Car Shakes At Idle But Smooths Out While Driving

Car Shakes At Idle But Smooths Out While Driving

Although it might seem strange, this situation isn’t as uncommon as you might think. When your car shakes at idle but smooths out while driving – something you might notice when driving through busy cities with all their stoplights, for example – it’s not necessarily the end of the world.

As with all problems, it’s best to approach them logically. If the car shakes at idle, is it because of low revs – is that car physically struggling? – or is it running just fine but is still shuddering? Are there any well-known problems with your model, or have you noticed something playing up recently? All of these should be taken into account when you’re trying to figure out what’s going on.

If this happens to you, the first and most important thing is to keep your cool. If you can tell that the car is likely to stall, keep your foot slightly on the throttle pedal to give the engine enough power to keep ticking over.

However, if the car does stall, do your best to restart it and pull over to a safe location out of the flow of traffic.

In this article, I’ll be explaining some of the most likely things that could mean the car shakes at idle but smooths out again when you start driving. This is actually something that happened to me a couple of years ago – click here to skip down to the second on throttle bodies.

What Causes It?

A car shaking at idle, especially, is usually a sign of either

Of all these, most will lead to reduced engine power, especially at idle. However, for problems with engine mounts, the output might feel about the same. Therefore, engine mounts should be the first thing to check if your car shakes at idle and smooths out when driving but isn’t experiencing any drop in power.

One of the main things to look for is the rev counter when you’re at idle. When the car starts shaking, is the rev meter lower than it usually is? For example, most cars have a tick over of somewhere between 600 and 1,000 rpm. Suppose it’s 100 or 200 rpm lower than usual. In that case, the shuddering feeling is the car trying to maintain the engine’s turning momentum and struggling to do so.

When a car moves, any changes in speed produce a smaller shift in momentum – relatively – within the motor. That could be why the car shakes more at idle but smooths out when driving more quickly.

For an easy comparison, think of someone doing a workout. As they get more tired, their movements become slower, more strained, and require more effort. Sometimes their arms or legs will start shaking, even. Previously, momentum might have been helping them to keep going, but now, without it, they’re struggling. This is a bit like what’s happening when the engine is at idle, and your car has a related fault, leading it to start shuddering.

Misfire – Car Shakes At Idle

A misfire could be caused by one of literally hundreds of things, but there are some common culprits:

  • Spark plugs.
  • Ignition coils.
  • Lack of compression.
  • Injector issues.
  • Valve failure.

The main signs of a misfire are a significant drop in power – you’ll feel this both at idle and while driving – and a general shaking to the car. For more information about engine misfires, check out a couple of these articles:

The engine shakes while there’s a misfire because its natural balance has been disrupted. For smooth and efficient power production, engines are naturally in balance. That’s why most engines have an even number of cylinders (for example, Inline-4, Straight-6, V8, W16). Usually, while one cylinder travels up, another travels down. This balances out the forces and makes for more power and a better ride for the occupants.

(Note: Boxer/Flat engines are the most naturally balanced of the three main engine types. They’re followed by Inline/Straight engines, with V and W engines taking up the rear. V and W engines need extra components called balancing shafts to make them, well, balance.)

The engine shaking from a misfire transmits through the rest of the car by literally shaking it, a little like somebody standing in front of your car and pushing it up and down (just really quickly). The vibrations transmit exceptionally well through the powertrain.

An excellent way to spot a misfire in a manual car is to park up somewhere safe with the handbrake on, turn the engine on, and put the gearstick into neutral. With most misfires, you’ll notice excessive vibrations coming through it.

Why Might I Notice A Misfire More At Idle?

A misfire is more noticeable at idle because the engine has to work harder to maintain its speed.

You might also notice the shaking more at idle because the car is sitting still. It’s possible that, when you’re moving, these forces act to dampen the shuddering effect of the engine. Even the road noise might subconsciously impact you and distract you from the shaking when driving. However, when the car is stationary, you might be more likely to notice everything.

If you think you might have a misfire but aren’t sure, check out the two articles linked up above. After reading those, if you’re still a little unsure, it might be best to take your car to a trusted professional mechanic.

Engine Mounts – Car Shakes At Idle

Engine mounts… well, they mount your engine in the chassis. They hold it there and act as a sort of mix of legs, arms, and suspenders.

Your engine doesn’t just sit bottom-down on a flat piece of metal. The reason? The vibrations and shaking would be so unbearable that nobody would ever choose to drive their car more than a couple of miles. It would make a horse and cart look like a luxury transport option.

(It also just wouldn’t make any sense at all, but for the purposes of this article we’ll focus on the vibration aspect.)

