How to solve Volvo S80 Engine Idling Rough

My workshop received a Volvo S80 with more than 200,000 km a couple days ago. According to the customer’s description, the engine idle was rough all the time.

Fault code diagnosis:

The running speed was about 1400r/min when I startup the car, the engine ran at rough idle. And then I used Volvo Vida Dice 2014A( I bought it on aobd2shop) to read the trouble code. It showed that something wrong with air throttle.

The following possible reasons are inferred 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, I took out the air throttle. After finish cleaning, I used the diagnostic tool Volvo Vida Dice 2014A 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. Therefore, 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 whether the wire harnesses are workable3, which were between the plug of air throttle and engine control module. The results showed that six harnesses can work.

So, harnesses had no problems.

Next, I used multimeter to measure the voltages of air throttle plug terminals. The results showed that the voltages were normal.

Then 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 again which is in normal work. Trouble was figured out via Volvo Vida Dice 2014A.