Volvo S80 Engine Idling Rough

Yesterday, my garage received a Volvo S80 which has run 340,000 km. The engine type is B5254T2. The transmission type is AW50/51. The user said that the engine idle was rough all the time.

Trouble diagnosis:
I started the car. The engine ran at rough idle. The running speed was about 1400r/min. Used Volvo dice 2013A to read the trouble code. It showed that something wrong with air throttle. I thought that there were some reasons for this trouble.

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.

First, I took out the air throttle. It was so dirty. After cleaning, I used the diagnostic tool Volvo Vida Dice 2013A to read the trouble code. It still showed that air throttle had problems.

Then, I used multimeter to measure the resistance of air throttle.

#1-#2 was 1.4Ω ( the standard value is 1.2～3.5Ω)
#2-#3 was 1368Ω (the standard value is 1000～1500Ω)
#2-#6 About 820Ω when closing the air throttle. About 1580Ω when it opened completely.
#2-#5 About 1480Ω when closing the air throttle. About 500Ω when it opened completely.

They were all normal. So, the first two reasons can be deserted. I found out the circuit diagram from diagnostic tool. And I tested the wire harness. I wanted to know that whether the wire harnesses can work, which were between the plug of air throttle and engine control module. The results showed that six harnesses can work. It meant harnesses had no problems.

Then I used multimeter to measure the voltages of air throttle plug terminals. The results showed that the voltages were normal. So, I tested the wire harnesses of air throttle carefully. I found that the harnesses were twined by gummed tapes. I 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.

Trouble shooting:
I welded the two harnesses again. I started the car. It can run normally. Read the trouble code with car diagnostic tool – Volvo dice 2013A. The air throttle had no troubles. So the trouble was figured out.

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