56 engine knock with hot engine only

[Hillbillycat]

Hi,

my ´56 SDV  has this low end, steam engine-like, knock at idle only when the engine is up at operating temps. No noise when cold or accelerating or keeping speed. Doesn´t go away after a 2 hour freeway drive at 80mph.


Sounds exactly like in this 56 Buick youtube video https://www.youtube.com/watch?v=1ZexzYxaldo

Is this carbon knock like assumed in the video? How to be cured?

Thank you in advance

[Lexi]

Try running some Seafoam in your crankcase and gas tank. If carbon knock related it may help. Clay/Lexi

[Caddy Wizard]

Frankly, that sounds like lower end issues, not detonation in the head.

I would try changing the oil and using 15w40 Shell Rotella T4 oil in it.  If the knock sounds even slightly less pronounced immediately after changing the oil, then you have a problem in the bottom end (rod bearing, main bearing, piston pin).  If there is no difference, it is inconclusive.

[Daryl Chesterman]

How is the oil pressure at idle when the engine is warm?  Art's idea of going to a 15w40 oil is a good idea, and to go even further, you could try a 20w50 with zddp additive.  The only problem with 20w50 oil is when the weather gets cold it doesn't want to crank very fast and there is marginal lubrication on the upper end until the oil warms up a bit.  I see you are in Germany, so that is probably not an option if you drive it in cold weather.  I'm with Art, I think it is a lower end problem if it sounds like the YouTube video—too much bearing clearance somewhere on the crankshaft.

Daryl Chesterman

[James Landi]

This knock may be a "thrust bearing" issue.  You can easily diagnose this by using a "pry bar" to assess whether you can move the crank pulley "fore and aft." There SHOULD be no play, and if there is, that deep knock may well be your issue   Hope this helps.  James

[The Tassie Devil(le)]

To add to what James said about there should be no play, there should be no more than  about .010" of play.   When new, the tolerances are a lot tighter.

I think what James was referring to was no excessive play.   As without any play, the engine would destroy itself.

Bruce.

[robert G. smits]

if you have a sound similar to the video stop driving your car immediately and get your checkbook/credit card ready or start tearing it down.  It would be helpful to post a video of what your car sounds like?  Let us know what you find.  Thanks

[Jon S]

Just add some ZDDP to your existing oil and see what happens.

[Caddy Wizard]

How is the oil pressure at idle when the engine is warm?  Art's idea of going to a 15w40 oil is a good idea, and to go even further, you could try a 20w50 with zddp additive.  The only problem with 20w50 oil is when the weather gets cold it doesn't want to crank very fast and there is marginal lubrication on the upper end until the oil warms up a bit.  I see you are in Germany, so that is probably not an option if you drive it in cold weather.  I'm with Art, I think it is a lower end problem if it sounds like the YouTube video—too much bearing clearance somewhere on the crankshaft.

Daryl Chesterman

Regardless of what oil the owner prefers regularly, I was suggesting temporarily using a heavier oil (15W40) just to see if it eliminates or lessens the knocking sound initially.  If it does, the problem is a clearance issue in a bearing or some other part lubricated by oil -- it eliminates self-detonation as the cause of the noise.  For this "test" 20W50 might be even better, although I don't normally run such a thick oil as my everyday oil in these cars.

[fishnjim]

I suspect the noise remains, you just don't hear it inside when driving with the hood/bonnet down.
Could be any number of things.   The old timey mechanics would put a stethoscope on and see if they could hear/pinpoint the source, but I never had much faith in that.   Hence the name Dr. of motors, came to be used loosely.
What you should do, in my opinion, is pull the valve covers and oil pan and inspect it for sludge build-up, loose, broken, bent, not moving, etc.   Start at the top as it's more work to open the bottom end.   If nothing there, then it's probably in the mid-section or camshaft area or an accessory PS, FP, WP, generator; bearing, bent pulley, etc..   
I wouldn't call that sound in the video a "carbon knock".   My "hunch", I suspect you'll find a nice assortment of sludge in that motor, if it's never been rebuilt after leaded gas.
If you continue those 80 mph cruises, thinking nothing is 'wrong', I think the problem will identify itself, when it flies apart.   But some of these old irons are pretty tough, and won't give up easily.
With no info given on the miles or condition of the motor, masking a serious problem with a heavier oil, may lead to bigger issues down the road, so just be aware/cautioned.   It's most helpful with ring wear, seals, etc.  If the oil pump is weak, it can make things worse.   I see no issue "trying" Art's change oil as a "test" as he suggests, but the use of diesel oil is an old motor with "sludge" build up, can make it worse, if it's an oiling related issue.   Powerful detergents in the diesel oil for carbon/soot removal which is normal there, but can dislodge accumulated sludge into the oiling system.   I'd suggest/prefer a "racing" labeled oil as it has the extra zinc and less detergents.  But I'd defer and do the inspection first.  If it's clean, go ahead.   Nothing to lose but a few liters of motor oil and possibly some noise.

[Hillbillycat]

Well, thanks ofr the many answers and I immediately see I forgot to add some info.

77000 miles.

Engine not rebuilt.

I had the oil pan and valve covers off when I bought the car 4 years ago - about half an inch of lead sludge in the pan. Rocker shaft full of sludge and rust. Shop cleaned everything, and flushed the passages.

Main bearings were checked wth plastigage by the shop when the pan was off - they were ok. Shop guy said that cylinder walls showed some wear - I wasn´t able to see that when peeping up from under there, but I´m no expert. So I had to believe them.

New oil pump with gaskets was installed, since initially the oil pressure was too low when hot. This was a leaking pump to block gasket.

