|
Most cars have stock faults. These are the problems which will beset every car of a particular model sooner or later.We've had a Peugeot 206 for about a year or so now and I can report on a few stock faults, one or two of which appear to defeat most people.After searching the Net for hours looking for a cure for tickover problems, I almost gave up and followed a trail no doubt trodden by most plagued with this fault.I'll describe it... the car drives well enough but dies when coasting to a road junction or coming to a stop. It always starts up again when the key is turned, and will continue till slowing down to rest again when it will die again. If the car was started it will tick over quite happily but blip the accelerator and the engine races then dies as its tickover speed drops to an unsustainable number of revs.Pundits advocate a new stepper motor and/or a new throttle potentiometer, even a new ECU.Let me tell you what these items are. First understand what tickover is. At rest the engine should be held at a speed which is not too fast as to cause a problem with engagement of first gear and not too slow as to make it difficult to move away cleanly without stalling. Let's say 1000rpm is a good target to aim for.With an old fashioned engine tickover was set in the carburettor by selecting a mixture and throttle setting by careful twiddling of a pair of screws. This is a reliable way of doing things and served us well for the best part of a century. There are small drawbacks with such a simple system, namely that over a period it will need to be reset to deal with wear and deposits of carbon or oil, and, more importantly, a heavy engine load such as headlights and windscreen motor, not to mention a rear window demister and air conditioning will make the engine slow right down to the point of stalling. Not good, if indeed it's a very wet night and the battery is past its best.The designers came up with a new system, based on feedback and dynamic corrective action. Once the engine was managed by an ECU containing a microprocessor, it could be tailored to also tick over at a precise speed, no matter what the load and state of wear. Built into the carburettor is a small electric motor, in fact a stepper motor, which responds to the engine speed at tickover and maintains it at a preset figure. If the speed is 1100rpm the motor steps down a notch and a mini throttle control (a plunger with a tapered end fitting into a hole) lessens the speed. If too low, say 900rpm, the mini throttle steps up a notch and speeds up the engine. In fact the feedback loop, subject to a damping factor to deaden the speed, or response, of the effect will continuously hunt to get exactly 1000rpm.If this system works as it is designed then nothing should deflect the tickover speed from its target of 1000rpm.Why is it then that all Peugeots sooner or later have stalling problems at tickover?Clearly, if any part of the tickover control loop is faulty, you will have problems. For example, if the engine microprocessor gets the engine speed wrong it cannot control the tickover speed. The fault could be in the circuit that measures engine speed, or the throttle potentiometer which tells the microprocessor that the throttle is not being pressed, and therefore failing to indicate that tickover is required.If the stepper motor fails, either because its bearings are stiff, or the windings have failed, or part of its mechanical arrangement has broken, then clearly it cannot provide the correct setting for the mini throttle.Hence, as a rule these two items will be changed in order to fix tickover problems, and no doubt in some cases this will do the trick, but in a lot of cases the fault will remain, until a garage mechanic eventually fixes it. Expect a bill of a couple to several hundred pounds. First a new stepper motor, then a new potentiometer, then horror of horrors a new ECU. This conjures up much the same concern as the dreaded words when the TV lost its picture... it's the tube guv.Let me give you a tip. Blip the throttle and watch the rev counter (or just listen to the engine speed if you don't have one). The speed goes upto 3000rpm then drops to 500rpm, then hestitates as if fighting for breath before correcting itself or just giving up with a shudder and stalling.There's the clue... fighting for breath. Slacken the screw holding the strap holding the air cleaner pipe to the carburettor and pull off the pipe.Try again and see if the problem has vanished... blip the throttle and see if the revs return fairly quickly to 1000.If the problem is fixed, replace the pipe and remove the top of the air cleaner housing. Remove the air cleaner and fit a new one.... cost just a few pounds, or make your own from a thickish piece of scrap plastic foam.. cost nothing.Of course you may be unlucky. Try the stepper motor first. Slacken its fixing screws and, with the engine running, gently ease it away from the carburettor. The engine should speed up: then as the feedback loop works the motor will move its plunger outwards and attempt to block the mini throttle aperture and the engine will slow down. Now push the motor back into place, ever so gently, and the engine will slow down and stabilise. Repeat until you get the feel of the feedback loop in operation. If nothing much happens fit a new motor, but note the number on the old motor first so you can get the right one... there are loads of different types. If a new motor doesn't fix the problem, fit a new potentiometer.I understand that there are quite a few other potential faults ranging from a faulty ECU, a loose battery earth bolt, poor sensors. All I can suggest is a trawl round the Net and then if all remedies are tried and else fails, visit your local garage or sell the car! |