In the past month, we have had two cars – a 2008 Honda CR S2000 and a 2007 Subaru Outback i 2.5-liter – come in with similar problems. Both customers complained about a rolling vibration.
The Honda owner said that when he accelerated, his car would have a bad vibration from the front and rear. The Subaru owner stated that once she was on the freeway, her car would vibrate badly between 45 and 70 mph. During our test-drives, we confirmed the vibration in both cars.
Handling the Honda
The Honda S2000 was the first car in the shop. After performing a test-drive and inspection, we found that the right-side engine mount was broken. Without engine mounts, the engine would literally rip away from the frame. The job of the engine mounts is to keep the engine in its position and insulate the frame from engine vibration. If an engine mount breaks, the engine will move (lift) itself out of position. If the engine moves into a different position, it will push or pull on other components of the drivetrain.
When you accelerate the engine, it delivers power or torque. The power is distributed to the transmission, then to the propeller shaft, then to the differential and then to the rear axles. If the propeller shaft or rear axles are out of position, they can vibrate.
We explained to the customer that the first step was to replace the motor mount. After we replaced the right-side mount, we test-drove the car and noticed right away that there was still a vibration, though it was not as bad as before. The vibration was no longer in the front of the car, however, it was only in the back.
We then inspected the propeller shaft, differential and rear axles. The propeller shaft and rear differential did not show any signs of wear. The rear axles, on the other hand, had a small amount of movement (1mm) that was not normal.
We called the customer and explained that we would have to remove one of the rear axles and disassemble it for inspection. When the axle was taken apart, we found pitting on the wall of the inner axle joint wall (channel). The pitting was causing the inner axle joint bearing to move out of its channel and the axle to vibrate. We explained to the customer that we should replace that axle and possibly the other side as well. We found the same problem on the opposite axle. The car ran just fine after we replaced both of them.
Servicing the Subaru
One week later, the Subaru came into the shop with a similar vibration. We test-drove the car and brought it in for an inspection. The first thing we found was that the transmission mount was broken. Similar to the Honda, the powertrain was moving out of position when the car was accelerating.
We called the customer and got the OK to replace the transmission mount. The transmission mount on the Subaru sits at the back of the transmission in the middle of the car. The propeller shaft connects to the back of the transmission, so if the transmission is moving around, it puts a lot of strain on the propeller shaft.
We then replaced the transmission mount and test-drove the car, only to discover that the vibration was worse. We then inspected the drive line again and concluded that there may have been too much strain on the propeller shaft because of the broken mount. Because the transmission mount was new, it was holding the propeller shaft tighter and was amplifying the vibration. We then removed the propeller shaft and found two of the universal joints binding badly. We called the customer and got approval to replace the propeller shaft. After doing so, the vibration was gone.
In the case of the Honda S2000, there were two things going on – the mount was broken and the axles were worn out. After looking at the S2000 repair, we deduced that the two problems were isolated incidents. The right-front mount may have caused some strain on the rear axles, but because the S2000 is rear-wheel drive, those components are farther apart.
As for the Subaru, we concluded the two problems were related because the two components were located so close to one another.
These cases show us how different components can work with or work against one another.