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Old 10-17-2015, 11:17 AM   #1
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Knocking under engine

My gf has a 94 gt. We recently replaced her transmission seal which had a bad leak and the engine was knocking so we parked it fearing it was a bad rod bearing. Friend says it was the smog pump so we bypassed it. Still knocks, friend then says sounds like the starter is knocking against the flywheel. Can't hear the knocking while stationary or in reverse only when accelerating. Looking for some opinions on this. I've researched what a bad flywheel sounds like it's slightly similar..


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Old 10-18-2015, 10:59 PM   #2
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There are numerous causes for knocking sounds & there are some tests you can do to help you find the cause.You're problem sounds like its more related to detonation vs engine knock.Detonation usually occurs during acceleration under a light load not at idle.Detonation is caused by one of the following things::
Defective egr valve
Too much compression due to carbon buildup in the cylinders
Over-advanced ignition timing
Lean air fuel mixture-vacuum leak
Overheated engine
Low octane fuel
You can remove the egr valve and clean it,if you suspect it as the cause.Timing is advanced while the egr valve is functioning,so a dirty or defective egr valve that's sticking in the open position(when it shouldn't be)will cause detonation.One thing you can do to eliminate it as the cause is to remove the vacuum hose from the egr valve, plug the hose then take it for a test drive.If the problem clears up,the egr valve or evr solenoid is causing your issue.
BTW-the egr valve is only supposed to be open during part throttle cruise,it shouldn't be open at idle or WOT.
Detonation is usually detected by a knock sensor & the computer retards timing to rid the detonation,but your Mustang doesn't have a knock sensor.If the noise you hear is more like a pinging or rattling sound,its detonation- not knocking...Trouble shooting the things listed above will help you get rid of the noise.

The next thing I'll list are causes for engine knock.
Engine knock is normally heard at idle,but it can also be heard when accelerating at low speeds.Let the engine idle & listen for a dull knock then gradually open the throttle up and listen for the knock to speed up.Now grab a spark plug wire (using insulated pliers)& pull it off & away from the coil post.If the noise is still present,reattach that plug wire and move to the next plug wire.If you remove a wire and the noise is *reduced* or *disappears*,you've now reached the cylinder with the worn bearing.
A heavy dull knock=worn main bearing
A metallic knock=worn rod bearing
A overlapping metallic noise that increases in intensity as the engine is revved is usually caused by a worn piston or cylinder wall.The noise is louder when the engine is cold.
A rapid clicking or double knock indicates a worn piston pin.The noise will *increase*when removing the spark plug wire.
A steady metallic rattling or rapping sound coming from the timing cover area indicates a worn timing chain.
If you hear a noise similar to a worn bearing,but its intermittent & does not change when the spark plug wire is removed,that's caused by a loose flywheel or excessive crankshaft end play.This noise is only heard at idle or during rapid deceleration.

I would definitely check your timing with a light.Its best to use without and draw a line on the harmonic balancer at the 10° mark.20° 10° 0° 10° 20°
The scale shown directly above is what the harmonic balancer looks like,from left to right if you're standing in front of the car looking down at the balancer.Your line should be drawn on the 10° mark on the right side of this scale.If your gf's cars timing is set above 10° and she's using 87 octane fuel,that is most likely what's causing the detonation.If it is set too high and you wanna ****** it a few degrees,be sure to remove the spout plug before you adjust timing.Plug it back in afterwards.
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Old 10-19-2015, 01:33 PM   #3
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Quote:
Originally Posted by wbrockstar View Post
There are numerous causes for knocking sounds & there are some tests you can do to help you find the cause.You're problem sounds like its more related to detonation vs engine knock.Detonation usually occurs during acceleration under a light load not at idle.Detonation is caused by one of the following things::
Defective egr valve
Too much compression due to carbon buildup in the cylinders
Over-advanced ignition timing
Lean air fuel mixture-vacuum leak
Overheated engine
Low octane fuel
You can remove the egr valve and clean it,if you suspect it as the cause.Timing is advanced while the egr valve is functioning,so a dirty or defective egr valve that's sticking in the open position(when it shouldn't be)will cause detonation.One thing you can do to eliminate it as the cause is to remove the vacuum hose from the egr valve, plug the hose then take it for a test drive.If the problem clears up,the egr valve or evr solenoid is causing your issue.
BTW-the egr valve is only supposed to be open during part throttle cruise,it shouldn't be open at idle or WOT.
Detonation is usually detected by a knock sensor & the computer retards timing to rid the detonation,but your Mustang doesn't have a knock sensor.If the noise you hear is more like a pinging or rattling sound,its detonation- not knocking...Trouble shooting the things listed above will help you get rid of the noise.

The next thing I'll list are causes for engine knock.
Engine knock is normally heard at idle,but it can also be heard when accelerating at low speeds.Let the engine idle & listen for a dull knock then gradually open the throttle up and listen for the knock to speed up.Now grab a spark plug wire (using insulated pliers)& pull it off & away from the coil post.If the noise is still present,reattach that plug wire and move to the next plug wire.If you remove a wire and the noise is *reduced* or *disappears*,you've now reached the cylinder with the worn bearing.
A heavy dull knock=worn main bearing
A metallic knock=worn rod bearing
A overlapping metallic noise that increases in intensity as the engine is revved is usually caused by a worn piston or cylinder wall.The noise is louder when the engine is cold.
A rapid clicking or double knock indicates a worn piston pin.The noise will *increase*when removing the spark plug wire.
A steady metallic rattling or rapping sound coming from the timing cover area indicates a worn timing chain.
If you hear a noise similar to a worn bearing,but its intermittent & does not change when the spark plug wire is removed,that's caused by a loose flywheel or excessive crankshaft end play.This noise is only heard at idle or during rapid deceleration.

I would definitely check your timing with a light.Its best to use without and draw a line on the harmonic balancer at the 10° mark.20° 10° 0° 10° 20°
The scale shown directly above is what the harmonic balancer looks like,from left to right if you're standing in front of the car looking down at the balancer.Your line should be drawn on the 10° mark on the right side of this scale.If your gf's cars timing is set above 10° and she's using 87 octane fuel,that is most likely what's causing the detonation.If it is set too high and you wanna ****** it a few degrees,be sure to remove the spout plug before you adjust timing.Plug it back in afterwards.



Definitely informative I'll do them one step at a time and keep u posted thanks


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Old 10-21-2015, 10:07 PM   #4
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I also failed to mention that if you hear the knocking sound only during idle & you also see the oil pressure gauge dropping down alot ,when idling or stopped at a red light,that's definitely a indicater of a worn main or rod bearing.When bearings wear down oil pressure loss,at idle,is one of the first you'll see.When this happens,there is more room between the bearing face & journal,so pressure goes down.When an intake gasket started leaking on my LX and I waited 1000 miles to replace it,the coolant ate my bearings alive in no time.Everytime I stopped at a red light,my oil pressure would drop to 0 & the engine would immediately start knocking.The bearing no longer has that thick layer of oil between it & the journal,so the rod starts moving back & forth and that causes the knocking sound.The noise gets louder and the pressure drops more as the engine warms up because the oil viscosity gets thinner with heat.
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Old 10-22-2015, 07:19 PM   #5
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Now my 88 does drop in pressure during idle. Never had any noises or trouble out of her performance. I'll look into it anyways better safe than sry


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Old 10-27-2015, 02:59 PM   #6
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I hope you are using a real gauge and not the factory idiot gauge.
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Old 10-27-2015, 09:39 PM   #7
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It's an aftermarket gauge. They need replacing anyways Click image for larger version

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