THE INFAMOUS cold engine knock/tap HELP
#11
04-06-2009, 02:42 PM
With 2 '94 Civics and 2 '95 Civics, and two out of the four having the condition known as "COLD ENGINE KNOCK"; yesterday I found it-- I found the answer to this long-standing dilemma while resetting a timing belt on one of my 94's.
Last week I had a mechanic install a new timing belt in this car (total mileage 188,000, last belt at 100,000) and afterward heard a terrible knock that would not quit after the motor got warm... I complained and was told to bring it in, the belt would have to be repositioned and it would be the same three hours and same labor charge... I looked at my Haynes repair, decided to try it myself, got half way through the job, discovered all that's really needed is take off the top belt cover, slip the belt off, make a mark on belt, make mark on cam sprocket tooth that you want the belt mark to line up with (one tooth to the left) then turn the crankshaft two inches, slip the belt on again making sure the two marks line up-- total time required 20 minutes.
But my situation wasn't that simple... I first moved the belt in the wrong direction, then had to move it two teeth the other way; this should have solved it, but no way-- turned out it needed to be moved yet another tooth! How did the mechanic get it so wrong.
And here's where it gets really interesting and why the answer to the mystery was finally revealed...
1) By the third time I removed the belt after running the motor sufficiently to warm it up to see if the knock would go away, I found the belt was much more difficult to remove and reset.
2) The thought occurred that the belt had shrunk due to the temperature increase-- garbage thought-- no, what happened was the motor expanded due to the temp increase, and therefore the distance from the cam sprocket to the crank pulley increased, causing the belt to tighten.
3) OK, the rest is obvious-- the motor warms, the belt tightens, it moves the cam shaft timing just enough to overcome the knock-- which is an IGNITION KNOCK, NOT A MECHANICAL PISTON SLAP, BEARING NOISE OR ANYTHING ELSE MECHANICAL (which would invariably get louder as the engine gets warmer).
4) If you suspect the truth of this analysis, all you have to do is loosen the distributor and rotate it counterclockwise 1/16 inch and see how much louder it makes the knocking sound when the engine is cold-- this will prove to all but the most stubborn that what you are hearing IS an ignition knock.
5) Finally-- (and now I'm in uncharted water needing more investigation); it might be better to let the "cold engine knock" live, since retarding the distributor could cause a slight loss of power, I'm just not sure about this part of it.
6) If you go through the usual procedure of timing with a timing light, and if the cold knock persists, I say leave well enough alone since you're probably getting the proper power level.
7) The real reason older high-mileage motors develop the knock might be some microscopic wear in the cam line that is happily overcome by the expansion when the motor warms up.
8) Do we care about the real reason-- I say it's enough to know it's ignition knock, not mechanical; and if you don't want to hear it you can turn the distributor clockwise 1/16" and put up with a possible slight loss of power (counterclockwise makes the noise louder and serves to identify it as ignition knock).
Case closed!
Have a great day, Richard
Last week I had a mechanic install a new timing belt in this car (total mileage 188,000, last belt at 100,000) and afterward heard a terrible knock that would not quit after the motor got warm... I complained and was told to bring it in, the belt would have to be repositioned and it would be the same three hours and same labor charge... I looked at my Haynes repair, decided to try it myself, got half way through the job, discovered all that's really needed is take off the top belt cover, slip the belt off, make a mark on belt, make mark on cam sprocket tooth that you want the belt mark to line up with (one tooth to the left) then turn the crankshaft two inches, slip the belt on again making sure the two marks line up-- total time required 20 minutes.
But my situation wasn't that simple... I first moved the belt in the wrong direction, then had to move it two teeth the other way; this should have solved it, but no way-- turned out it needed to be moved yet another tooth! How did the mechanic get it so wrong.
And here's where it gets really interesting and why the answer to the mystery was finally revealed...
1) By the third time I removed the belt after running the motor sufficiently to warm it up to see if the knock would go away, I found the belt was much more difficult to remove and reset.
2) The thought occurred that the belt had shrunk due to the temperature increase-- garbage thought-- no, what happened was the motor expanded due to the temp increase, and therefore the distance from the cam sprocket to the crank pulley increased, causing the belt to tighten.
3) OK, the rest is obvious-- the motor warms, the belt tightens, it moves the cam shaft timing just enough to overcome the knock-- which is an IGNITION KNOCK, NOT A MECHANICAL PISTON SLAP, BEARING NOISE OR ANYTHING ELSE MECHANICAL (which would invariably get louder as the engine gets warmer).
4) If you suspect the truth of this analysis, all you have to do is loosen the distributor and rotate it counterclockwise 1/16 inch and see how much louder it makes the knocking sound when the engine is cold-- this will prove to all but the most stubborn that what you are hearing IS an ignition knock.
5) Finally-- (and now I'm in uncharted water needing more investigation); it might be better to let the "cold engine knock" live, since retarding the distributor could cause a slight loss of power, I'm just not sure about this part of it.
6) If you go through the usual procedure of timing with a timing light, and if the cold knock persists, I say leave well enough alone since you're probably getting the proper power level.
7) The real reason older high-mileage motors develop the knock might be some microscopic wear in the cam line that is happily overcome by the expansion when the motor warms up.
8) Do we care about the real reason-- I say it's enough to know it's ignition knock, not mechanical; and if you don't want to hear it you can turn the distributor clockwise 1/16" and put up with a possible slight loss of power (counterclockwise makes the noise louder and serves to identify it as ignition knock).
Case closed!
