Strange Engine (?) noise

XK
1

Heading down a rabbit hole, thought I’d throw this out just before I jump…

A quick search on this topic turns up many problems and many solutions, not sure which one will be mine!

My 56 140 OTS has started exhibiting a funny noise… usually when loafing along, with engine warm, sometimes a gentle rattle, sometimes evolving into a louder more violent thwacking, after a minute or so seems to subside on its own, does not respond to plus or minus acceleration, ie does not get louder & faster if I accelerate when the noise happens, and vise versa does not get quiter and slower if I jump off the gas.

A short history on the car may lead to clues?

I assumed ownership (custodian-ship?) of the car about 3 years ago from my dad who bought it @ 1982. Overall condition at the time was questionable as that was an era when these cars were not appreciated and service tended to be the quickest and cheapest route back to the road… as illustrated by the C Type head tossed aside for a 120 head when repairs where needed many years ago!

When I adopted the car dad had been driving it as a sunny day car, but in rapidly diminishing amounts as at 90 years of age he found it was getting harder to get into much less drive! and his usually very high standard of self-service was diminishing also as he found it equally hard to get on and off the ground to fix things! (He currently still maintains his 60 Alvis and 57 Bentley… he’s no slouch!)

Over the last couple of years I have given it moderately thorough mechanical inspection and repairs where needed to make it road-worthy…
New front sway bar bushings to replace the nonexistent ones
New water pump, hoses, thermostat, flush rad
Remove , clean and inspect distributor, new points, condenser, cap
Rebuilt carbs including throttle shaft bushings as I could not get idle down below 1000 rpm.
Changed engine oil, filter, spark plugs and rear axle oil
New (to me) 54 spoke wheels, new tires and tubes, broken spokes replaced, tire and tubes self installed
Lubed all chassis points
Dismantled, cleaned and inspected all brakes: no signs of leaks, wear and dust accumulation seemed even amongst all wheels so back together with nothing more than the cleaning and adjustments.
Cleaned under chassis, engine, transmission, OD, rear axle, inspect and tighten all nuts and bolts to minimize leaks… OD leaks significantly at actuator arm, nothing else leaks more than expected.

Proceeded to adjust points gap (dwell), timing, carb settings… started having the "noises ". Noises were not present in similar drives before all the mentioned work…

Thinking this is some form of valve rattle (although in my experience that is usually most evident under hard acceleration which is not the case here) I went back through everything…

Pulled front wheels looking for loose or broken spokes, pulled front brake drums looking for improperly installed adjusters or anything that could have come loose…
Re-did Gap, confirmed with Dwell.
Re-did timing, confirmed function of mechanical and vacuum advance with additional marks I put on flywheel.
Drained and refilled gas tank with fresh 94 octane gas
Inspected all front body work, panels to ensure nothing was rattling

Since the second tune up the noise seems to have reduced in frequency and severity, or maybe I’m used to it and not as freaked out!? But it needs to be resolved… generally car is running smoothly, idling well at @ 600 rpm when warm, pulling strongly up hills, plugs are nice and light brown when inspected.

So the plan forward…

Pull valve covers, look for anything amiss… note tach is not reading past @ 2500 rpm but this has not changed in years, will inspect tach drive but unlikely to provide the "energy " needed to create such a loud and violent noise.

If nothing is evident, pull the head… looking for broken valve spring?, bent valve? Maybe timing chain tensioner?

Trying to think of what I have touched in my inspections and services that this noise could be related to… if it is a problem I have created then it’s not in the valve train or engine…

And having typed all this out I feel much more focused so thank you for listening!

Thoughts, related experiences, tips all appreciated!

2

When cam box covers are off, ascertain the tightness of the upper chain, the condition of the upper chain guides, and see if you can observe any slack in the lower chain.

Also inspect the tappet guides, and see if any have migrated up, out of their bores.

3

First try a mechanic’s stethoscope, or the low budget version is a long screwdriver in the ear, see if you can locate the noise in the engine.
If nothing is obvious, I would start looking for loose sheet metal, loose fan blades, exhaust hangers, etc.

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Thanks Paul, that is where I am headed!

5

Hi Rob, I had hoped to do this, but the noise happens intermittently and I haven’t been able to have it happen while in the garage. I did pull over promptly once when it was rattling, but by the time I got the hood up the noise was gone… I have gone through all the fan, sheet metal options a couple of times now, at first thought it might be something interfering with the fan…

6

The stethoscope still might detect something.
I would try eliminating things. Disconnect the tach cable at the camshaft and speedo cable at the gearbox. Run it up to a good speed, put it in neutral and shut off the ignition, and just coast, see what you hear while rolling with the engine off.
Remove the fan belt and run it without the water pump and generator turning and listen with the stethoscope. You can do that for a minute or two.
But I keep coming back to timing chain.

7

That would be my vote. The tappets will generate a ticking noise that varies with engine speed if something is amiss. If the chain guides are failing, it could account for the intermittent nature of the noise as the position of the guides and the chain changes. You could put an endoscope down past the upper chain and sprockets, but my guess is the damage would have to be significant to be seen.

8

Dave, does your engine have the coil spring lower chain tensioner or hydraulic tensioner?

