Ask a mechanic - 12/11/07

A: As far as the odometer and speedometer are concerned, either the cable that drives them has snapped or disconnected itself, or the plastic gears (in the gearbox) that drive the cable have broken up (not as terminal as it sounds). It's also possible, though less likely, that the speedo unit itself is faulty. The gear engagement problem is due either to the clutch not disengaging (probably because of failure of the clutch auto-adjust mechanism at the pedal) or to a misalignment of the main gearchange selector rod where it enters the gearbox.

STICKY THROTTLE, HEART-STOPPING MOMENTS
Q: The throttle of my N-reg Ford Fiesta 1.3 keeps sticking, so when I take my foot off the accelerator the engine keeps revving. Any suggestions would be most welcome. Posted by: w.macdonald

A: It should be easy enough to find an obvious point of friction in the throttle cable and linkages - that's just a question of checking by feel and by eye. If the throttle cable is kinked or trapped somewhere, that will explain it. Also check that the throttle pedal bush (its pivot under the dash) is okay (i.e. that it moves freely up and down).

FIND THAT VOLT-HUNGRY GHOST
Q: I have a Rover 416 that has occasionally surprised me with a flat battery, even since it was new. I know this can be caused by a hidden light such as a boot light, but that's been checked and eliminated. So what else might be causing this? Posted by: davidjones697

A: The fact that this only occasionally happens suggests that either there's an intermittent fault with the charging system, or that some electrical consumer is occasionally being left on when it shouldn't - maybe due to a faulty switch. An auto-electrician can carry out a current-draw test to find out what's going on.

PERSECUTED BY A JAMMING CLUTCH
Q: There is an intermittent fault with my Peugeot 206 2.0 HDi. If I press the clutch pedal down it stays stuck there. I can pull it up again and it flicks back, so I can use it again, but obviously it's difficult and dangerous to drive like this. So far the garage doesn't believe me! Posted by: sjrw

A: It is indeed hard to believe. And the trouble with intermittent faults is that according to Murphy's Law they will rarely show themselves when there's a professional around waiting to pounce on them. This problem is usually due to excessively nasty wear of the clutch diaphragm fingers, where the clutch release bearing has worn their tips so much that it sometimes gets pushed through the diaphragm, when it should be pushing against it. Before you get the clutch replaced though, have a mechanic check that it isn't due to some external fault of the release mechanism.

TOO LATE TO TIGHTEN YOUR BELT
Q: My Renault Trafic 1.9dCi van just broke down on the motorway. The engine shut off and a Stop light flashed on the dashboard. I tried to restart it but it wouldn't fire, and I found that it now turns over very, very quickly. Could this be the cam belt or alternator belt snapped? Posted by: m6wul

A: Regrettably, it sounds as if the camshaft drive belt has snapped. The reason the engine is spinning over so rapidly is that it's not having to drag the camshaft around with it, which takes a lot of force. As this is a diesel engine it's highly likely that there will be serious internal damage to the valves, and possibly the pistons. Was a belt-change service overdue, do you think?

REGRETTING MY REBUILD
Q: My Subaru Impreza has recently had an engine rebuild, but now the engine has a tickover problem - sometimes it revs up high, to nearly 3000rpm, then drops back down to just over 1000rpm, then up again, and so on. I was told it could be the airflow regulator so I fitted a new one, but that made no difference. Can you shed any light on this annoying problem? Posted by: www.p-humphries

A: No, not directly. But it shouldn't be too difficult for a Subaru fitter to diagnose. Erratic idling is most often due to an air leak on the induction side of the engine, so that should be looked for. Failing that there may be a problematic supplementary air valve, or some other aspect of the injection system at fault. Fortunately most obvious faults are pointed to by the system's electronic control unit, which stores fault codes that can be read by Subaru agents.

A MAN CLUTCHING AT HIS GEARS
Q: I just had new clutch fitted to my Vauxhall Corsa 1.6 16V and now I can't select reverse or fifth gears, and on some occasions I can only get one or another. Any idea what's gone wrong? Posted by: trendy1271

A: It might simply be that the clutch release mechanism is out of adjustment, so get that checked. Alternatively, given that the gearbox would have been disturbed to do the clutch job, it's possible the external gearchange selector linkage is misaligned. Failing these, there could be a gearbox problem, although it's unlikely.

A LITTLE BIT OF HEAT GOES A LONG WAY
Q: I need to change the track-rod end of an Audi A6 but I can't detach the track rod from the end of the steering rack - it's so tight that I'm frightened it will snap the end off the rack. Help! Posted by: julian.wallis

A: You won't snap the rack, promise. Assuming you've already tried the obvious, which is to soak the offending joint connection in penetrating oil overnight, your best option is to get perfectly tight-fitting spanners - one for the joint and one for its locknut - have an assistant hold the joint spanner while you belt the locknut spanner with a mallet. If that doesn't work, heat up the locknut and joint connection with a blowtorch before repeating the exercise.

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