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Jeep Cherokee Runs Rough Cold

Q. I have a 1987 Jeep Cherokee that has developed a problem of running rough at startup for a time. It is a 4.0 liter, automatic transmission, A/C with about 190,000 miles. It will start up and run rough for about 10 seconds and then clean up and run fine. It acts just as it does when I've had the fuel manifold off and it has air in it.

Jeep Cherokee Runs Rough Cold

It does this when it has sat for a time, it starts fine if it has run recently. If air is entering the fuel manifold where would it be likely entering?

Jim

A. It may not necessarily be air but a lack of fuel pressure due to an injector that sticks while it's cold. You can verify this with a fuel pressure gauge connected to the service port on the fuel rail.

Connect the gauge while the vehicle is cold and start the engine. Observe and record your initial readings and let the engine warm up until it clears up. If the fuel pressure readings change, especially if they increase, then you must have a hanging injector.

If you don't notice any differences in fuel pressure this condition could be caused by something as simple as the engine coolant temperature sensor. If this sensor is faulty the engine control computer won't know how to adjust the air fuel ratio to compensate for cold operation.

When the oxygen sensor gets warmed up enough it will provide feedback to the computer to adjust the ratio which could explain your cold running problem as well. Best of luck to you.

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