Engine Knocking Help

[Kenny H.]

I have a 400 ci engine with a flat tappet hydraulic cam. When I first start the engine it runs nice and quiet, from a "foreign" noise perspective. After the engine gets up to operating temperature, a knocking noise appears to come from the valve train, on one side only. I've used a stethoscope to isolate the knocking on one side of the engine, and the knocking is uniform the full length of the valve cover. Putting the stethoscope on the other valve cover gives a smooth, normal hydraulic lifter noise.


The knocking isn't the heavy knocking I've heard from a bad rod or main bearing, but rather a less heavy knocking sound. Any ideas? Bad lifter(s) that show up when the oil is up to operating temperature? Cam lobe(s) in the process of wiping out? I was thinking maybe a wrist pin, but I can't isolate the noise above any one cylinder; it seems to be uniform the entire length of one valve cover.


Thanks.

[Edward J.]

Re: Engine Knocking Help

Kenny, the first place to start is to remove the valve cover and nose around, check the rocker arm movement a bad cam lobe would show little movement ,could be a lifter adjustment, but generally they tend to tick when loose. wrist pins tend to be noisey when cold and go away when warm.

[Kenny H.]

Re: Engine Knocking Help

Thanks for the reply Edward. Before I posted I had already taken off both valve covers and adjusted all valves. Nothing remarkable stood out to me; nothing loose. When I put the valve covers back on and started the engine, same story. Very quiet until I drove it some and got everything up to operating temperature, then the light knocking sound returned. I'll open up the engine and check the cam if necessary, but just want to avoid minor surgery if possible.

[Mark E.]

Re: Engine Knocking Help

See if you can isolate the noise further by running the engine with the valve cover off. Use rocker splash guards or slice open an old valve cover and install that to prevent oil splash.

If you can identify the noisy valve train, troubleshoot by selectively swapping parts (rocker, ball, push rod, spring, retainer) from a quiet valve or use new parts. If the noise stays with the valve, it's the cam, lifter, valve, valve guide... or something other than the valve train.

Awhile ago, I read an insightful article about the subtleties of different engine noises, and what causes them. Not sure, but it may have been an old service manual (not Corvette). Engine noise interpretation is quite an art; maybe you can find a resource.

Post edit: Watch for a broken inner spring.

[Randy G.]

Re: Engine Knocking Help

Sounds have a habit of travelling and can be hard to isolate.

I have a knocking/tapping sound that gets louder when it warms up coming from the right front lower area of the engine of a C2 I bought last week. A friend of mine looked at it yesterday and he said it can be felt through the fuel line coming from the fuel pump when you grab it. He says he's seen it before, where the fuel pump is going bad and develops a knock. Possibly The pivot pin on the pump lever could be damaged. I've never heard of that happening before. He said to get another pump and a new push rod and try it.

When I inspected the car we noticed that there was an electric fuel pump mounted down by the mechanical one on the block. The owner was an elderly gentleman who is now in France for a month. He had someone maintaining his cars for him so it's doubtful he wold know anything about it anyway. But when he gets back I'll ask him.

[Frank D.]

Re: Engine Knocking Help

I agree a short run with the valve covers off to make sure the valve train isn't tapping the cover...that sound can emanate as a light knock. Sometimes I'll loosen or remove the fan and other belts and run the engine VERY briefly to see if that affects the sound. In one case it was the water pump bearing....

And yes, there has been a spate of fuel pump actuator rod pins 'walking' out of the pump body - but there are usually other symptoms related to that ... poor running, intermittent fuel delivery, etc. (example shown)..

An old mechanic's trick is to take plug wires off individually on the running engine and, once the 'load' is removed, the knock may disappear and that is the offending cylinder...

[Richard M.]

Re: Engine Knocking Help

Just to add to the ideas as these can be a bear to find.

I had a loose crankshaft pulley cause a knock on my '63 coupe. Strangely, it only knocked after the engine was warmed up. Simply tightening the 3 bolts fixed it. On another '63, a pulley that had cracks at the mount holes caused a vibration. Fixed with a new pulley.

Another common "knock or rattle" is from the clutch/bell housing cover upper circular flange being hit by the crankshaft.

Rich

[Dan P.]

Re: Engine Knocking Help

Pull the spark plug wires off each cylinder . If the knock goes away you have a loose rod bearing on that cylinder . Then if it still persist pour transmission fluid down the carburetor . This will remove any carbon . If it is a carbon knock.You will have to rev the engine up to do this .

[Duke W.]

Re: Engine Knocking Help

Does the knocking increase linearly with engine speed?Is it load sensitive?Does the loudness vary with speed or load?A wrist pin knock is usually load sensitive, but not so much with a bearing or valve train problem.DukeDuke

[Randy G.]

Re: Engine Knocking Help

I solved the engine knocking sound yesterday. It was the fuel pump. And it was pretty loud.

With power steering it was quite a job to get the mechanical fuel pump off but I finally did it (thank goodness for my 2 post lift). Sure enough, the fuel pump lever was bent and the push rod was scored where the lever went against the side of the push rod instead of working off the tip. At one point the lever got back under the push rod and was actually pumping fuel, but it must have had .060-.080 clearance.

I removed the electric fuel pump, installed a new mechanical fuel pump, replaced the push rod, replaced the rotted looking 3/8" fuel hose, and fired up. Quiet as can be, and ran fine as the new pump was doing it's job. I traced all the wires for the electric pump and removed them from the car. Bubba must have been proud of his accomplishments including the switch hidden under the dash. Thankfully the mickey mouse attempt at fixing it by Bubba, and the fact that it was driven like this, didn't wreck the camshaft or anything in the engine.