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Old 30th March 2013, 22:57   #1
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Default Knocking Problem at 1,500 rpm

Hii

Apologies for starting a new thread. Please merge with the appropriate one mods.

Ok, i am facing a small issue with my esteem.

First of all, please consider the list of all the activities carried out :

1. Replacement of rubber seal to stop oil leakage,
2. Replacement of gear oil
3. Oil Change to semi synth : total quartz 5000
4. Air filter clean
5. Change of spark plugs to Denso ones.Gap set : 0.6
6. Tightening of belt to get rid of squealing starts.


Now, the performance is great, car returned a mileage of around 21 on CNG but i am facing a small issue,

Whenever a step off the accelerator or coast, there is IMMENSE Knocking at 1500 RPM.
Suppose i am at 2100 and i coast, the engine knocks like anything from 1500 to 1200 rpm.
To get it stable, either i have to step on the gas or disengage the clutch.

Please help !
Is it belt too tight , or new spark plugs , or has the timing got off.

Mods i tried to find an appropriate thread, couldnt find any, please merge.

Thanks
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Old 31st March 2013, 08:51   #2
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Default re: Knocking Problem at 1,500 rpm

Get your fuel injectors checked, there is a chance that they are injecting less than the fuel required at lower RPMs.
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Old 31st March 2013, 11:56   #3
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Default re: Knocking Problem at 1,500 rpm

Knocking is a sign of premature ignition, the fuel is ignited before time. Here is what you can do

. Retard the engine timing. As the knocking is at low RPM it means that the fuel is igniting before it is supposed to do. As ignition of fuel takes a finite time, that time lag has to be adjusted at all RPM ranges so that the combustion is at the proper time (wrt the piston position), hence at high RPM the ignition is advanced, while at low RPM it is retarded. As each fuel has different time between the initiation of the spark and combustion, the automatic advance has to be tailored to each fuel.

. If retarding the engine stops the knocking but reduces the performance, then your automatic advance is not working. In most of the cases there is a vacuum system, taken off the inlet, which ensures that as the engine RPM increases (the air inflow also increases, hence creating a lower pressure at the take off point), the timing is advanced. So an effective and proper range of the automatic engine advance is required.

It so happens that the range required for CNG operation is much more than that for petrol engines. That is why the ignition timing in CNG is retarded a bit, resulting in slack performance, the factory fitted CNG have a wider range advance system.
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Old 31st March 2013, 13:11   #4
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Default re: Knocking Problem at 1,500 rpm

Quote:
Originally Posted by Aroy View Post
Knocking is a sign of premature ignition, the fuel is ignited before time. Here is what you can do

. Retard the engine timing. As the knocking is at low RPM it means that the fuel is igniting before it is supposed to do. As ignition of fuel takes a finite time, that time lag has to be adjusted at all RPM ranges so that the combustion is at the proper time (wrt the piston position), hence at high RPM the ignition is advanced, while at low RPM it is retarded. As each fuel has different time between the initiation of the spark and combustion, the automatic advance has to be tailored to each fuel.
That is just an amazing explanation brother ! I went back to get it checked. Told him the same thing and he basically increased the spark plug gap. That did the trick. Ride is less jerky and very smooth. A small uneveness is still there at 1300-1500 rpm but it barely gets noticed. I asked him for reducing the advance as well but as per him, this will also do it for now.

Now, your explanation also answers one of the question i always had in my mind. In my air filter assembly, there is a twisted 30 cm something tube going nowhere. Its all twisted and curvy and is closed at the top. I always wondered what kind of joke is this, people try to maintain an even air intake and they have installed a tube twisting and turning and going nowhere, I guess this vacuum explanation is the basic reason. A similar tube is connected to the engine top as well.





Quote:
Originally Posted by EagleEye View Post
Get your fuel injectors checked, there is a chance that they are injecting less than the fuel required at lower RPMs.

I guess injectors affect performance when the car is being run by petrol. I am sure they are pretty clogged as my car runs better on CNG than on gasoline ! For me spark plug gap did the trick. If it persists, i'll go for a slight retarded timing.

Thanks Guys !
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