I have a fleet of concrete truck. A driver complained of harsh vibrations in truck and further diagnosis showed that said truck idles at 500 wish rpm, while the others are at 625-650 rpm smoothly. When idled up to 625-650 truck is smooth. Where to look? Driver states truck does not run poorly or have any other symptoms when driving... if low fuel pressure was the cause, wouldnt the computer pick up the issue and make necessary adjustments to put idle at desired range? idle does not fluctuate with a/c on or off