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  #1  
Old 12-05-2007, 12:21 PM
Meridious Meridious is offline
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Unhappy Odd but scary engine knock

My T-bird started a scary knock yesterday. It is only at a very low rpm, as when the car is idling in gear.

In Park, the knock ends as the engine increases a bit in RPM...or in gear, when RPMs increase, the knock ends.

And when I say it ends, I mean it ends. You can stick your head alongside the engine at the low-idle and the knock is scary...but rev up a little, or place it in park, and you can't hear a THING.

The engine was rebuilt 1700 miles ago, but unfortunately that 1700 miles was also about 9 years ago. The car was a very occasional Sunday-driver until I decided to grab it and turn it into a daily driver.

I have changed the oil and filter, and tried the Seafoam additive (Which I don't have much faith in, anyway, but tried as this was a strange knock) but it hasn't slowed it down.

I have had rods/rod-bearings and mains go bad before, but I have never had a situation where the knock disappears completely with just a touch of a higher RPM.

* Does not knock when cold at all
* Begins when car is warm and at a low-idle. I can leave it in park and idle it down and it begins, or I can have idle set normally, place car in Drive or Reverse and the knock begins.
* Knock completely ends with RPM increase.

Anyone have any experience with this? My first guess is that the oil-pump is not working properly at the low-idle, but I don't have a gauge. (And I believe more than one rod would be knocking were that the case, and this sounds like a single booger, not multiple) Engine-oil light functions properly and does not come on or blink, etc.

My second guess is a rod knocking, but I have no prior experience with rod-knocks being temperamental...they pretty-much knock or don't knock.

The knock seems in-time with the engine idle and not a fast knock...if I had to choose between a rod, main, or lifter, I'd choose rod because of the degree of noise and the speed of the knock.

It is just weird that is completely goes away with the slightest idle increase.

Ideas?
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  #2  
Old 12-05-2007, 01:28 PM
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tbird430 tbird430 is offline
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Well it still sounds like a bad bearing to me. By giving it a slight throttle - "ideling the engine up", this increases the oil pressure just enough to fill in that gap in between the worn bearing tolerences.... Just my 2 cents though.
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  #3  
Old 12-05-2007, 01:38 PM
tarps3 tarps3 is offline
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I had a water pump that was going bad on me that exhibited the sound you describe. the bearings/shaft were shot. I replaced the water pump, the knock went away. I only ever heard it at idle. It wasn't terribly loud but was noticible.
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Old 12-05-2007, 05:23 PM
bcomo bcomo is offline
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If you want a better idea about the sound, you can use the "Poor Mechanic's Stethescope".

Cut a 2 or 3 ft piece of garden hose, or 3/4" heater hose. Put one end of the hose on your ear, and the other end on the oil pan, block, or wherever the sound seems loudest. You can check the top end and water pump area the same way.

If it's that loud at idle, the hose should amplify the sound so that you get a better idea of what it is.

P.S. What weight oil are you using?
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Last edited by bcomo : 12-05-2007 at 05:27 PM.
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  #5  
Old 12-05-2007, 06:21 PM
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Dan Leavens Dan Leavens is offline
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Meridious Just following your post on knocking. Just a thought what is coming out of the pipes when you start the bird up
and what colour ???

Dano Calgary,Alberta Canada
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Old 12-05-2007, 07:28 PM
tbirds8 tbirds8 is offline
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Sounds like a piston pin. but just a guess.
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  #7  
Old 12-05-2007, 08:12 PM
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Have you changed the fuel pump recently? Sometimes the arm on those reman pumps isn't always in contact with the eccentric cam. That sounds like a lifter tick (usually)

This sounds wierd too . . . It could be carbon. 1700 mi in 9 yrs??? Take a glass of water (Seriously), hold it at a fast idle and slowly pour it down the carb throat. It can't hurt anything, but it might loosen up a piece of carbon that may have worked loose on a piston. This worked for me once. An old "Ford guy" told me about it. I was just getting ready to pull the motor out of my Dodge Charger.

