Engine Knock

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Forum topic by Eric posted 04-02-2012 12:53 AM 1763 views 0 times favorited 10 replies Add to Favorites Watch
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221 posts in 2684 days

04-02-2012 12:53 AM

What is engine knock?

Tonight I was putting the lawn mower in the shed after dark. I started it at an idle and left it there as I drove it into the shed so as to not bug the neighbors.

With the engine at the slow idle I hear what I thought was a metalic popping sound sound from the engine.

-- Eric

10 replies so far

View whitedog's profile


652 posts in 3630 days

#1 posted 04-02-2012 02:00 AM

Check the oil.

-- Paul , Calfornia

View schloemoe's profile


709 posts in 3111 days

#2 posted 04-02-2012 02:10 AM

I have a jeep thats been doing that for months and it don’t mow the grass….............Schloemoe

-- schloemoe, Oregon , http://www.

View JollyGreen67's profile


1676 posts in 2936 days

#3 posted 04-02-2012 02:35 AM

Maybe it wanted out?

-- When I was a kid I wanted to be older . . . . . this CRAP is not what I expected ! RIP 09/08/2018

View Rick  Dennington's profile

Rick Dennington

6239 posts in 3367 days

#4 posted 04-02-2012 02:50 AM

Is this a riding mower? I take it that it is…...may have water in the gas….....spark plug fouling….????

-- " It's a rat race out there, and the rats are winning....!!"

View fussy's profile


980 posts in 3223 days

#5 posted 04-02-2012 02:59 AM


Engine knock is pre-ignition—the engine fires early on the compression stroke causing the piston to hit the flame front as it expands. The effect is similar to hitting something solid and a clank or knock is the audible result. It’s caused by improper ignition timing, lean mixture or carbon deposits that raise the effective compression ratio. As it almost allways occurs under a heavy load, it’s unlikely knock is your problem. It could be excessive clearance between the crank and connecting rod or main bearings, the piston and wrist pin, or just a funny noise some engines make.

It could be the bolts mounting the engine to the frame are loose allowing it to shift back and forth. Take a long screwdriver and touch one end to the engine (NOT a moving part) and your ear to the other end and try to determine if the noise is coming from within the engine—usually bad news. Check oil level as suggested above. You should do that before mowing every time. My vote is loose engine mount bolts. I’ve heard it before and it can be impressive.


-- Steve in KY. 44 years so far with my lovely bride. Think I'll keep her.

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Rick Dennington

6239 posts in 3367 days

#6 posted 04-02-2012 03:14 AM

Now why didn’t I think of that…........???? I can tell fussy is an engine mechanic..just look at his avatar..

-- " It's a rat race out there, and the rats are winning....!!"

View jmos's profile


890 posts in 2542 days

#7 posted 04-02-2012 12:43 PM

Not to be too picky, but knock isn’t usually the engine firing early due to timing, although that could certainly cause it too.

The piston rises on the compression stroke, compressing the air fuel mixture. As gasses are compressed they heat up (think how hot a manual bicycle pump gets after a few minutes of pumping.) Ideally, the piston finishes compressing as the spark fires causing rapid expansion propelling the piston down for the power stroke. With knock, the air/fuel spontaneously detonates due to the heat of compression causing the piston to be pushed down too soon. This causes a bit of a jerk in the cycle, and robs the engine of power, since the detonation is pushing against the rotation of the engine, instead of with it.

This is usually the result crud in the cylinder that heats up unevenly (compares to the cylinder wall.) Higher octane fuels requires higher temperatures to ignite, so moving to a higher octane can reduce/eliminate knock. Some newer cars can automatically detect knock and adjust the engine operation to compensate, but this also reduces power output. This is why high compression engines usually require high octane fuel in normal operation. Knock is also called dieseling, as a diesel engine uses the heat of compression to naturally cause combustion, eliminating the need for spark plugs.

If the mower is knocking, I’d try cleaning. Either a fuel additive to clean it out (although many of those are snake oil), or a mechanical cleaning.

-- John

View Doss's profile


779 posts in 2437 days

#8 posted 04-02-2012 10:15 PM

Actually, I’ll clear this up as jmos and fussy seem to be just slightly off (some of what you said is true and sometimes describes knock by the nontechnical definition) on the differences between pre-ignition and knock.

Knock is an event that takes place when the compressed air-fuel mixture is ignited at a time that is not what is defined as optimal. This event occurs in the same manner as a normal ignition-controlled event, it’s just that the timing is incorrect. The result is a shockwave of the characteristic PING! or knocking/clanging type.

Pre-ignition is not knocking; though some use them interchangeably. Pre-ignition (not due to spark plug) occurs due to hot spots, lean mixtures, deposits, low octane fuel, etc. in the combustion chamber igniting the air-fuel mixture before the sparkplug does.

Diesel engines run on a form of controlled “pre” ignition where the air-fuel mixture is compressed to the point of ignition (compression heats up the air-fuel mixture).

While an engine can last a long time (sometimes) when it is knocking (it’s why cars have knock sensors to adjust for it), pre-ignition can destroy an engine fairly quickly. Both of these are less common now in computer-controlled, fuel-injected vehicles (although I have had and have watched people have several effects of pre-ignition or lean air-fuel mixtures when running nitrous :-) ).

Both situations cause the cylinder pressure to spike beyond normal, but since pre-ignition is essentially battling the motion of the piston, it’s even more damaging than what knock does.

Clean the carb and the cylinder on your mower pretty good and do a good general tune up and it should go away.

-- "Well, at least we can still use it as firewood... maybe." - Doss

View jmos's profile


890 posts in 2542 days

#9 posted 04-03-2012 12:42 PM

Doss – I looked it up and you are correct. According to what I read knocking is actually when a pocket of air/fuel is detonated outside the desired flame front caused by the spark plug. What i described is pre-ignition.

Thanks for the clarification.

-- John

View Doss's profile


779 posts in 2437 days

#10 posted 04-03-2012 04:23 PM

No problem jmos.

What to take away from this? If your engine is making a funny noise, get it looked at no matter if it is knocking or pre-ignition.

-- "Well, at least we can still use it as firewood... maybe." - Doss

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