Noise, vibration may have different causes

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This article was published 23/04/2004 (7915 days ago), so information in it may no longer be current.

QUESTION — I own a 1987 Ford Ranger 4×4 with lots of noise coming from the back differential. I changed the u-joints as well as the driveshaft hanger bearing, but now I am getting a lot of vibration from the back end as well. The vibration peaks around 80 kilometres per hour (50 miles per hour). Could this be caused by a pinion bearing or is it worse?

ANSWER — I am assuming that you still have noise from the back end of the truck as well as the vibration. These two problems likely have different causes. The vibration only started after you changed the u-joints and hanger bearing so let’s start there.

Three driveshaft problems can cause the vibration: out of balance, excessive runout, or incorrect phasing. During the u-joint replacement, one of the balance weights on the driveshaft may have been knocked off. Look for marks on the driveshaft where the weight was spot welded on. If a weight is missing, have the driveshaft balanced. Driveshaft runout can be created if a u-joint is not centered in the driveshaft yoke. Inspect the u-joints visually to see if the u-joint cups are positioned equally from one side to the other. A dial indicator can be used on the driveshaft as it is turned to measure the runout if required. Typical runout specifications are a maximum of .025 inch to .030 inch (.06 millimetres to .075 millimetres).

I am guessing your vibration is caused by incorrect phasing. As the driveshaft turns through an angled u-joint, the shaft speeds up and slows down twice each revolution. The yokes on each end of the shaft must be in line with each other so the acceleration of the shaft through the first u-joint is cancelled out by the deceleration created by the second u-joint. During the hanger bearing replacement, the driveshaft components may have been installed “out of phase.” A visual inspection can identify this problem and realigning the yokes correctly will fix the vibration.

The noise you are hearing from the rear could be caused by several bearings in the rear axle. Noisy pinion bearings will change their sound during acceleration and deceleration. Rough axle bearings sound louder as a load is placed on them during cornering. Differential side bearings also sound like bad axle bearings, but don’t change as much as load varies.

I suspect your problem is caused by a bad pinion bearing because the noise occurs at 50 mph (80 kph). This speed is where differential gear noise is most noticeable and a bad pinion bearing would permit the pinion to move out of position, making that noise louder. If caught quickly, only the bearings and seal require replacement. After driving a few thousand kilometres (miles) with bad bearings, then the ring and pinion gears may be worn. The only way to correct gear noise after the gears are worn is to replace them. This is expensive, so repair bearing noises quickly.

QUESTION — I have a 1991 Mazda MPV with a 6 cylinder engine and I am having idle speed problems. The idle has been adjusted several times and once there was a code for the idle system. It was fixed, but it started doing it again the next day. Most times, it stays revved up in park or neutral and sometimes while driving. My fuel economy is also very poor. Help!

ANSWER — Your Mazda’s symptoms are a classic example of what happens to fuel-injected engines with a vacuum leak. As extra air enters the intake manifold through the leak, the computer compensates by injecting more fuel. Fuel economy drops and the engine speed goes up. The computer tries to lower it with its idle air control valve. If the leak is too large, the idle cannot be controlled and a code may be set.

Your problem sounds intermittent, so I suspect a cracked vacuum hose or a sticking PCV valve. A careful inspection of all hoses will probably find the cause. If the hoses are OK, replace the PCV valve and check for leaking intake manifold gaskets. Spray a little water on the gaskets, and if the idle speed changes, then the gasket is leaking.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca