Think I've got a rod knock...how to check it?

While tuning my fuel injection installation. I noticed a slight knock on light throttle cruise with the engine warm. Sound would go away during accel and decel. Got under the hood and could reproduce it by throttling up to 1500 rpm approx then slightly let off on the throttle gently and there it is. Not there during idle. Whatcha guys think?

 

Sounds more like 'piston noise' to me than a rod. I wouldn't think that a rod knock would come and go. Piston noise doesn't either, but is easily masked by other sounds. just my opinion. We are sure this isn't spark knock you're hearing is it?

 

Thats what I thought it was at first.
Comes on when I let of on the throttle while the jeep is sitting in neutral.
Stuck my head under the hood...sounds like its comin from the enigine deep inside.

 

Disconnect the ignition wire to that cylinder and ground it. Run it without the ignition wire, if the noise goes away it's a good chance it's a rod or wrist pin. If not, look further. Nickmil.

 

If theres slop in a wrist pin or rod bearing...What does stopping combustion on the bad cylinder do? Wouldn't it make the noise as the rod changed directions anyways??? I will still try this method to diag. the problem, but was just wondering.

 

It makes the direction change much less violent, and therefore the noise will change. It won't always go away, but it will definately change.

 

the noise caused by any slop is magnified during the powerstroke of the engine. no spark, no power stroke, no (or at least less) noise.

my f250 had the exact same symptoms you described. i took off each wire one by one and sure enough the noise was nearly non existent when i removed the wire leading to cylinder number 7. by the way, i've had this knock for over 2 years now and the truck pulls a boat and pushes a lot of snow. the noise hasn't gotton any worse and the power is still there. i really should open the motor up, but i haven't gotton around to it.

 

Sound like your not too worried about the problem. Should I be worried? Thanks for the help guys....what other things could it be? If not a rod or wrist pin?

 

i was very worried when it started happening. luckily it was in april when plowing season ending, so i figured i'd see what happened. well 2 years later, nothing has changed. i'm not too worried anymore, but clearly something ain't right. parts for a ford 302 are cheap and plentiful and i got a spare block. worst comes to worst, i blow a motor and drop in another one. the truck only travels a 100 mile radius from the house at the most. taking it on a road trip isn't gonna happen. if you have the ambition, i recommend taking it apart and fixing it. if your not so motivated, keep close tabs on the symptoms you are currently experiencing and see if they worsen over time. either way, the right thing to do is undoubetdly to fix the knock.

 

take a long screw driver or piece of rod (or a stethascope if ya have one) and listen to varius parts of the block. lay under it if ya have too. place the screw driver on the block up front and then put the other side on your jaw bone beside your ear...sound travels niecly. its a great way to listen internally without the use of a magnetic bearing amplifier. listen along the block and up it. if you dont hear it, then have someone replicate the sound and listen then. if its louder on the top end by the head than it is in the bottom...i think youve found the culprit.
watch the fan! watch the exhaust! watch the neighbor sunbathing next door. =)
block the tires..
whats your oil pressure like?

 

Oil pressure @ 20PSI.

 

Use a Piece of garden hose to listen for the noise. Much less chance of getting something burned or cut off or smashed that way.

 

garden hose wont resonate the sound through will it?
A piece of copper pipe works great to.
is your oil pressure cold or hot?

 

garden hose? sound propagate down a hose pretty well?
never heard that. will have to check it out.

oil at 20 psi always? does it flitter or is it nice and smooth?
if it flitter back and forth, prolly a worn spot on a bearing. it hits the spot and loose pressure for a sec until it rotates alittle more and releases force on that side (or something like that).
if its steady, still might be a bearing, but i think its prolly the piston pin.
altho a main bearing might knock like that. a certain RPM causes it to vibrate or resonate enough for you to hear it. higher rpm make it go away.
just a thought.

 

20 PSI always. A little higher when cold. I have a stethescope I will try to use. Knock is pretty conisistent. Not at idle,, not at stead run in neutral, but comes as soon a I release the throttle. Pretty steady. frequency of knock drops with RPM of engine.

 

If I can find the rod that may be making this noise...Can I pull just the one rod and piston with out removing the crank?

 

what nick says x2
easiest way
you can remove just one; if it is a rod, most likely the rod journal will be bad also
but you have to find the problem first
do the diagnostics, then figure out the cure

 

Here's a more extensive discussion
http://remanufactured-engines.com/page4.htm

Usually a rod knock is louder under acceleration and quiets when coasting. With a cast iron crank, like the V6, the crank journal is typically damaged as much as the bearing, so the crank has to come out for machining to fix it. Forged steel cranks are another story.

You should expect at least 10 psi per 1000 rpm; 20 psi at idle is good, but 20 psi at all speeds is too low.

 

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