Drain your oil. Look closely at it (the oil in the fltr too - maybe cut it open). If you see shiney (Like metal flake) in there, it prob is a bearing.
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  #8  
Old 12-05-2007, 08:27 PM
Meridious Meridious is offline
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Quote:
Originally Posted by Dan Leavens View Post
Meridious Just following your post on knocking. Just a thought what is coming out of the pipes when you start the bird up
and what colour ???

Dano Calgary,Alberta Canada
The car does not smoke AT ALL. I can't emphasize that enough. Not even a puff. When I had the oil changed, the guy looked up the oil and asked me if I wanted the 'listed' weight for the vehicle/engine and I said "Yes." I don't recall what weight it was. And also, I had the oil changed BECAUSE of the knock, not the other way around. The knock started before the oil change.

I stuck my head in there real well and it definately sounds like it is from the front-side of the engine, whatever it is. I didn't have a hose, but I had an old Christmas-paper cardboard tube, which I tried to get into some tight spots.

It is definately at the front of the motor, and less noise at the intake, more at the center of the engine near the water pump.

When I laid somewhat under the car and listened through the tube in various spots on the oil pan (Really all I could get to was front and back) it sounded weaker underneath, but still on the front-side of the engine.

It sounds like it is coming from very near the water-pump, but I do not think it is the water-pump itself.

It is like it may be a front rod or piston pin....and it can get LOUD.

It is just amazing that with just a touch of throttle it completely goes away.

I took a screwdriver and idled the engine up a bit...and you could not hear a thing. I idled it back down and it sounded like something was about to burst through the block.

I think I am just going to have to tear it down.

Sigh.

I have an opportunity to snag a 64 390 long-block re-manufactured at a great price from a local machine-shop...but I hate moving away from the original motor. I will probably just have it rebuilt if it comes to that.

It is so weird I am tempted to film it with my digital camera and a good microphone so I could post it at low-idle and just a tiny bit higher. You'd be astounded to hear it go from horrendously loud to kitten-purr with what seems like less than a 1000 rpm-change.

Last edited by Meridious : 12-05-2007 at 08:30 PM.
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  #9  
Old 12-05-2007, 08:29 PM
Meridious Meridious is offline
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Quote:
Originally Posted by byersmtrco View Post
Have you changed the fuel pump recently? Sometimes the arm on those reman pumps isn't always in contact with the eccentric cam. That sounds like a lifter tick (usually)

This sounds wierd too . . . It could be carbon. 1700 mi in 9 yrs??? Take a glass of water (Seriously), hold it at a fast idle and slowly pour it down the carb throat. It can't hurt anything, but it might loosen up a piece of carbon that may have worked loose on a piston. This worked for me once. An old "Ford guy" told me about it. I was just getting ready to pull the motor out of my Dodge Charger.

Drain your oil. Look closely at it (the oil in the fltr too - maybe cut it open). If you see shiney (Like metal flake) in there, it prob is a bearing.
I've not change it, but it was changed...I can see it is not the original. I have poured 1/3 of a can of Seafoam additive in the carb/intake as directed after this started in hopes it would help (and added it to the oil as directed).

No change.

Last edited by Meridious : 12-05-2007 at 08:32 PM.
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  #10  
Old 12-06-2007, 12:28 AM
bcomo bcomo is offline
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You said that the noise is localized to an area in the front near the water pump (not the pump itself), and around the fuel pump.

I wonder if the timing chain (stretched) could be something to look into? Can anyone comment on that as a possibilty? Things usually vibrate at low RPM and don't at higher RPM.

The other thing (already mentioned) is the fuel pump arm. That arm is pretty long, and goes into the same area. If the pump was changed -- who knows what's in there.

This might sound dumb, but check that the tranny cooling lines aren't slapping against the frame. At low RPM they would vibrate. (I had one of those lines hitting against the fan belt -- almost cut through one of them).

Don't give up yet -- take your time and try to eliminate the possibilities.
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Last edited by bcomo : 12-06-2007 at 12:33 AM.
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