9

Doubtful the upper chain guides are implicated if they’re just worn, though possibly one has broken off. When the aftermarket guide on the left side of my 4.2 rebuild broke off there was an intermittent rattle as the thing bounced around. The rattle of course disappeared when I fished the piece out and the engine sounded normal again, with only the right side guide still in place. (So, if the engine sounds normal with only one guide in place, it stands to reason that wear alone will not be manifested by a rattle.) I contacted the parts supplier and was told that they’ve seen original, unrebuilt XK engines with broken guides, so it happens with OEM upper chain guides too. Easily enough to check out after removing the cam covers. Wear or bonding failure in the lower guide may be another story, since unlike the upper chain the lower chain’s tension is variable and the long run up to the intermediate sprocket may be subject to excessive lash, though if the rattle is from the lower chain I would sooner suspect a tensioner issue.

10

Don’t know! It’s engine # G3892-8S if that helps!

11

Ok most likely you have the coil spring type.
The coil spring is the yellow arrow.
It pulls the blade (red arrow) which pushes against the chain to hold it tight.

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I had my coil spring break once and it made a rattly noise which could be heard down low with the stethoscope.

12

…and that is almost always a result of a loose upper chain.

As part of periodic maintenance on customers’ cars, I checked/tightened the upper chain every 10,000 miles.

13

I finally have a little time to dive into the engine and start investigating. Cam covers and Breather cover off, everything looks right… I haven’t actually tried to tighten or adjust the upper chain yet but it feels tight, no flex when I push on it with my finger. I can clearly see the upper guides in place, and peering down the timing chain cover from the top I can see the top of the tension spring on one side and the guide on the other side of the lower chain, something might be loose of broken, but its not obvious yet!

One item of note is that the release button for the upper chain adjuster was protruding maybe an 1/8th of an inch or so in front of the adjuster, I pushed on it and it easily moved back to be flush with the face of the adjuster and has stayed there so far… could that be indicative of anything moving around incorrectly?

But now I’m wondering again about the cooling fan. when these noises first started - somewhat softly or quietly compared to more recent occurrences - my first thought was something interfering with the fan blades. A quick poke around with the engine off showed nothing that could be interfering so I moved on… but more on the fan blades…

One of my habits (maybe a bad one?!) over the years while working on ignition tuning has been to rotate the crank by pulling on the fan blades (sparkplugs out, usually requiring pushing on the fan belt to tighten it so it does not slip…) Did that on this car seemingly months ago now and realized pretty quickly that I had bent a fan blade - maybe two? So I gave it a preliminary straighten while it was still on the car - a casual eyeball alignment with the generator pulley so that all blades looked equal and moved on to using a wrench on the pulley going forward. Now that I’m back under the hood investigating this noise issue, one of the first things I did was remove plugs and grab the fan to rotate the engine into TDC and of course felt the blade bend! and so I bent it back… and then when I was under the car rotating the crank with a wrench I heard a suspicious scrapping noise… of course it was a fan blade scrapping over the pulley.

This all leads me to wonder if my occasional noise could be a fan blade that is warping during driving, a quick inspection of the blades (fan still in the car as of now) shows two blades with some scuffing on the tips… but who knows how long they have been like that, could be from an event years ago…?

any thoughts? do fan blades bend and flex during normal operation? maybe a series of careless bending and re-straightening has weakened a blade or two?

A new fan from Moss is quite expensive so I’m not keen on just sticking a new one on to see what happens!

TIA for any further insights!

14

That button is supposed to stay out. It has locking teeth which lock in the tension adjuster. Pushing it in releases the tensioner.

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Fan blades certainly can flex more if they have been bent and then straightened without restoring the curve in their shape. The curve gives them more stiffness to resist bending.

15

thanks for the quick reply Rob!

I managed to easily pop the adjuster button back out!

I now have the fan on the bench and it is obvious that some of the blades are at different angles and have different curvature than others, but I don’t see any obvious signs of fatigue… cracking etc, so I will attempt to get the ones that show signs of reshaping to match those that look pristine. The two obviously miss-shaped blades do also have some abrasion marks on their tips on the engine side… so likely interfering with the crank pulley.

does it make sense that the poorly shaped blades would flex towards the engine, only once warm, and only at low revs? the noise seems to go away when driving faster / at higher revs, only appears when idling along at low speeds and low revs.

I would have thought that if there was any flex it would have been towards the radiator if the blade was not strong enough to “pull” the air…

16

I had a similar noise several years ago. Upon dismantling, the tensioner spring and its mount were both broken. Replaced with hydraulic type.

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and there’s the crux of it!

If it’s believable that the noises could be from my fan, I don’t really want to pull the head, sump, and timing cover to discover nothing is wrong…

If it’s unlikely that my fan is the culprit I’m prepared to dive in, and now is the time to do it as it’s winter and no driving is happening.

If I go with the fan as the culprit and find out on the first Spring drive it was not, then Ill be spending valuable driving season time back under the hood!

18

Run it without the fan for a minute and see what sounds you hear.

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I had a generator bearing cause a rattling sound. Also had bent fan blade hitting the shroud. Measure from a point on the breather to each blade. Suggest you loosen the belt rather than turn the blade while tight. Check if the fan blade rivets are loose. Check the fan blade bolts are tight. Run the engine with the fan/gen belt off. Just some stuff to try before heading to surgery.
Pat H

20

All good suggestions… unfortunately running the engine without the fan does not seem easy - the noise does not occur until the engine warms up, not sure what effect the warm engine has on the shape of the fan blades. I guess I could drive it around to warm it up, and then pull the fan or the belt, I’m sure that would be a fun adventure on a hot engine! but I’m not sure how long I’d be comfortable driving around with out water pump and generator - how long do I drive to see if the sporadic noise is gone? I think I’ll install a new fan and go from there… unfortunately they seem expensive, although I found them through DMG in the UK for about 1/2 the price of Moss in the US…