I´m using a mineral 15W40 already. Had used straight 40 weight non detergent for diagnositcs but apart from much harder starting no difference in this knock.

Knock is also heard from inside the car with hood down when hot at idle.

Can´t buy Seamfoam over here. But read it´s 100% pure Petroleum, which I have on hand for lanterns.

Will check the endplay and report back.

[Hillbillycat]

Well, just came home from taking the Cadillac to the shop to check compression. I wanted to know if there´s something going wrong up there, since besides the knocking when hot, I have an exhaust leak AGAIN after only a few hundred miles on new planed surfaces and gaskets (intake and exhaust manifolds). Why would it leak again if not getting warped by unbalanced heat?
That pt-pt-pft noise is annoying, too.

We also endoscoped the piston face via the spark plug holes. Never had done that before nor seen any endoscope pics. So I didn´t know how to interpret. They all had a speckled face that was looking like lichen growing on them, or like bacteria on agar-agar plates.
Shop guy said they looked clean except #3 had some carbon, size of a quarter. That I was able to tell apart.

While there we also checked the crankshaft endplay like suggested: zero play.


Next approach will be checking carburetor setting with a CO-gauge and readjusting. Engine could be getting too hot from wrong carb setting.
Then oil system flush and oil change with Liqui Moly classic car oil 20W50 that has ceramic additives who are said to seal up any slight wear rather then washing all away like modern oils do. I think this is similar to what you guys over in the States use by adding ZZDP. Neither can get that ZDDP over here nor can I find info about ZDDP levels on the oils sold over here.

[James Landi]

Well, Mr. Stark, you've done a great deal of homework, but before I'd mess with the carburetor, I'd be looking at the following: ; I'd check the vacuum level on the manifold with a vacuum gauge--I'd make certain to note exactly what the vacuum gauge is indicating--- it's a excellent dignostic tool--- a vacuum leak can cause overheating and pre-ignition knock ---how about the vacuum advance on your distributor?--- connect an 1/8 rubber hose to it--- can you determine if you can blow through it--- the vacuum advance is OFTEN overlooked during tune ups .  Will the vacuum advance hold vacuum when you suck on it? I'd want to know the coolant temperature (if the radiator is partially plugged, you'll have overly heated combustion chambers under load--a deep dive into the radiator may be necessary, (perhaps, start with a hand held temp reader) and finally,  I'd want to know if the brakes are dragging (using that temp gauge).   Sounds like your engine is working mighty hard, getting hot, and knocking.    Hope this helps, James

[Jon S]

Have you tried adding ZDDP to the oil before you tear everything apart???

[35-709]

Have you checked the heat riser valve to make sure it is not frozen shut?

[Hillbillycat]

Thanks for all the input.
To answer your questions:

ZDDP - no I didn´t. All the ZDDP stuff found over here is declared as break-in oil additive that requires replacement after a few hours.

Heat riser was removed by PO while they did a new exhaust front to end.

Carb had been fully cleaned (ulatrasonic bath) and rebuilt with Daytona parts by myself a few years back. I had set the carb to highest vacuum with my vacuum gauge then left the setting alone forever.

Vacuum advance is working.

No vacuum leaks present.

Brakes all new and free, no drag.

Water pump is new, radiator is old. It didn´t look too dirty so I didn´t sent it out for a new insert. Never checked coolant temp but temp gauge always creeps around 1/4 mostly less. That was different with the old worn pump. Then I had 1/2 to 3/4 temp readings.
So you suggest using a infrared temp tool and going over the radiator top to bottm, side to side, right?

About the compression readings. Shop said the readings are acceptable for this kind of old engine. So I beleive them.

Interesting and maybe a hint for you was spark plug color. All are relatively new, gapped to the correct spec. They all looked the same with the tip being grey and one half of the insulator being colored light brown. The one from cylinder #3 (with the little carbon buildup) however was snowy white all over the insulator. No trace of brown. Does this indicate intake valve not sealing properly or ignition trouble?

I plan on running seafoam through the engine to hopefully get rid of the carbon in cylinder #3

[Roger Zimmermann]

The compression cards are strange to me. The orange card for the cylinder #1 is showing 7 bar. The other card, with a different scale, has cylinders 3,4,6 and 8 at about 10 bar, while cylinders 5 and 7 have 12 bar.
Was the test done with the same tool and different cards or with two different tools?
The difference between the lower value and the higher one is rather large, but that does not explain the knock.

[Hillbillycat]

Hi Roger,

Yes, that was done with the same Motometer tool and different cards.
The 7bar on #1 cyl is not 100% accurate. The alternator was prohibiting proper access to the hole to seal the plug with the rubber cone.  Don´t know why they used a orange card for this one. Maybe because they used a rubber hose not the metal rod, or they ran out of blue cards? Can´t tell and didn´t notice they had different scales while I was there.
So the 7 bar resembles the 10 on the blue card.

[DaveO]

For what it's worth, I had the exact same engine knock problem with my '55 and I swore there was an exhaust leak from the exhaust manifold, based on what I was hearing. I started trouble shooting by going from least expensive to most expensive fix. I put a can of Seafoam in the gas tank, and that cured the knocking for an hour or two, then it came back. So I changed the oil and filter and, instead of 6 quarts of oil, I put in 5 quarts of 10W-40 and one quart of RISLONE engine treatment. Problem solved. I have been driving the car for 3 months now and no knocking whatsoever. If you can't get Seafoam or Rislone where you are, I don't know what to tell you, except that you need to figure out how to get it.

[Hillbillycat]

Hi Dave,

Rislone I could get. Seafoam I´d mix myself.
I plan on using the can of Seafoam in the intake not the tank.