engine speed oscillations - Mitsubishi-Forums.com
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post #1 of 2 (permalink) Old 06-04-2019, 04:52 PM Thread Starter
Join Date: Jun 2019
Posts: 4
engine speed oscillations

This past day I went through a series of events with a 4 cylinder 2002 Mitsubishi Eclipse.

I am looking for help in trying to understand my experience. Maybe there is no clear answer as I probably am not offering enough detail for any meaningful diagnosis to be made.

The sequence of events begins last fall. My friend, who owns the car, decided to take the car off the road for the Connecticut winter. I removed the battery and put it on a trickle charger in a warm
room for the winter. The car had been working just fine (owner statement) up to the time I took the battery out in the fall. The battery appears relatively new.

Today, (4 Jun 2019) I put the battery back in the car. Upon turning on the ignition, the engine would turn over, but labored in doing so. Eventually the engine would start but not stay running. if I repeated the startup, the exact same sequence would occur engine would turn over, eventually start up and then stall. it seemed as if the engine was starved for gas.

I pulled off the cover exposing the coil over plug arrangements and changed the spark plugs. The plugs were dry and the porcelain was coated with a fine tan powder. I turned the motor over with a spark tester (neon bulb inductively coupled) for one of the cylinders. The periodic bursts of light suggested at
Least that one cylinder was getting adequate high voltage to create a spark across the spark plug.

I could not figure out how to open the air cleaner. It appeared as if two push pins at the opening of the intake needed to be removed, but I could not figure out how to remove them. (there was a phillips head hole in the middle of the inner pin but I could not figure out how to use it). Regardless glimpses of some of the air filter appeared reasonably clean.

I put two cans of dry gas in the gas tank followed by 5 gallons of fresh fuel. I jumped the battery. The engine did start but only if I pressed the pedal near full throttle. Upon start up I would back down on the throttle to a point where the engine oscillated between 2,000 and 2,500 rpm. I would say the cycle period was 2 seconds (guess). If I backed down on the throttle, the idle became rough and the engine eventually died. I could repeat the experience as many times as I wanted.

So at this point, I did not know what to do. Perhaps the fuel line was clogged at a location where there was just enough fuel to cause the car to run but not sustain it. On the other hand, the recovery was amazingly fast as I could duplicate the results by turning the engine off and then on again and the entire
Engine behavior would repeat. I sat down and ate a sandwich and drank some fresh mint tea.

Reluctantly I got up and tried starting the car again. Amazing. It started up and ran at any rpm. Smooth as butter.

Perhaps the dry gas had time to get to a point in the fuel system to extract water from the region? I am
Baffled. Any ideas?
whburling is offline  
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post #2 of 2 (permalink) Old 06-05-2019, 07:06 PM
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Join Date: Mar 2019
Posts: 121
Petrol can go "stale", which is when the lighter petroleum fractions have evaporated. What's left is difficult to start a car with. I expect the fresh petrol swamped the stale petrol, and you just needed the pipes to flush through.

I have just had experience of this. My latest 2001 Galant acquisition had been left for months, quite possibly 18 months, and was very low on fuel. It's a wonder it started at all.
Ian61 is offline  

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