Minus one: easy ways to find the cause of engine thrashing


This “boo-boo-boo” is for a reason!

There will be very little theory today. Engine thrashing is when a four-cylinder engine experiences ignition skips for some reason in one of the cylinders. Although I once came across an amusing theory on the Internet that the thrashing is when a four-cylinder motor is missing one of the clock cycles. Well, it happens. I will not comment on this scientific discovery.

Anyone who started his way of a motorist with a “nine” or something similar spiritual and staple, will immediately roll up his sleeves and in most cases find the cause of the malfunction. Those who have recently sat down on the wheel of an injected car and mix up the DPKV (which we wrote about not so long ago) and the oxygen sensor, have a harder time. It seems that there are too many clever things, blocks, sensors and other “kompuhtors”, and it is difficult to understand in such engine. There is a part of truth in it. Death of the same dpcv can lead to insecure start of the engine, and to its throbbing. But let’s face it: it’s not so often that these sensors die on their own, and the cause of the TNT is much more often much simpler. And finding it yourself is not only easy, but pleasant.

It’s impossible not to notice the TNT itself. The engine works unevenly, instead of a uniform “trrrrrr” from the exhaust pipe you hear some uneven “trrr-boo-trrr-boo”, and sometimes it can slam quite loudly (if gasoline accumulates in the exhaust, evaporates and rattles from the heated iron). Sometimes (but not always) the Check Engine light will flash or be on. And it is already a good reason to engage in diagnostics.

I want to warn you right away: computer diagnostics is a necessary thing in itself, but do not hope that someone will connect his Chinese ELM 327 scanner to your car for 500 rubles and immediately diagnose. Most likely the result of such a diagnostic will be the phrase “ignition failure in such a cylinder” and that will be all. Why the skip, where the skip comes from, what to do with it – all this puts such an ill-diagnostician into a deadlock. In response, you will see the look in his eyes, the way a retired recluse looks at LJ, and you will hear inarticulate babble. So, if there is no money or desire to go to a good, but expensive diagnostician at once, it makes sense to disassemble the engine yourself. As an example, I will disassemble the simplest four-cylinder gasoline engine, which is in the majority in modern budget cars.

The approach to solving the problem I propose is not quite standard. We will go not from cause to effect, but vice versa – the way we face it in life. That is, first of all, we will carefully analyze under what conditions the TNT occurs, and then, based on this information, we will think about why it may occur. Let’s begin with the simplest case – the TNT after morning start in wet weather.

Morning, rain, no spark

The situation: in warm and dry or rather frosty weather the engine starts easily in the morning, but in wet or rainy weather it is difficult to start. After starting the engine is trembling, but as it warms up, it starts working smoothly. It is the simplest and classic case, which is quite simple to overcome.

Almost always in this case the ignition system is to blame – high-voltage wires or ignition coil. Overnight they can get damp, and the spark that escaped through the breakdown starts to walk on their wet surface. As it warms up and dries out, the problem disappears on its own, but in the morning it appears again. In dry weather, of course, much less often.

Of course, it would be best to have a scanner for this case: it would allow you to determine exactly in which cylinder the skips started. The rest would be easier: you know where to look. But if you do not have it, you can go the old-fashioned way: start the engine, give it a little touch, then take turns unscrewing the plugs. A wet from gasoline spark plug will indicate the cylinder, which is lazy to work in wet weather. At the same time, by the way, you can look at the plugs. Replacement of plugs in this situation helps seldom, but if they are bad – it’s an occasion to put new ones (only qualitative and suitable to a motor, but not the first ones you see).

Now it’s time to find out what’s the reason for the lack of spark. Don’t laugh, but the old old-fashioned way works not only on Zhiguli and Moskvich, but also on quite modern injection engines: you need to look at the tripping motor in the dark, in the morning or evening. Well, or at night, if the tripping does not let you sleep peacefully. A broken high-voltage wire can be seen at once by beautiful sparks, which should not be on it. If for some reason it can’t “get” the spark to the spark plug, that spark will still come out somewhere. Most likely, in the place where the wire insulation is worn or cracked from old age. They haven’t learned how to repair wires yet, so you’ll have to replace them. It is better to replace them all at once, because the life of all wires is the same, and if one wire dies, all the others will die soon.

The second reason is the ignition coil breakdown. There are some engines that have it practically a generic disease – the coil “sews” to the ground because of the case breakdown. For example, very popular Renault K7M/K4M engines or Ford Duratec 16V Sigma (for example, on second Focuses) and some others like to indulge with it. What can you do: in these units, the coil housing is so close to the motor that it constantly overheats and begins to crack with age. Super-saving people in this case do not change coils, but repair them. For example, they douse the housing with epoxy. Of course, it is a good dielectric, but it is still better to replace the coil.

And then we get to the main question: is it the wires or the coil to blame? Which should be changed first? If the way of checking in the dark did not help, you can check both with a multimeter. You can inspect externally: if there are areas of different color with incomprehensible dark dots, then these are the places where the coil or wire “stitches” the spark. Well, the most reliable way is to ask someone to change your wires or coil with the ones you know are working.

The spark plug is less often blamed for the engine stalling in wet weather. It is not difficult to determine its fault: you need to swap plugs. If along with a spark plug ignition skips in the other cylinder, the fault of a spark plug is proved. Repositioning will also help you find the faulty individual coil – the one that is placed on each spark plug separately.

