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A collection of helpful articles based on reader submissions
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Below is a collection of Jim Grant's Tech Tips sorted by Vehicle Make. These Tech Tips were answers to questions submitted to Jim by users over the course of many years.
If you have any questions, please feel free to email us.



Lincoln - Mercury
  Jim Grant's Tech Tips

'95 Toyota 4-Runner, Rough Cold Weather Idle

Q: My ‘95 Toyota 4-Runner runs rough when it's cold. As soon as it is warmed up, it runs great, but it does take some time to warm up. It also seemed to have some roughness when it wasn't being pushed (accelerated) above 50 mph or so. I had it in to Toyota and had the fuel injection serviced. The problem wasn't as severe for some time, but with the cooler weather coming, the problem when cold seems to be increasing. What do you suggest, or what do you think might be the problem?

...Paul W.

A: I would suggest inspection of two items. First, the coolant thermostat. The thermostat is designed to maintain coolant temperature to a specified temperature. If a thermostat is failing to bring and engine up to the correct operating temperature the vehicle’s computer will make incorrect fuel adjustments. In simple terms, the engine would get too much gas for too long of a time. This could result in spark plug fouling and poor performance. So a properly operating thermostat is critical. Second, the spark plugs. The wrong heat range spark plug can cause rough idle and misfire problems, when the engine is cold, that will be much more noticeable in colder weather. There is a lot that goes on in engineering a spark plug. A spark plugs tip temperature must remain between 500C-850°C. If the tip temperature is lower than 500°C, it will not be hot enough to burn off carbon and combustion chamber deposits. The result is a fouled spark plug causing an engine skip or misfire. As the weather gets colder the spark plug tip temperature is more critical. The difference from one heat range to the next is approximately 70°C to 100°C. With that in mind you can see that a cooler heat range spark plug can result in cold driveability problems. Also not all spark plugs are created equal. Some manufacturers try to get one part number to fit a broad range of engines. The problem with this? What works in one application may be too hot or too cold for another, which results in a performance/driveability complaint.

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