Jim Grant's Tech Tips
Toyota 4-Runner, Rough Cold Weather Idle
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?
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 vehicles 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.