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I can feel a good shake of Engine when I start the car in the morning and specially after the engine has completely cooled down. The shake reduced a bit after I replaced the spark plugs some three months back, however it has not gone completely.

The initial rev starts at 1100 rpm and gradually reduces to 750rpm and the shake is severe at rev 1000 rpm. (both above and below 1000K the shake is minimal). Also the shake disperses completely when I put my car into gear. After I take it for a spin for about 5 mins and after the engine temp reaches just below the half mark on the temp gauge (1 mark below hot and cold) the shake disappears even on idle.

I feel this is a typical misfiring case, but would like to get the opinion from experts here before I take it for servicing.

NB: Engine and Transmission mounts are all healthy and in-place. No Fault code and Check engine is displayed

Specs - Mazda 3 1.6 ltr DOHC 4-door sedan AT 
  • You have mentioned changing the spark plug helping, maybe try to take a look at the spark plug wires – method Oct 18 '16 at 7:16
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    Are you sure the motor mount are healthy? Mazda 3 motor mounts are notoriously bad for failing. mechanics.stackexchange.com/questions/36103/… – vini_i Oct 18 '16 at 10:29
  • I have got it checked through two different service centers (and I was personally present), the mount bush is all in place and there are not torn outs... – Youbaraj Sharma Oct 18 '16 at 13:08
  • Has the mount ever been replaced? – vini_i Oct 18 '16 at 19:33
  • I would clean the throttle body and check/clean the Idle Air Control asssembly. – SteveRacer Oct 19 '16 at 4:18
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Sounds like it could potentially be a misfire caused by either poor air flow at idle, worn components, timing chain or a fuel system problem, along with anything from carbon build-up, clogged injectors or worn engine/box mounts. Watching an engine running on 3/4 cylinders lets you physically see the engine:

  1. Shaking around - The engine's crankshaft is balanced with each explosion from each cylinder in mind. If one's missing, it'll throw it out of sync.
  2. Slowing down - The engine is only having three quarters, so for the first three quarters, the engine is essentially keeping a constant speed, then the missing combustion would cause the engine to slow down a tiny amount, which also increases number 1.

However, you've also stated that it feels fine when the engine is warmed up. This tells me something could be worn. When things get hot, stuff expands. That's the same reason F1 engines need heating up before a race, and why train tracks have gaps between each piece of metal rail. When the engine is cold, there could be a minute gap on something, causing it to allow parts to move. This may or may not throw a code as it would be classified as a knock-detection sensor.

It could also be to do with an idle air valve; At idle, the butterfly valve that allows air into the inlet manifold is completely sealed, allowing only a tiny amount of air through either a specific valve or a tiny, tiny channel along the edge that bypasses the valve itself. This might be clogged or akin. However, this should throw a code.

It could also perhaps be a timing belt/chain, depending on which your car has. If it's even slightly worn, it could be either advancing or retarding your timing depending on how severe it is. If this is allowing fuel into the chamber AFTER the spark plug has already been set off, it's not combusting the fuel, therefore creating a miss-fire problem. However, this should throw a code.

It could also potentially be an injector problem or a fuel pressure problem. If the fuel pressure isn't high enough, it may not disperse the fuel into the combustion chamber optimally, and if the injector is having the dreaded squirt-squirt-squirt-sneeze problem that some Mazdas suffer from (inside joke from some Mazda workshops where I live) it could be that one of them is allowing too much fuel or too little into the combustion chamber, not allowing a total combustion or any whatsoever. However, this should throw a code.

There's probably many, many more problems, but these are the basic diagnostics for most petrol vehicles that run a timing chain, 4-cylinder configuration with a poor idle.

Good luck, I've always been a fan of the Mazda vehicles, though not necessarily their engines.... :P

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I am coming back to this almost after a year, but it actually took me a lot of time to do multiple items to partially solve the puzzle. I took advise from multiple dealers and service centers (local and branded), here are the steps I undertook. I am documenting this for others to refer

  1. Throttle Body and MAF sensor cleaning - It was dirty and plenty of carbon deposit, but it helped me more on the Miles per liter side rather than reducing the vibration
  2. Spark Plug Change - Though I was doing it regularly, but this time I made sure I replace it with exact specification as mentioned in the booklet
  3. Ignition Coil Replacement - This was an expensive affair so I started with cylinder 1 and 4 (as suggested by the dealer) and boom it reduced the vibration almost 85%, and I got a super boot in acceleration and power, so I decided to change all 4 (but the performance was almost same as what I received with first two changes)

  4. Another suggestion given by local dealer was to clean the Injectors or do a Sea Foam which I haven't tried yet. I believe this will clean the Piston heads and Injectors. Other even suggested me to do some water therapywhich they suggests cleans the Piston Heads better than Sea Foam

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