What could cause noise in car since clutch kit fitted?

I HAD a clutch kit fitted to my 1300 Toyota Tazz and now I have a noise from the gearbox that was not there before.

If you apply the clutch the noise is gone. The dealer who supplied and fitted the kit maintains that the noise is not from the thrust bearing, but cannot tell what causes the noise in the gearbox. I would appreciate your assistance.

Derrick (e-mail)

Derrick, I would start with all the small things that can be overlooked during a clutch replacement. Has the gearbox oil been topped up to the correct level? Has the clutch linkage been properly adjusted?

Do you have the correct amount of free play on the clutch pedal? Has the engine's idling speed perhaps been adjusted so that it now causes a vibration somewhere (perhaps at the release fork pivot) that wasn't there before?

Has something been displaced during the clutch replacement and not properly fastened afterwards, allowing it to vibrate against a metal surface?

If the noise persists after you have ruled out all the trivial causes, you have to delve deeper. If I understand correctly, the noise goes away when you depress the clutch pedal. As you know, the gears in the gearbox will stop spinning when you depress the clutch pedal.

So if the noise is the typical whining or growling sound of a worn bearing, it could be coming from one of the bearings inside the gearbox.

Are you sure, though, that the noise is coming from the gearbox and not from the clutch? It's easy to mistake the two because these components are so close together.

Though not impossible, it would be a strange coincidence if something inside the gearbox decides to pack up during a clutch overhaul.

If the noise originates in the clutch, one possibility that comes to mind is that some of the spring fingers on the pressure plate are protruding further than the rest.

This will cause a chirping sound as the release bearing touches only the protruding fingers during idling. The sound will stop when the clutch pedal is depressed and the bearing is pushed into firm contact with all the fingers.

This problem sometimes occurs when poor quality remanufactured parts are installed instead of a matched kit from a reputable manufacturer.

It can also arise when the pressure plate is distorted by incorrect tightening of its attachment bolts during installation.

A good test to find out if the workshop knew what they were doing is to enquire whether the flywheel was skimmed.

(Ask to see the receipt from the engineering firm that did the skimming.) In a first-rate workshop this will be standard practice.

Can you please find out for me the function of a rear diffuser and the reason for the fuss over it in Formula One racing.

Magosi (e-mail)

Magosi, the "rear diffuser" that has been causing so much controversy in Formula One circles is a carefully shaped extension of the lower bodywork at the back end of a racing car, between the rear wheels.

It consists of a central channel flanked by side channels and its purpose is to direct and organise the high-speed airflow underneath the car in such a way that down-force on the rear of the car is increased (which improves agility and cornering ability) while drag is reduced, allowing higher top speed and better acceleration.

The controversy arose when at the start of the 2009 season, three teams, Brawn, Toyota and Williams, had diffusers that worked better than those of the other teams, giving their cars a significant advantage.

They achieved this by interpreting the regulations governing the shape and dimensions of the diffuser in a way that their rivals did not.

The other teams, finding themselves at a disadvantage, and unable to simply copy the successful diffusers at short notice - the diffuser is an integral part of the design of a F1 car; if you change it, you have to change a lot of other things as well - naturally could not allow this to go unchallenged.