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1991 Jaguar XJS has wild fluctuation when idling as it goes from 700rpm to 1500rpm and back constantly like a heartbeat and cruise does not work any suggestions?
Wiki User
∙ 14y ago
The cyclic fluctuation of your idle from 700rpm to 1500rpm is caused by a vacuum leak. This leak is causing a very high idle. The ECU has a fuel cut-off feature that temporarily cuts the fuel supply if the rpm exceeds about 1800rpm while the engine is at closed throttle. The idle drops as the fuel is cut off, but rises again when fuel is restored, causing the cyclic fluctuation. The solution is to find the vacuum leak. The most common cause of this on the XJ-S is the AAV. (Auxiliary Air Valve) This is the valve that lets in extra air during warmup, then slowly closes off as the engine gets up to operating temperature. The AAV is in the metal housing below the short 90 degree hose at the back of the driver side intake manifold. The AAV can stick open. If the car's idle fluctuates only after warmup, it is likely the AAV. If the idle fluctuates immediately on cold start-up, it is more likely a different air leak. To track down a vacuum (air) leak on the Jaguar V-12, warm up the engine, shut it off, then remove the driver side air filter cover and air filter. Restart the car and cover the AAV air intake hole with something large enough to plug the hole, but NOT something that could be sucked in. The AAV hole is smaller and in front of the throttle body opening. If the idle drops down and the fluctuation stops, the AAV is the problem. If not, listen for a hissing and track down the vacuum leak. Brittle, broken vacuum hoses are a common cause. Good luck. Tony Evans, Omaha NE
Wiki User
∙ 14y ago
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