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Temperature sensor

When I start my car (Saturn SW2 1999, 90k miles), about 20-25% of the time the engine will rev up and down, going dangerously low before stalling out. Then when I try to start it again, the engine floods. A mechanic told me that the best way to start the car after this was to actually push the accelerator all the way down, causing the valves to lock and the ignition to ignite whatever fuel is flooding the engine. I tried this once, and it worked, although my engine flew up to 10k RPM before coming back down, and even then I had to hold it at 3k RPM for a while in order to keep it from stalling out.

My stop-gap solution has been to just gently nudge the gas as soon as I turn it on and hold it at 2k RPM for a minute or two. At first the car shakes a lot and I can hear the engine sputtering before it finally stabilizes.

The computer produces no error codes, however the mechanic thinks that it is a temperature sensor, but doesn’t think its necessary to replace yet. A note, this occurs 20-25% of the time whether it is cold or warm outside (has not been hot here yet since this started). Also, it is worse when it is damp or raining heavily outside (suggesting perhaps an electrical problem).

Any suggestions would be highly appreciated.

The output from the temperature sensor can be tested. There’s no reason to guess about it. If the sensor is bad it should be replaced. I don’t understand why your mechanic doesn’t want to replace it if it’s causing you trouble 25% of the time. What’s he waiting for? 75%?

The fact that wet weather aggravates the problem makes me think it’s time to replace the spark plug wires. Old, dry wires are often the cause of difficult wet weather starting.

Is all other maintenance up to date according to the manufacturer’s schedule? Spark plugs, filters, coolant replacement, etc, etc? If not, bringing everything up to date would be a good idea.

The original reason I took it in was because I thought the spark plugs needed to be replaced. They tested the spark plugs and said that they were ok, suggesting that the problem was a dying battery. I replaced the battery, and have always been on schedule with oil changes, coolant replacement, and all other maintenance. The problem, however, continues.

I wanted to make sure that this description sounded like a temperature sensor problem to enough people, and was not a warning flag of something more expensive that is about to break.

An incorrectly performing engine coolant temperature sensor can lead to the engine starting (or, cranking), and running, too rich. That isn’t the ONLY thing which can cause a rich, or flooded, engine. A fuel injector not holding the pressure, and leaking into the intake manifold, can cause flooding.
Your mechanic can check the fuel injectors, and fuel pressure, if he has, and USES, a fuel pressure gauge. To do so, connect the fuel pressure gauge to the fuel line, shut off the engine, and observe the fuel pressure. The fuel pressure should not fall off for several minuets, or longer (I’ve seen hours!). If the fuel pressure falls quickly, in a few minuets, the pressure is bleeding off somewhere. The “somewhere” could be at the fuel injectors (into the engine intake manifold), through a bad fuel pressure regulator, a fuel pump check valve, or, a leaking fuel line.
Once the cause is found, he can decide on a fix.