What causes vibration in my cold truck engine?

Written by chris stevenson
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What causes vibration in my cold truck engine?
Checking cold-engine vibration in your truck requires a logical process of elimination. (truck image by Andrew Breeden from Fotolia.com)

Engine vibration can be a mystifying problem to solve even for the seasoned auto repair mechanic. Engine vibration can be associated with component failures that relate to engine performance while idling cold, or the cause can remain elusive until the suspect part responsible for the problem can be located and repaired. Narrowing down the problem with logical steps can be the only way to pinpoint malfunctioning parts, including systems that show signs of beginning failure.

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Determining Engine or Chassis Vibration

Start your engine and let it idle normally. After the first few minutes of start-up, listen and feel for any vibration in the vehicle. A sound like a buzz or rattling during cold running will most likely be engine-related. Any vibration felt in the steering wheel or on the clutch pedal, brake pedal or floorboard will also point to an engine component failure or out-of-balance engine part. If the noise decreases during driving, you can conclude that the problem cannot be related to the drive train, suspension or wheels.

Motor Mounts

Check the condition of the motor mounts. Most vehicles have two front engine motor mounts, with an additional mount that supports the weight of the transmission. Motor and transmission mounts consist of thick pads or rubber shock absorbers designed to cushion the weight of the engine between the vehicle chassis. Any rubber or pad that has worn away will cause metal to metal contact between the engine and frame, causing a serious vibration. Bad motor mounts can often be detected during sudden acceleration, at which time the engine will lift up suddenly and drop, causing a very heavy clunking sound

Electrical Engine Misfire

Any electrical engine misfire will be most prominent during cold start-up and idle. Check the distributor points in vehicles that have breaker point ignition. Burnt or improperly gapped ignition points will cause a constant engine miss that can be felt through the chassis. A burnt rotor or distributor cap, bad plug wire or defective spark plug can set up a vibration, since the misfire will unbalance the power stroke of the engine.

For HEI (high-energy ignition) plug wires, rotor, cap and spark plugs should also be checked for proper firing, as well as the air gap setting in the distributor.

Fuel Misfire

Make sure the choke setting has the proper adjustment for vehicles equipped with carburettors. A choke that does not close for cold-start conditions will not allow enough extra fuel into the intake manifold for combustion. A choke mechanism that does not disengage properly (open) will flood the engine and cause a noticeable stumble. The stumble will cause a cold-running vibration.

For fuel-injected vehicles, the injectors must be checked for proper firing. Any injector that has shorted out will not fire and will cause a stumbling vibration. Fuel-injectors line should be cleaned, as well as the injector heads. Determine if the fuel pump regulator or fuel pump delivers enough fuel (pressure in pounds per square inch) to the intake manifold by using a fuel pump pressure gauge.

Fan Blades

Cooling-fan blades can wobble on worn-out hubs or fan clutches, causing a very noticeable vibration. Any cooling-fan blades that have chips or are missing or broken should be replaced.

Pulleys and Balancer

Check the harmonic balancer for any perceptible wobble or noise. The harmonic balancer has been designed to rotate at a specific engine frequency, providing the crankshaft with a smooth, even rotation.

The crankshaft pulley at the front of the engine, sometimes called a dampener, can come loose from its keyway shaft and wobble. A misaligned crankshaft pulley can cause a very heavy vibration during cold running and disappear with higher engine rpm.

Clutch, Pressure Plate and Flywheel

If your vehicle has a standard manual transmission, the clutch, pressure plate, throwout bearing and flywheel should be inspected. These heavy transmission parts can be bent, be loose from their mounts, or be broken and out-of balance. Cracked flywheels can rattle, causing chassis vibrations. Bad flywheels can sometimes be detected when the starter makes a loud noise upon engaging. Clutch, throwout-bearing and pressure-plate problems can be felt through the clutch pedal and cause the entire chassis to vibrate during idle in neutral, but disappear when the clutch becomes engaged or the vehicle moves.

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