351C - Engine knock at idle

[TheDude]

Howdy everyone, my 351C 4V engine has had a knocking issue since I got it a couple years ago. When warm and at low idle, the engine knocks a bit. It happens when idling at around 800 RPM. Pick up up just a bit to 950-1000 RPM and it goes away. Not just overtaken by noise but goes away. Oil pressure looks good. I've been told by a couple different mechanics that have heard it that it's not a rod knock or anything to worry about, but never really got a straight answer about what is actually causing it, if it's doing any harm or how to stop it other than increasing the idle speed. 

Any thoughts on what could be doing this and what if anything can be done to get to the bottom of it and stop it?

Thanks!

 

[Tnfastbk]

Found this on a search for knock at idle. Interesting theory if nothing else.





 








Sounds like a rod knock. Disable one cylinder at a time by disconnecting a plug wire or fuel injector to isolate which cylinder it is.
 




 

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dishdude

 









Originally Posted By: mehullica
Sounds like a rod knock. Disable one cylinder at a time by disconnecting a plug wire or fuel injector to isolate which cylinder it is.

Huh?? I don't understand this diagnostic procedure.


 




 

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OVERKILL



 











Originally Posted By: mehullica
Sounds like a rod knock. Disable one cylinder at a time by disconnecting a plug wire or fuel injector to isolate which cylinder it is.

Huh?? I don't understand this diagnostic procedure.


When you remove the ability for that cylinder to contribute to the power the engine makes it will stop loading the rod bearing and subsequently not knock. It is an easy way to isolate which cylinder is making the noise.


 





 

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Thank you for the explanation. I figured the crankshaft was still turning so it would still make noise. Although I'd think with the engine missing it would really make noise no matter which cylinder you have disabled.

 

[droptop73]

A rod knock will typically get louder and increase in frequency with an increase in RPM. I'd still investigate it further. 

 

[jpaz]

I could maybe just be a rocker arm.

 

[TommyK]

Get a mechanic's stethoscope and try to isolate the area the noise is coming from.

 

[Don C]

Loose crankshaft pulleys and loose flywheels or flexplates can mimic a crankshaft knock. 

Rod and crankshaft knocks generally occur at all speeds, with main bearings louder under load.

Lifter and rocker arm noises are generally more of a click than a knock or thud.

I would try two things, one, pull the belts to eliminate the possibility of it being an accessory noise. You can run it for a few minutes without the water pump. Second, connect a mechanical oil pressure gauge, make sure you have warm oil pressure at idle. You can also get an idea of the condition of the main bearings with a good mechanical oil pressure gauge. If the oil pressure fluctuates on engine acceleration compared to deceleration if you blip the throttle to 2000, or so, RPM the main bearings are likely worn. 

Edit: I have never had an engine noise fix itself. Better to find it now before the engine grenades and ruins your day, not to mention your bank account.

 

[TheDude]

Thanks for the replies and for some reason I never seem to get notifications when there is one so didn't know until I looked. It does sound lower than a rocker arm and I've run it without valve covers and didn't hear anything there. I tried the old long screwdriver to the ear trick to try and hear it but couldn't narrow it down. Thought it might have been the water pump, but replaced that recently as well. I'll see if I can find a cheap stethoscope.

Don, I like the idea of pulling the belts. That gets rid of a lot of variables and easier to narrow things down from there if it does or doesn't go away. The sound has been there and been the same basically since I got the car. It's pretty subtle so most people don't even notice it. It had occurred to me to speed up the idle a bit to make it go away but that seems like a cop out and I really need to try and get to the bottom of it. 

 

[midlife]

Possibly an exhaust manifold leak?  Just throwing it out...

 

[jpaz]

It could even be the fuel pump arm. 

 

[Stanglover]

Thanks for the replies and for some reason I never seem to get notifications when there is one so didn't know until I looked. It does sound lower than a rocker arm and I've run it without valve covers and didn't hear anything there. I tried the old long screwdriver to the ear trick to try and hear it but couldn't narrow it down. Thought it might have been the water pump, but replaced that recently as well. I'll see if I can find a cheap stethoscope.

Don, I like the idea of pulling the belts. That gets rid of a lot of variables and easier to narrow things down from there if it does or doesn't go away. The sound has been there and been the same basically since I got the car. It's pretty subtle so most people don't even notice it. It had occurred to me to speed up the idle a bit to make it go away but that seems like a cop out and I really need to try and get to the bottom of it. 

Stethoscope, Harbor Freight. 

The rest, no idea, but man, it's annoying when there is something you know is just not right and you can't put your finger on it.

 

[TheDude]

Possibly an exhaust manifold leak?  Just throwing it out...

That could totally be. I have no end of trouble getting my headers to stay sealed. I have to tighten the bolts constantly, one of these days I'll get some proper locking bolts. 

It could even be the fuel pump arm. 

Good call, but running an electric pump. 

Stethoscope, Harbor Freight. 

The rest, no idea, but man, it's annoying when there is something you know is just not right and you can't put your finger on it.

That's what she said? Yes, no doubt on that and just ordered one and a bunch of other odds and ends from HF. 

Thanks for the comments and ideas. I'll post back if I ever get this figured out.

 

[Don C]

 just ordered one and a bunch of other odds and ends from HF. 

That's one nice thing about our hobby, gives us lot's of excuses for tool acquisition.

 

[Carolina_Mountain_Mustangs]

If you do not have a scope to listen to the engine with just take an old wooden broom handle and cut you a piece about 3 feet long. You can put against water pump, PS, alternator, heads and block and keep the other end against your ear. It will help you isolate the area the noise is coming from.
My F-150 2002 model I bought new has had a ticking noise since brand new still has it with over 306,000 miles.
One other thing you can do easily is to take the valve covers off one at a time and with the engine running push down on the rockers over the push rod and see if noise changes. Use your broom handle. Make sure you do not have loose rocker. I have seen them come all the way off and toss push rod thru the valve cover.

 

[TheDude]

That's one nice thing about our hobby, gives us lot's of excuses for tool acquisition.

Yes my wife keeps reminding me about this. Although she doesn't call it a nice thing. 

If you do not have a scope to listen to the engine with just take an old wooden broom handle and cut you a piece about 3 feet long. You can put against water pump, PS, alternator, heads and block and keep the other end against your ear. It will help you isolate the area the noise is coming from.
My F-150 2002 model I bought new has had a ticking noise since brand new still has it with over 306,000 miles.
One other thing you can do easily is to take the valve covers off one at a time and with the engine running push down on the rockers over the push rod and see if noise changes. Use your broom handle. Make sure you do not have loose rocker. I have seen them come all the way off and toss push rod thru the valve cover.

Thanks. I had tried some of this, but didn't have a long enough stick for get to the bottom of the engine. The lose rocker thing is making me think a bit. The valve covers that were on it when I got the car didn't have enough clearance and the rockers on the front edge of both sides were knocking against the valve cover to the point that the covers eventually cracked. Everything seemed good at the time and nothing was obviously lose, but I should double check that. 

Thanks!