intermittent misfiring/coughing while in D gear

Refer to earlier posting where my Honda Airwave 2009 overheated due to a thermostat issue : after replacing
thermostat, for the first few days, the car had very bad frequent 'coughing' which became almost unnoticeable
after a few days.  

For the last 1 week, this 'coughing' / misfiring returned but not as 'violent' or frequent as when the first few
days after the thermostat got replaced.

As I'm trading in this car, I'll need to get it fixed quickly otherwise the dealer is going to (mis)claim it's a major fault
& offer a much lower trade-in value.  At lower gears like S or L, I don't notice this 'coughing' (or perhaps it's not
noticeable), it's only at D gear that it's noticeable the last 1 week.

What are among the lowest cost components I could replace first?  
Spark plugs or ?    I just got the gearbox replaced about 1.5 months ago.
Is engine mounts the likely cause & if so, shouldn't it manifest in this 'coughing' at S or L gears as well?

Or how can I narrow down the root cause?
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JohnBusiness Consultant (Owner)Commented:
1. Replace spark plugs and check ignition wires for cracking and wear.
2. Make sure you are using premium gasoline,
3. Not likely engine mounts.
4. Get the timing and exhaust checked at a garage for possible catalytic convertor clog.
sunhuxAuthor Commented:
>1. Replace spark plugs and check ignition wires for cracking and wear.
Ok, will ask workshop to check ignition wires;  will spark plugs give such intermittent issue or it's more likely consistent issue?
This morning, it was completely smooth with no noticeable 'coughing' but I'm not sure if it will return.

>2. Make sure you are using premium gasoline,
Had been using  ESSO Supreme last 2 months.

>4. Get ...
Ok will get workshop to check this as well
The main symptom of worn engine mountings is excessive movement of the engine.  The engine is always going to move a bit every time it is revved, but the mountings (top one sometimes called "struts") are there to restrict this movement to a minimum.  The most common ones have a lump of stiff rubber in them to absorb vibration while still allowing but damping the small amount of engine movement.  There are some fancy and more expensive fluid-filled "hydro-mounts" around, and even active electronically controlled ones that use a vacuum controller and react dynamically to changes, but I would suspect that your car as standard metal and rubber mounts.   Remember that everything that is fixed to the engine is moved every time the engine rocks slightly when revved, so your rubber coolant hoses are being repeatedly flexed, your fanbelt is probably being stretched and loosened, the exhaust manifold to exhaust connection via a flexi-pipe is being repeatedly bent, and even linkages like throttle and transmission (less so on a front wheel drive than a rear wheel car) can be affected if the engine is moving too much.  A sticking throttle cable every time the accelerator pedal is pressed and the engine moves excessively could cause a jerky ride and what might seem like "coughing".

I would think that you would have been aware by now if the engine was being allowed to move too much because of worn engine mounts.  With the car in "N" get somebody to repeatedly press the accelerator pedal sharply from idle speed.  Watch the engine and listen for clonking. You might be able to do this all yourself if you have long legs or arms.  Compare the amount of engine movement with a newer car of similar type.

Your car was only produced for the Japanese and far East markets, so I am not really familiar with it, but from Wikipedia it looks as though it has CVT (continuously variable) transmission.  This is "stepless" and somewhat similar to a bicycle's derailleur gear change.  You normally don't even know what gear the car is in and when it changed, so worn engine mounts would be less of a problem than in a car where you feel the gears changing and the car engine would move during these changes.

From my limited knowledge of automatic cars, the D-rive mode is standard, the L-ow mode keeps the car in the lowest gear, and the S-port mode allows the engine to reach higher revs before changing to a higher gear.  I understand that on some newer cars the Sport mode also stiffens suspension and makes the car more like a low-slung sporty car in terms of ride, but I am not sure about a 2009 Honda that is designed as a "people carrier".

If you had bits of debris in your fuel line that had been sucked up from the tank they would be caught in the fuel filter.  If the fuel filter was clogged it would affect the car and make it cough and splutter, especially when the engine needs more fuel like going up a hill and starting up.  If it was very blocked the car would misfire even when idling.  I would have thought that a blocked fuel filter would have manifested itself more in L and S modes than in D mode, which is the opposite of what you are experiencing.  You can get problems where there are bits of debris that get sucked into fuel lines when the car is revved up, but then float away when the car engine slows down again, and this could cause intermittent stuttering or stalling of the engine speed.  A bad batch of petrol can occasionally contain water or debris.  Filling stations won't normally volunteer responsibility unless lots of people are affected and have the proof to make claims, so generally you wouldn't know if this was the case.

Fuel filters on older cars were usually easy to replace because they were just a cannister connected between fuel lines that were accessible.  The problem is that many were replaced by filters inside the fuel tank and it looks like your fuel tank is in a less accessible place up under the front seats somewhere.

One thing you probably should check is the air filter.  In warmer, and often more dusty, country these can get clogged up quite quickly.  Restricted air to the petrol mix can cause spluttering and coughing.  Air filters aren't terribly expensive, but if you are trading the car in it might be just as easy to knock the existing filter against a fence to clear the dust from it.

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JohnBusiness Consultant (Owner)Commented:
There are several things that can cause "coughing".  I would put ignition, fuel delivery and exhaust near the top of the list. But you need to visit the shop and ask for a hands on opinion.
dbruntonQuid, Me Anxius Sum?  Illegitimi non carborundum.Commented:
Could be just about anything.

The garage can check and replace the spark plugs.  That should be a cheap job to do.  But I suspect that's not the problem.

Check air filter as suggested.  Check all tubing and connections from air filter to engine.  Look for loose connections, split rubber, split plastic, anything that might permit air to leak out of the tubing or into the tubing.

Check petrol filter as suggested.

Check all electrical connections to engine including battery terminals.

But again this could be almost anything ...
Thank you sunhux
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