Slipping clutch can make car suddenly rev

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This article was published 10/07/2014 (4431 days ago), so information in it may no longer be current.

Q. I have a 2004 Chevrolet Cobalt and I am having problems. It just started to have the engine rev suddenly when I am driving down the street even though I am not stepping on the gas. The car doesn’t accelerate but the engine revs just like I shifted down a gear. It has a five-speed manual transmission and otherwise seems to work fine. Can you tell me what could be wrong with the car please?

A. It sounds like you have a slipping clutch. The clutch is designed to slip a little while you start the vehicle moving but hold tightly when you are driving so the engine power is transmitted to the transmission and drive wheels. If the clutch slips when it is supposed to hold, then the engine will rev up and feel like it is disconnected from the wheels — like you have placed the transmission in neutral.

There are a couple problems that can cause the clutch to slip. The first is a worn-clutch assembly. Every time you start the vehicle moving, the clutch has to slip a little and this creates a lot of heat. The slipping parts and heat do wear the clutch plate and eventually it wears to the point it can no longer hold. I have seen clutch plates wear out in a few hours because the driver was slipping the clutch excessively while trying to accelerate a high horsepower vehicle quickly, but typically a clutch will last many years. Stop-and-go city driving is much harder on a clutch, while highway driving places virtually no wear on the parts.

A worn or damaged clutch will also usually start to slip excessively in the higher gears before it begins slipping in all gears. This is because the lower gear ratios provide a mechanical advantage to the clutch so it doesn’t have to provide as much holding power.

Replacing a clutch can be expensive. The transmission has to be removed, the pressure plate, clutch disc and release bearing have to be replaced, and often the engine flywheel also needs to be replaced because it is part of the friction surface the clutch disc applies against. This can be four or five hours work plus parts on many vehicles.

Another potential problem that can cause a clutch to slip is that the clutch isn’t being allowed to engage completely by the clutch pedal and linkage assembly. Some vehicles use a mechanical link or cable to operate the clutch but a hydraulic clutch is now more common and your vehicle has hydraulics. If there is dirt or corrosion in the clutch master cylinder, then the clutch apply fluid may not be able to return completely to the master cylinder reservoir and it keeps pressure inside the system. This pressure will hold the clutch only partially applied, so it slips when power is demanded from the engine. Before going to the expense of installing a new clutch assembly, have the technician check the hydraulic system to ensure it will allow the clutch to apply fully.

Q. My 1998 Volvo V70 sometimes will only click when I go to start the vehicle. The lights on the dash come on, the headlights seem to be bright enough, but the engine won’t crank over. The battery is only three years old but if I give it a boost from another battery, the car cranks over and starts right away. Do I need a new battery already?

A. Even a relatively new battery can sometimes develop a weak connection internally and this can create intermittent starting problems, but if the headlights are bright while trying to crank the vehicle, then the problem is likely in the starter. The armature in the starter likely has some worn or burned contacts or the brushes are worn and by boosting with another battery, you increase the voltage just enough to enable the starter motor to turn over. If the armature is worn, once the starter has moved a little, it will usually continue cranking till the engine starts. Have the battery tested, and if it is good, then look at the starter as the main problem.

 

Jim Kerr is a mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net