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BMW 635 CSi: Rough idling
A BMW 635 CSi imported from Japan brings its owner a lot of joy, but one day it annoys him with a rough idle. The first repair measures do not bring the desired result; it is only the high-tech diagnostic system from AVL Ditest that tracks down the perpetrator.
BMW 635 CSi: Rough idling
The BMW E24 luxury coupé, built from 1975 to 1989, was revised for the second time in 1987 and sold under the name 635 CSi for around 80,000 German marks. Today, a well-preserved example of the luxury sports car sells for around 30,000 euros.
A lover of the BMW classic imports a particularly well-preserved example from Japan that only has 58,000 kilometers on the clock. The gasoline-powered 3.5-liter six-cylinder in-line engine produces 211 hp and 305 Nm of torque and accelerates the 1.5-ton sports car from 0 to 100 km/h in 8.1 seconds; the top speed is 225 km/h. The catalytic converter and lambda sensor on board ensure exhaust gas purification. One day the owner brings his dream car into the workshop with a rough idle.
A diagnostic crime thriller begins.
▶ Initial diagnosis: The vehicle runs normally when cold, but when the engine is warm the idling becomes increasingly rough. If you turn off the engine and let it cool down for 15 minutes, it will be difficult to start when you try to start it and will give off a distinct smell of gasoline.
▶ First measure: The idle control is replaced. Conclusion: the idle speed improves somewhat, but it still doesn't run completely smoothly.
▶ When the engine is warm, an emissions check is carried out using the four-gas tester on the tailpipe. Result: All values are perfect, but a cyclic enrichment in connection with a change in the engine running smoothness can be noticed about every 30 seconds.
▶ The AVL Ditest oscilloscope is used to check the voltage of the lambda probe: The curve shows no deflection and, after greasing with a cold start spray, voltage spikes into the negative. Conclusion: either the probe is defective or a cable is broken.
▶ The lambda sensor connector is checked, secured and sprayed with contact spray. With success, the control circuit shows a perfect picture on the oscilloscope, but also a problem: cyclic voltage dips (lean dips) in the lambda sensor voltage.
▶ The injectors are sprayed with cold start spray. Conclusion: In cylinders 3, 4 and 5, enriching shows a change in the lambda sensor voltage, which leads to the conclusion: The seals on the injectors are defective. In addition, the fuel pressure regulator is identified as the cause of the escaping gasoline vapors.
▶ After replacing the valve seals and the fuel pressure regulator, all problems are resolved, the engine runs smoothly in the entire speed range, the case is solved.
PETER SEIPEL FROM YOUNG TO VINTAGE CAR
The 30th birthday is a magical date for a car. It marks the end of his “bool years” as a young timer and from then on ennobles it as an old timer. If you want to dedicate yourself to the beautiful hobby of classic cars, you certainly can't go wrong with purchasing a BMW 635 CSi. Since the luxurious sports car was built for over 13 years, it is BMW's longest-running model series. This ensures the supply of spare parts for many years to come. A total of 86,219 vehicles were built by 1989 before production was discontinued in favor of the larger E31 8 Series Coupé. More mature examples of the long-maintained 6-series also have the advantage of being free from childhood illnesses or the sins of youth. The legendary 635 was last seen as the company car of the Bavarian Commissioner Benno Berghammer in the television series “The Bull of Tölz”.