A couple days ago, I put my Volvo S80 (over 180,000km) into a local workshop, for which the engine idle was rough all time.
The running speed was about 1200r / min when the repairman startup the car, the engine ran at rough idle. And then he used Volvo Vida Dice 2014A to read the trouble code. It showed that something wrong with air throttle.
The following possible reasons are inferred by him from the problem:
1. Air throttle was so dirty.
2. Air throttle was damaged.
3. The wire harnesses of air throttle had problems.
4. Engine ECM was damaged.
To begin with, he took out the air throttle. After finish cleaning, he used the diagnostic tool Volvo Vida Dice 2014A to read the trouble code. It still showed that air throttle had problems.
He found out the circuit diagram from diagnostic tool and tested the wire harness in order to know whether the wire harnesses are workable, which were between the plug of air throttle and engine control module. The results showed that six harnesses can work.
Therefore, harnesses had no problems.
Next, he used multimeter to measure the voltages of air throttle plug terminals. The results showed that the voltages were normal.
Then he tested the wire harnesses of air throttle carefully and found that the harnesses were twined by gummed tapes. And then took out the tapes and found that two harnesses were twined together. When the engine temperature got higher or the engine was shaking, the voltage of these two harnesses increased. It caused the signal difference. That was why engine ran at rough idle.
He welded the two harnesses again and started the car again which is in normal work. Trouble was figured out via Volvo Vida Dice 2014A.