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Sluggish V6 Engine - Mk1
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Sluggish Mk1 V6 Engine Below 3200rpm
Background
This is common with aging Mk1 V6’s. The secondary valves are vacuum operated and open and close to allow the engine to switch between 18 valve(12 exhaust and 6 inlet valves below 3200rpm) and 24 valve (12 exhaust and 12 inlet valves above 3200rpm) mode. The reasoning for this is that 18 valve motors typically have more torque at lower revs, while 24valve motors deliver more power at the top end.
The vacuum pipes are made from plastic which over time get brittle from the heat in the engine bay. This causes them to crack which in turn allows the vacuum to leak, and leads to vacuum failure, where the secondaries cannot close themselves. As a result they are ‘stuck’ open and the car runs poor below 3200rpm.
Method
This is relatively straight forward to fix - the vacuum hoses need to be replaced. This is not a difficult job, do-able for the DIY mechanic. Just be sure that when you pull off the old hoses, the new ones go on the right parts! Also take care when applying pressure to install the new hoses, that you don’t damage or crack them.
Other Components
Two other components in this system are the solenoid switch that sends the signal to open and close the secondaries, and the mechanical diaphragm which physically pulls them open and closed. Of these two other components, it appears that the solenoid switch is the more likely component to fail. When I got the vacuum pipes replaced on mine, the solenoid switch had already, so I replaced that too. The UIM (Upper Intake Manifold) has to come off to get at all the vacuum pipes, so its worthwhile replacing the solenoid switch if you suspect its faulty.
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