Instead of sitting the engine on something, manufacturers use engine mounts to sort of hold it in space. This means vibrations are far less intense. The mounts are also designed to dampen the forces going through them, reducing the feeling of the vibrations even further.

If one breaks, it’ll be unlikely to have too much of an effect on the vehicle’s overall performance. That is, you wouldn’t usually expect to see a significant drop in power output (although it could be possible if the engine has literally dropped slightly, I suppose). However, you would certainly feel the vibrations coming through from the engine much more intensely.

You might be more likely to feel this shaking at idle simply because there’s less going on to distract you from it. If you drive an automatic, try putting the car into neutral (not while you’re moving! Park up somewhere safe first!). If the shaking seems to subside, engine mounts (or transmission mounts) might be the issue.

To get an engine mount repaired, you will probably need to take your car to a professional. The engine will need to be lifted out of place, and a not-insignificant replace or repair job carried out. Unless you have access to an engine hoist and know your way around engine mounts, I’d recommend leaving this one to the experts.

Fuel Intake – Car Shakes At Idle

Issues with fuel and combustion would very much link into the misfire and air intake sections of this article.

If there’s a problem with the fuel entering the cylinder, it could cause a reduction in power due to insufficient amounts being burnt. For example,

  1. The fuel injector could be malfunctioning.
  2. Significant amounts of dirt or debris could have compromised the quantity and quality of fuel, indicating a problem with the fuel filter.
  3. The fuel pump might not be functioning correctly.
  4. An intake valve might be malfunctioning or affected by debris.

These fuel-related issues could mean a varying quantity of fuel gets delivered into the engine with each revolution. As such, you’ll notice high-frequency vibrations, relatively correlating to the engine’s cycles per minute (rpm level).

Again, you can probably feel these vibrations at all levels, but they might be more noticeable at idle. The engine has to work harder to keep going. You’ll notice consistent poor performance levels at all times.

Timing Belt – Car Shakes At Idle

A timing belt (or chain) – otherwise known as a cambelt – has the job of matching the pistons’ positions to the corresponding valve positions. By linking the crankshaft with the camshaft through an ingeniously simple mechanical gear/pulley system, the engine continues to function accurately for hundreds of millions, if not billions, of revolutions.

Inevitably, though, things sometimes go wrong.

Nowadays, most manufacturers use timing belts over chains. They do that simply because it’s cheaper, although they’re also quieter and lighter. With the modern-day automotive market being as competitive as it is, manufacturers need every penny.

Timing belts often suffer from the curse of all belts, including those you wear on Christmas Day: they start off tight but lose tension and slacken out over time. Occasionally, they tear or crack. When this happens – and it will, at some point – you’ll need to take it in for a professional repair. This one isn’t something I’d recommend trying to do yourself. Misalign the timing, and you could be in for a new engine as a minimum and possibly a written-off vehicle altogether.

A bit like a bike chain, the timing chain can sometimes jump or slip. Chains tend to last longer than belts, as you might imagine, but they too can stretch over time. Again, you should take your car into a shop to get this fixed.

Because things aren’t lining up properly, you can get vibrations throughout the car. The engine will feel sluggish and almost pained, too.

Diagnosing A Faulty Timing Belt

It can be hard to diagnose a misaligned timing belt or chain if you don’t know your way around an engine. However, if it’s damaged or snapped altogether, it’ll certainly be playing havoc with your revs, one way or another.

(Note: people commonly get confused between the timing belt/cambelt and drive/fan belt. The timing belt connects the crankshaft to the camshaft and is responsible, in essence, for valve timing. You shouldn’t be able to see it on most engines without taking the engine apart a bit. By contrast, the drive belt is the one you can see when you open your hood. It connects the crankshaft to things like the power steering pump, air conditioning compressor, and alternator.)

A damaged or stretched timing belt could cause vibrations that you feel more at idle than at speed. This would be due to similar reasons to those mentioned before. Things are more noticeable when fewer forces are acting on the vehicle. The engine is also mechanically relying on itself to maintain momentum without manual throttle input from you, the driver.

The overall lesson? If in doubt about a timing chain, take your car to a shop that you trust. If your car shakes at idle but smooths out when you’re driving at speed, a timing belt could be the issue.

Timing belt replacement jobs can be relatively expensive. Follow the link for more information.

Spark Plugs – Car Shakes At Idle

Spark plug failure is usually down to carbon build-up. When it “fires” (produces a spark), this ignites the air/fuel mixture within the cylinder. Fuel, whether gasoline or diesel, is a set of long hydrocarbon chains. When these ignite, carbon is a by-product. Most of it escapes through the exhaust valve, but some will coat the spark plug.

This carbon coating can stop the plug from sparking entirely or simply reduce its effectiveness.

