Check hydraulic clutch to fix Miata woes

Hey there, time traveller!
This article was published 11/07/2008 (6474 days ago), so information in it may no longer be current.

QUESTION — My 1993 Miata five-speed has only 116,000 kilometres. At least once a month when driving, I noticed that the rpm increase dramatically when accelerating. The clutch is not engaged (pedal pressed) and I am driving around town in third gear at about 40 km/h. When given the gas, the rpm go almost to red-line. When I ease up on the gas, they go back to around 3,000 rpm. Could it be the clutch?

ANSWER — You usually notice a clutch slipping by a sudden increase in engine rpm during acceleration in higher gears. A tire spinning would cause the same symptom, but I doubt your Miata has the power to break the tires loose during straight-line acceleration at 40 km/h. A worn clutch disc or pressure plate could be causing the problem, but you should be able to duplicate this repeatedly and in fourth gear as well. Your problem only occurs occasionally, so I think the problem may be in the clutch hydraulic system.

When you step on the clutch pedal, hydraulic fluid (brake fluid) is moved from the clutch master cylinder to a slave cylinder on the side of the transmission bellhousing. The fluid in the slave cylinder moves a piston, which moves mechanical linkage to release the clutch. When you take your foot off the clutch pedal, the fluid from the slave cylinder is pushed back into the master cylinder.

Small holes between the body of the master cylinder and the fluid reservoir allow the fluid to return from the slave cylinder. If these holes are plugged with sludge, then the clutch may not be engaged fully, as pressure still remains in the slave cylinder. A similar problem could be caused by a leaking master cylinder at the pedal end. The leaking fluid can build up in a varnish deposit and prevent the master cylinder piston from returning properly. This also prevents the clutch from being fully engaged. Your clutch may be worn, especially if you drive most of the time in town, but I think the rpm increase is caused by the master cylinder not working properly.

QUESTION — I had an immobilizer installed in my 1994 Bonneville in 2006 (68,230 kilometres). A few days later, the car would not start. The dealer suggested the immobilizer was not at fault and blamed the crank sensor, which they replaced. On Aug. 17, 2006 (68,329 km) the “Service Engine Soon” light would stay on for prolonged periods, then the light would turn off. The dealer installed another CMP sensor at no cost. Since then, the car was returned to the dealer several times for the same problem. Finally, the SES light is now OK. However, I am often having trouble starting the car. After waiting until the immobilizer LED blinks slowly, I insert the key and nothing happens, not even a clicking sound. On the second or third try, the engine turns over but does not start. Leaving the car for an hour or two does not help, but usually starts after sitting quite a few hours or overnight. It then operates normally for a few days and then the problem recurs. I returned the car to the dealer on June 10, 2008 (76,593 km). They could not duplicate the starting problem, but gave me a tuneup. Two weeks later, the car would not start, but did start after sitting for several hours. We are reluctant to take the car anywhere. It is going back to the dealer, however, I have almost lost my faith in their ability to analyze and correct the problem. Any suggestions?

ANSWER — The Service Engine Soon light may likely have been caused by a problem other than the immobilizer. A code should have been set in the engine computer that would help the technician locate that problem. The tuneup you had done may help if the problem was always a crank but no-start situation, but because your car often doesn’t crank, then the immobilizer would be a likely suspect.

I would have the immobilizer temporarily disconnected to see if the problem goes away. If it does, then you have found the problem. Like any other module on a vehicle, it can go bad intermittently, which is always difficult to diagnose. Either substituting another module, or in this case, disconnecting it would be the easiest way to diagnose it.

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

kerr.jim@sasktel.net