Mechanic comes through in the clutch, solves Miata problem

Nov 12, 2023

In the auto repair business, it is common to see repetitive vehicle failures, but it's extremely rare to see a problem for the first time.

Two weeks ago a customer brought in his 1991 Mazda Miata with what he believed was an intermittent clutch disc problem. His car had been a bit of a thorn in my side, because whenever he brought it in, the problem could not be identified. In addition, his car had an unusual clutch repair history.

There are two main groups in a clutch system: the mechanical clutch parts and the hydraulic clutch parts.

The mechanical clutch parts transfer the power of the engine to the transmission. The mechanical parts consist of clutch disc, pressure plate, throwout bearing, pilot bearing and flywheel. The flywheel is bolted to the engine's crankshaft and is spinning whenever the engine is running. The clutch disc (friction disc) is placed between the pressure plate and the flywheel. Once in place, the pressure plate is bolted to the flywheel, sandwiching the disc between the flywheel and pressure plate. The throwout bearing sits between the pressure plate and the clutch fork so that when the clutch hydraulics are engaged, the clutch fork is able to depress the pressure plate, thus releasing the clutch disc. The driver can then select a gear.

The clutch hydraulic system operates the mechanical pieces of the clutch system. The hydraulic clutch parts consist of clutch master cylinder, clutch slave cylinder and clutch lines (hoses and pipes). Pushing on the clutch pedal depresses a rod that builds pressure in the master cylinder (located on the firewall of the car). The fluid pressure is then distributed to the clutch slave cylinder located on the transmission. Another rod that extends from the slave cylinder then operates the clutch fork, the transition from hydraulic to mechanical.

So let's get back to that Miata. We replaced the clutch in 2005 and again in 2015. The first time we replaced it, the car had 128,600 miles on it; the second clutch was replaced at 175,900 miles. This car had gone through the second clutch after 47,300 miles. The life expectancy of a clutch disc, under normal driving conditions, is between 80,000 and 120,000 miles. But if one were to aggressively push the clutch system (racing, regularly driving on hills or using the clutch pedal incorrectly), one could burn up a clutch within 1,000 to 20,000 miles.

Once a new clutch has been installed, the clutch pedal free-play must be checked or adjusted. Most modern clutch hydraulic systems do not have to be adjusted. Correct free-play adjustment gives the clutch pedal approximately 1 inch of movement before the pressure plate is engaged.

One thing that was strange was that we had to perform one free-play adjustment on the Miata between clutch replacements – at 172,800 miles. Another strange thing was that our customer said the clutch would slip intermittently. This is virtually impossible for a clutch system – once it starts to slip badly, it cannot correct itself.

We checked that all of the mechanical parts were OK and the free-play adjustment was correct, which left us with the hydraulic system. I have changed thousands of clutch slave cylinders, clutch flex hoses and clutch master cylinders. I changed all of them because the internal seals failed and the cars lost clutch hydraulic pressure. Once the hydraulic pressure is lost, the pedal will just go to the floor and it will be impossible to disengage the clutch disc or shift.

A thousand miles after the second clutch replacement, the customer said the clutch was slipping badly, so he brought the car back in. I test-drove the car and the clutch didn't slip. We again checked the free-play; it was fine.

At this point, we began to think that the customer might be miscommunicating the symptoms. We told him to bring the car back only when the problem was happening. After three more visits, we finally caught the car in the act. We discovered that the clutch flex hose had fallen apart on the inside and was blocking hydraulic pressure to the slave cylinder – like a one-way valve. When the problem occurred, it was as if the clutch pedal were engaged.

Once we replaced the clutch flex hose, the slave and the master were so worn out that they too had to be replaced. This problem was so rare that I doubt I’ll ever see it again. It was great to finally get to the bottom of it.

Matt Pataky owns Sunnyvale Foreign Car Service, located at 15 Pioneer Way in Mountain View. For more information, call 960-6988, email [email protected] or visit sunnyvaleforeigncar.com.

Log In

Keep it Courteous. Be respectful, truthful, anduse no threatening or hateful language.Be Proactive. Use the 'Report' link oneach comment to let us know of abusive posts.Share with Us. We'd love to hear eyewitnessaccounts and the history behind a news event.Read our full comments policy: losaltosonline.com/comments