Technically, this would also fall under the realm of misfiring. See above for more detailed information about why it means your car shakes at idle but smooths out while driving more quickly.

Spark plugs are one of the most straightforward jobs on most cars unless you have a big, old V-engine. Cheap spark plugs aren’t a worthwhile investment – they’ll fail much earlier and can cause a whole wealth of other problems. It’s best to get the mid-range or premium brands.

Good spark plugs generally cost about $3 to $5 each for standard ones, while the more expensive iridium plugs can come in at up to $8, $10, or even $15 each.

Throttle Body – Car Shakes At Idle

When my car, a 1.2-liter Vauxhall Corsa C (2005), developed this very problem, the cause turned out to be the throttle body. It behaved fine most of the time, but it would suddenly start to struggle when sitting still with the engine running. It was almost as if it would suddenly panic, shudder and gasp for air, sometimes for a while, with the revs dropping well below the standard idle rate.

Then, eventually, it would either reset itself, or I would blip the throttle, raising the revs and seemingly resetting the system.

I was also experiencing occasional inconsistent throttle delay.

For a while, this puzzled me, but I then set to work checking a few things. After ruling out the spark plugs or any misfires, I turned my attention to the throttle body.

I popped down to the local parts store and got myself a £1.99 bottle of throttle body cleaner (which is all you need). I then took the throttle body off the top of the engine – it’s not too complicated on most cars – and cleaned it up, including the throttle plate.

The butterfly valve is the piece of metal that turns when you press the throttle pedal. The more you press the pedal, the more it turns, allowing more air to pass by it and into the engine.

(Note: although the butterfly valve – the rotating metal face within the throttle body – might feel sensitive and breakable, it’s a fairly tough piece of kit. It shouldn’t break. He says. Not that you shouldn’t be cautious!)

After cleaning everything up, I reconnected all the parts (including the TPS – Throttle Position Sensor – don’t forget about that) and started her up. And she worked. This was three or four years ago, and I haven’t had a problem since.

I must confess, to this day, I’m not sure what exactly was wrong. Perhaps the parts cleaner removed some dirt, dust, or grease interfering with the throttle plate or the TPS. Maybe simply unplugging and plugging back in the TPS (turning it off and on again, I suppose) reset everything.

Either way, you should check the throttle body and clean it up. It shouldn’t cost you much. Just make sure you don’t break anything or lose any of the connecting bolts.

It might just solve your problem.

Air Intake – Car Shakes At Idle

Problems with the air intake might be directly related to the throttle body. If you think that might be the case, scroll up slightly to read that section.

The air intake of a car is one of two parts that combine to form the power. The other is, of course, the fuel, whether gasoline or diesel.

If something is going on to prevent the air from reaching the cylinder, it’s a bit like covering a candle with one of those snuffer things. Remove the oxygen from the fire, and it goes out. That’s quite literally what happens in these situations.

For it to cause shaking, it’ll probably only be affecting one or two cylinders, thus leading to an imbalance. To my mind, the most likely reason behind this would be a faulty valve.

If a valve doesn’t open all the way – or, indeed, at all – it could choke the one cylinder of oxygen while leaving the others fully functional. This would create a misfire and a vibration noticeable when your car’s sat at idle.

Reduced airflow, in general, could make the car struggle, too. This could happen if there were issues with vacuum seals or even a ridiculously clogged-up air filter. If the MAF (Mass Air Flow) sensor records a lower volume of air entering the engine, the fuel amounts will also reduce, dropping the engine speed down towards tick over.

If the engine speed drops below its usual tick over, it will cause shaking and shuddering. With a substantially reduced airflow, it may even struggle to bring the revs back up and stall. To stop this from happening, you’d need to have your foot slightly on the gas at all times when stationary.

Conclusion

Many things might cause your car to shake at idle.

In summary, you should ask yourself the following questions.

  • Is the engine’s idle speed (tick over) lower than usual?
  • Does the engine have the same levels of power output as usual?

If yes to either of the above, the cause is probably mechanical and needs further diagnosis. You could try working on a few more simple repairs yourself, such as replacing the spark plugs or air filter or cleaning up the throttle body. You may be able to check the engine mounts yourself, too.

If you’ve answered no to both of the questions bulleted above, it likely suggests issues with the engine mounts or transmission mounts.

As with all things automotive, it’s possible to work on these yourself… but not necessarily recommended. Rather than make a costly mistake, I’d recommend taking your car in for a professional diagnosis if you’re at all unsure. In the long run, you might actually save money by doing it this way.

Thanks for reading this article. If your car shakes at idle but smooths out while driving away, please let me know down below if you’ve managed to fix it and, if so, what the cause was. Likewise, if you’re still trying to figure it out, post your symptoms in the comments to put it out to the community.

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