Nissan Repair: warm up idle engine shakes., nissan quest, suspect one

Question
Hi, I have 1999 Nissan Quest with a 3.3 V6, 136K. The engine is strong and I have replaced plugs, plug wires, distributor, cap & rotor, fuel injectors, fuel filter, air filter, PVC and checked timing. The engine seems to miss or shake a little when warming up. After reaching operating temperature it evens out and runs okay. What could cause the shaking when warming up? Thank you for your help.

Answer
Hi Tobias -

When the engine is warming up, it is in a "cold loop". Meaning that certain vaccuum lines are ported open until operating temperature is reached. Because it only runs rough during warm up, I suspect one of these vaccuum lines is broken or cracked. This is usually connected to some type of sensor that is screwed into the manifold of the engine. Look for a couple of vaccuum lines, and trace them all the way.

the other way you may check for a vaccuum leak is to run the car when cold, and spray carb cleaner very lightly around the rngine, particularly following the vaccuum circuit. When the idle improves, you have found the device or line that is the problem.

Hope this helps -
aloha
calvin