There are several other methods you can use to find a coil or wire breakdown. They consist of leaning the unscrewed spark plug against the “mass” and cranking the engine with the starter, or removing the wire tip from the spark plug and seeing if a spark appears between the wire and the spark plug. These methods work, but I would not recommend abusing them. First, if you touch the wire (even if not necessarily punctured), you may get a great shock, and that is not pleasant. Secondly, if in fresh air between the electrodes of the spark plug the spark is good, it does not mean that it will occur while the engine is running. There the spark may not have enough power to break between the electrodes, and it will go to the insulator or break through the life-beaten insulation of the high-voltage wire at all. Well, and thirdly, it is possible to burn the quite working ignition coil. So, the accuracy of this method is conditional. No spark is definitely bad, and if it is there, it does not mean that it is always there.

The engine is going haywire under load.

Here, everything is much more complicated. If the TNT starts at high revs and at high speed, you need to look at the plugs in the first place. Especially, if the recoil is noticed in any certain cylinder. If the plugs are inexpensive, it is easier to replace them. If the problem is gone – good, no – you need to dig further. Digging without experience is long in this case. There are a lot of reasons for the engine failure at high revs, and it is difficult to find the right one. There can be sensor failure, and breakdown of the same coils or wires (but in this case, as I said above, the trouble is more typical for wet weather), and the vagaries of the gas pump or throttle. It happens that at high revs the valve hangs up because of fouling, broken or tired spring or dead hydro-compensator, or ECU can not define the amount of fed gasoline because of incorrect data from dead lambda-sensor. In short, without experience and tools, it will be difficult to find the cause of the malfunction, and it is better to go to the service.

If you want to at least do something yourself, you can check for air leakage in the intake. It makes sense if all cylinders are affected in turn, not just one, and the color of the side electrode of the spark plug is unnaturally light, up to pale. Air unaccounted for by DAD or DMVR leads to too poor mixture, and it is most noticeable just under load. If attempts to do something on your own have not been successful, it is better not to delay visit to car service: driving on too poor mixture leads to strong overheating, burnt-out valves and glow ignition.

It’s a little bit hot in the morning, it stops after warming up, and then it’s hot again after you hit the gas

It sounds complicated, but the situation is quite common. More often than not the ignition is not to blame, but the power supply. The first thing to do is to unscrew the plugs and look at them. Most likely, one of them will be in dark soot. The reason in this case is a leaky injector. More often it is easier to change it, but someone prefers to clean the injectors or perform other resuscitation procedures with them of varying degrees of effectiveness. On many cars it is not difficult to check the serviceability of the injector: it is enough to remove the ramp from them and turn on the ignition. If the injector is dripping – it does not hold pressure and overflows. Hence, there is a fidget because of too rich mixture in a cylinder with such injector. And do not forget to put a new sealing ring when replacing the injector, otherwise the repair results will not be pleasant. In general, of course, not everyone will want to remove the ramp themselves, but in some cars it is really not difficult to do.

Similar symptoms are when the gasoline pump does not work efficiently. The spark plugs will not be distinguished by fouling, but the symptoms are similar. While the revolutions are small, there is enough fuel, but if you give more gas, it will not be enough. Unfortunately, to check the petrol pump by yourself you need at least a pressure gauge to measure the pressure on the fuel rail. A characteristic feature – ignition skips will not be in one and the same cylinder, but in all. With a faulty injector, especially when using a scanner, it is difficult to confuse.

After starting, it runs fine, it throws up as it warms up.

This rarely happens in life, and the search for the cause can take a long time. Most likely, it will be difficult to find the cause without a minimum set of tools. Electrics are rarely to blame in this case, the most probable reason is purely mechanical. First of all, it is worth checking the compression and valve gap adjustment. Most likely, the valve clearance is broken somewhere. However, it will be good to check plugs, wires and coil – what the hell. But then you still have to go to service for more serious diagnostics.

I’m an engineer at my mom’s!

Naturally, we have not considered many reasons for the engine thrashing. Low compression, sensor errors, phase mismatch and many other things I have omitted intentionally. All this is difficult to diagnose without equipment and experience. And such troubles are much less common than punctured wires and a half-dead gas pump. But the methods of elementary search of the TNT reason, described above, can help in many cases. Well, in order not to bring the situation to the necessity of looking for the spark that ran away, it is enough only to change the plugs and wires in time, to carry out maintenance and not to forget about the small defects in the work of the engine.

Even minor ignition misses due to an “almost” serviceable spark plug can lead to a wire breakdown, or even worse, a coil, which can already cost quite a bit of money. Driving with the ignition going off is not good for you, either. If only the gasoline consumption would be increased and the dynamics would drop, but no – the consequences can be much more diverse and very unpleasant. For example, the increased engine vibration kills both supports and poorly fixed wiring elements. And also the automatic transmission electronics can go mad, which does not understand, why crankshaft revolutions jump up and down sharply all the time.

In general, there can be all sorts of consequences, although all of this can be avoided by buying some candles for 300 rubles. So, if you can not do something yourself, it is better to give the car to professionals at once. Sometimes it comes out that way considerably cheaper.


Leave a Reply

Your email address will not be published. Required fields are marked *