Have a great day, Richard
#12
04-06-2009, 02:53 PM
The infamous cold engine knock/tap help
With 2 '94 Civics and 2 '95 Civics, and two out of the four having the condition known as "COLD ENGINE KNOCK"; yesterday I found it-- I found the answer to this long-standing dilemma while resetting a timing belt on one of my 94's.
Last week I had a mechanic install a new timing belt in this car (total mileage 188,000, last belt at 100,000) and afterward heard a terrible knock that would not quit after the motor got warm... I complained and was told to bring it in, the belt would have to be repositioned and it would be the same three hours and same labor charge... I looked at my Haynes repair, decided to try it myself, got half way through the job, discovered all that's really needed is take off the top belt cover, slip the belt off, make a mark on belt, make mark on cam sprocket tooth that you want the belt mark to line up with (one tooth to the left) then turn the crankshaft two inches, slip the belt on again making sure the two marks line up-- total time required 20 minutes.
But my situation wasn't that simple... I first moved the belt in the wrong direction, then had to move it two teeth the other way; this should have solved it, but no way-- turned out it needed to be moved yet another tooth! How did the mechanic get it so wrong.
And here's where it gets really interesting and why the answer to the mystery was finally revealed...
1) By the third time I removed the belt after running the motor sufficiently to warm it up to see if the knock would go away, I found the belt was much more difficult to remove and reset.
2) The thought occurred that the belt had shrunk due to the temperature increase-- garbage thought-- no, what happened was the motor expanded due to the temp increase, and therefore the distance from the cam sprocket to the crank pulley increased, causing the belt to tighten.
3) OK, the rest is obvious-- the motor warms, the belt tightens, it moves the cam shaft timing just enough to overcome the knock-- which is an IGNITION KNOCK, NOT A MECHANICAL PISTON SLAP, BEARING NOISE OR ANYTHING ELSE MECHANICAL (which would invariably get louder as the engine gets warmer).
4) If you suspect the truth of this analysis, all you have to do is loosen the distributor and rotate it counterclockwise 1/16 inch and see how much louder it makes the knocking sound when the engine is cold-- this will prove to all but the most stubborn that what you are hearing IS an ignition knock.
5) Finally-- (and now I'm in uncharted water needing more investigation); it might be better to let the "cold engine knock" live, since retarding the distributor could cause a slight loss of power, I'm just not sure about this part of it.
6) If you go through the usual procedure of timing with a timing light, and if the cold knock persists, I say leave well enough alone since you're probably getting the proper power level.
7) The real reason older high-mileage motors develop the knock might be some microscopic wear in the cam line that is happily overcome by the expansion when the motor warms up.
8) Do we care about the real reason-- I say it's enough to know it's ignition knock, not mechanical; and if you don't want to hear it you can turn the distributor clockwise 1/16" and put up with a possible slight loss of power (counterclockwise makes the noise louder and serves to identify it as ignition knock).
Case closed!
Have a great day, Richard
Last week I had a mechanic install a new timing belt in this car (total mileage 188,000, last belt at 100,000) and afterward heard a terrible knock that would not quit after the motor got warm... I complained and was told to bring it in, the belt would have to be repositioned and it would be the same three hours and same labor charge... I looked at my Haynes repair, decided to try it myself, got half way through the job, discovered all that's really needed is take off the top belt cover, slip the belt off, make a mark on belt, make mark on cam sprocket tooth that you want the belt mark to line up with (one tooth to the left) then turn the crankshaft two inches, slip the belt on again making sure the two marks line up-- total time required 20 minutes.
But my situation wasn't that simple... I first moved the belt in the wrong direction, then had to move it two teeth the other way; this should have solved it, but no way-- turned out it needed to be moved yet another tooth! How did the mechanic get it so wrong.
And here's where it gets really interesting and why the answer to the mystery was finally revealed...
1) By the third time I removed the belt after running the motor sufficiently to warm it up to see if the knock would go away, I found the belt was much more difficult to remove and reset.
2) The thought occurred that the belt had shrunk due to the temperature increase-- garbage thought-- no, what happened was the motor expanded due to the temp increase, and therefore the distance from the cam sprocket to the crank pulley increased, causing the belt to tighten.
3) OK, the rest is obvious-- the motor warms, the belt tightens, it moves the cam shaft timing just enough to overcome the knock-- which is an IGNITION KNOCK, NOT A MECHANICAL PISTON SLAP, BEARING NOISE OR ANYTHING ELSE MECHANICAL (which would invariably get louder as the engine gets warmer).
4) If you suspect the truth of this analysis, all you have to do is loosen the distributor and rotate it counterclockwise 1/16 inch and see how much louder it makes the knocking sound when the engine is cold-- this will prove to all but the most stubborn that what you are hearing IS an ignition knock.
5) Finally-- (and now I'm in uncharted water needing more investigation); it might be better to let the "cold engine knock" live, since retarding the distributor could cause a slight loss of power, I'm just not sure about this part of it.
6) If you go through the usual procedure of timing with a timing light, and if the cold knock persists, I say leave well enough alone since you're probably getting the proper power level.
7) The real reason older high-mileage motors develop the knock might be some microscopic wear in the cam line that is happily overcome by the expansion when the motor warms up.
8) Do we care about the real reason-- I say it's enough to know it's ignition knock, not mechanical; and if you don't want to hear it you can turn the distributor clockwise 1/16" and put up with a possible slight loss of power (counterclockwise makes the noise louder and serves to identify it as ignition knock).
Case closed!
Have a great day, Richard
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