Shakes or Wobbles problem
2001 Ford Laser 4 cyl Front Wheel Drive Automatic 130k miles
I just bought a 2001 Ford Laser (Lynx) sedan which is quite similar to Mazda Protege. When I first tested the car, there is no problem with the drive. The RPM dropped to 500 and the car shaked when I stopped at traffic light. The salesman said it was clogged throttle body and cleaned it for me.
After 2 days, I picked up the car and the RPM drop problem was gone. But then, I experience a sudden jerk (less than 1 second) when I drive in the city and highway. It is a pull back jerk, only happen once at a time. The RPM will drop slightly and rise back to previous level. Eg: 3000rpm to 2500rpm and back to 3000rpm in less than a second. Sometimes the RPM rise back quickly, sometimes the RPM stays at lower level and slowly rise up again. There is no gear change on this time.
The jerk force is moderate. More forceful than sudden brake. But my body just move forward slightly. And it is not caused by gear changing. The car is not shaking.
The jerk happens at speed from 20km/h to 120km/h. It may occur several times in a minute. And it may occur once in 5 to 10 minutes.
Later a software diagnostic was performed and one error is deleted. Immediately, city driving no longer jerks.
Now, the jerk only occurs above 70km/h. It happens quite consistently at 80km/h.
It mostly happen while cruising at steady speed. The car accelerates nicely with no loss of power. The gear changes smoothly, up shift and downshift. There is no clanking or explosion sound. When the jerk occurs, there is a slight " thump" sound. Recently, I realize the engine makes a high pitch sound during highway driving.
The car idles at 800rpm. I can see the RPM needle moves up and down slightly, around 790 to 810rpm. Fuel consumption is okay too. Highway gets 40mpg and city gets 24mpg.
The spark plug, gearbox oil filter, air filter, engine oil, gearbox oil was changed without any difference on the jerk. Fuel filter is still in good condition.
Side note: the ABS and parking brake light don't light up. The bulb is spoilt. When a new bulb is replaced, the parking brake light glows much brighter. Maybe this cause it to blow up in the first place. The panel electronic board was suspected to have spoilt. I don't know whether this will affect the car's other electronic system.
A recently computer diagnostic found no error. So what was the possible cause for this jerking. Thanks for your help.