Odd behavior V8, knocking sound

Mats Fredsson

Mats Fredsson

Member
Yesterday when I started my 3500 the engine first was as usual quiet, but after a minute it started to knock while idling (I was waiting for my Mrs).
It hasn't been started for over one week, and I suspect that one of the lifters played me a trick. A couple of revs didn't do any difference.
I parked the car in my garage again.
But.... when I started it today - no odd noise or knock! It ran nice as usual! I took it for a longer spin and it ran perfect.
I'm confused. (more than usual...)

Anyone else who experienced similar behavior?
 
Keep an eye [ear] on it. It is a bit random and not uncommon. If its from the top [rocker area] possible a little oil starvation until the pressure builds up. Old engine malai.
 
GRTV8 said:
Keep an eye [ear] on it. It is a bit random and not uncommon. If its from the top [rocker area] possible a little oil starvation until the pressure builds up. Old engine malai.
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Thanks for your reply. I got fast and good oil pressure, and it was not until after a minute the sounds arrived.
 
Mats, how loud was the knocking? Was it a light sound or a heavy sound? Next time you start your engine and during your drive, with the engine running at idle, place your ear down onto the gear selector. Can you hear the knocking? If you can then like my Rover's engine back in the late 1980s, one or more of your crankshaft main bearing caps have come loose. This is not uncommon with the factory engine as supplied for the P6B.
You will need to remove the sump and re torque the bolts to specification.

Ron.
 
SydneyRoverP6B said:
Mats, how loud was the knocking? Was it a light sound or a heavy sound? Next time you start your engine and during your drive, with the engine running at idle, place your ear down onto the gear selector. Can you hear the knocking? If you can then like my Rover's engine back in the late 1980s, one or more of your crankshaft main bearing caps have come loose. This is not uncommon with the factory engine as supplied for the P6B.
You will need to remove the sump and re torque the bolts to specification.

Ron.
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Hi Ron,
It was pretty obvious. I don't believe it's the crankshaft bearings, in that case I would have low oil pressure and the noise would be consistent.
There is no noise now.
 
Mats Fredsson said:
Hi Ron,
It was pretty obvious. I don't believe it's the crankshaft bearings, in that case I would have low oil pressure and the noise would be consistent.
There is no noise now.
Click to expand...
My engine didn't have low oil pressure when the knocking was evident. I don't remember if I could hear it all the time, I just know that when I placed my ear to the gear lever at idle, I could hear it.

You have nothing to lose by checking, but it is up to you.

Ron.
 
SydneyRoverP6B said:
My engine didn't have low oil pressure when the knocking was evident. I don't remember if I could hear it all the time, I just know that when I placed my ear to the gear lever at idle, I could hear it.

You have nothing to lose by checking, but it is up to you.

Ron.
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Thanks Ron,
I will check it up. One have to look in every way. But it is a problem when it only occurs random. Well, it is soon time to put the car in for the winter, before the salt on icy roads. Then I will give it a better check, new oil and such.
 
Ron has a good point there.
I had a rattle on the right bank of my V8. Couldnt pin point it.
I removed the rocker cover and used a long piece of thin wood [drum stick] and put ear/wood /rocker, with the motor running
The errant rocker/tappet was very noticeable.
metallic noise will travel so identifying the source can be a journey.
 
GRTV8 said:
Ron has a good point there.
I had a rattle on the right bank of my V8. Couldnt pin point it.
I removed the rocker cover and used a long piece of thin wood [drum stick] and put ear/wood /rocker, with the motor running
The errant rocker/tappet was very noticeable.
metallic noise will travel so identifying the source can be a journey.
Click to expand...
Thanks for the tip. I already tried the "rod-listening methode", but I have no noise now, and not before either. That was just at that very moment, very confusing.
 
Its pretty hard to find a knock when it isn't there. There are some things to check. Quite possibly the engine was running a bit rough and shaking enough to make something hit something else. Likely culprits are around the fan area, the exhaust pipe and the shock rubbers (you can reach under the guard grap the shock and twist it. If it moves rubbers need replacement or fitting properly). Check for loose exhaust clamping as a rocking engine will cause it to hit. Less rock, no hit...Hence went away next day
 
Check all the way down the exhaust path - failures of any of the hangers can cause knocking on the body. Considering the way my LT77 is supported and the closeness of the rear mufflers to the body I am continually surprised I dont get any body knocking.
 
My P5b has a doc doc doc when i first start but goes away after a few minutes. Oil pressure always good.
 
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