accelerated pretty well, but that was no consolation when trying to get through Minneapolis on a hot summer day when all three bypasses where down to one lane for road construction. Took four hours to get through the city, then it ran fine on the highway.

The cause was a plugged fuel strainer, (pickup screen) inside the gas tank. Had I known it at the time, I could have disconnected and plugged the vacuum hose going to the fuel pressure regulator on the engine. It would have run too rich, but the stalling would have stopped occurring. The reason for this is the stalling occurs when the highest volume of fuel is pumped, and that is during coasting. I can explain tomorrow night if necessary. I think your fuel pressure regulator is inside the gas tank, so that isn't an option for you, but to find this, monitor the fuel pressure while you're driving. I had a fuel pressure gauge clipped under a wiper arm for over a year while trying to find the cause of another elusive problem. Yours won't take that long. You should be able to borrow a pressure gauge from an auto parts store that rents or borrows tools. On my van, normal fuel pressure is around 50 psi, and it varies up and down a little depending on engine load. If I dragged my tandem axle enclosed trailer that's bigger than the van, under that heavy load the fuel pressure would slowly drop to less than 20 psi. The engine didn't sputter until 15 psi, then the fuel pressure would shoot back up as soon as I let off the accelerator a little.

Many people jump on the fuel filter for this problem, and that is a good suspect on GM vehicles, but except for diesel engines, you will never solve a running problem on a Chrysler product by replacing the fuel filter. They typically last the life of the vehicle.

One thing I should mention but I don't know if this is still an issue in 2005, is if the battery was disconnected or run dead, your Engine Computer will have lost its memory. All the sensor data that was stored will be relearned very quickly as you drive, ... Except for "minimum throttle". Until the computer relearns that, the idle speed will be too low, the engine may be hard to start unless you hold the accelerator down 1/4", you won't get the nice "idle flare-up" to 1500 rpm at start-up, and it will tend to stall at stop signs. You need to meet a specific set of conditions for that relearn to take place. That is to drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals.

The second thing to consider is driving with a scanner attached so you can view live sensor data. One important piece of data is "desired idle speed steps". That goes from 0 to 256 "steps" with step 32 being typical for a well-running engine. This will be different if you have the "throttle-by-wire" system instead of the common sense throttle cable. If you find it's at step 50, for example, then you need to look at why the actual speed is too low. The computer is calling for a higher speed but the engine isn't responding. If you find the desired step is real low, like "10" or "20", that is in response to something the computer is seeing. You have to look at the other sensor readings to see what doesn't look right. You'll see the desired step is "0" if minimum throttle hasn't been relearned.

Related to this is looking at the other sensor readings to see what doesn't look right. When the problem is intermittent, most scanners have a "record" feature that allows you to record a few seconds of sensor data to be reviewed slowly, later. Because the data goes through the scanners memory, the recording actually starts a couple of seconds before you pressed the button. The first things to look at are the input sensors the computer uses to calculate fuel metering and injector and spark timing. The next things to look at are the output specifics, meaning the oxygen sensors and the engine operating parameters. A simple one to understand is engine speed. When the speed dropped off, did an input sensor value change at the same time. Perhaps the EGR valve was open and a chip of carbon is preventing it from closing fully. You would see that no other sensor values had changed. If you see a front oxygen sensor go suddenly very lean while the other one didn't, you might suspect a spark-related misfire. The O2 sensor would detect the excessive unburned oxygen in the exhaust.

There are many more things to look for but you need the scanner to see what the Engine Computer is seeing and responding to. Sometimes the least expensive choice is to get a mechanic involved, particularly before you start throwing random parts at the problem.
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Sunday, September 4th, 2016 AT 11:32 PM

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