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68 Bentley T engine rattle under load.

3959 Views 104 Replies 12 Participants Last post by  Jim Walters
Hi all, I have been restoring my T1 5139 for the last 6 months and have completed the interior etc, however I have an annoying engine rattle under load which I cannot diagnose. L believe engine mileage may be 143k.

It feels like it is a collapsing hydraulic tappet, but all nice and quiet at idle or steady load or decellerating. I have had all the tappets out, tested them with the proper RR leak down tester and found 3 that would leak down very quickly at 2 seconds, 5 seconds and 7 seconds. Minimum should be 20 seconds maximum 90seconds. So the 3 weak ones were replaced with known good ones, put it all back together and the same rattle persists.

The noise is engine speed related, feels like it’s coming from the engine gallery area and is more like a hollow rattle than a knock. The noise increases if the load on the engine increases. It’s not the hydraulic brake pumps by the way. Those have been removed and the braking system has been converted to a conventional dual master cylinder and remote servos.

Would anyone have some ideas of where to look next? Weak valve springs? Cam looked good, no lobe wear.
Any assistance appreciated.
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Did you pull the front pump on the gearbox for a look see?
Hi, yes I pulled the gearbox oil pump, took it to pieces, no damage, rebuilt, set the shaft end float correctly with new shims.
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One or more small end bearings?
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Yes it could be little end bearing(s), which would be unusual I would say. I ran the car with each individual HT lead removed and it made no difference what so ever. I would have expected the noise to reduce at least in volume if it was little end bearings. I could take the sump off and check big end and little end play?
When the problem is the little end bearing, you must hear them when the engine is warm and you push the gaspedal several times.
That's pity - it would have been a welcome relief after 85 posts. Good luck with your diagnosis.

I also recall that I spent hours hunting for a clonk from the rear end of a car when braking hard. A fellow petrolhead sitting in the passenger seat said "That's from under the bonnet, not the rear end". It was a loose wiper bottle that banged on the bulhead.

Sigh... it's a good job I'm retired.

Alan D.
I've got a Shadow in the shop right now that I've been trying to find the source of an engine knock. I've posted a short video on Youtube, link below. This is the first time I've posted on YouTube, and when I view it the text doesn't show so I've copied and pasted the text that accompanies the video below the link. The difference between this thread and my video and ultimate diagnosis of the car in question is that the knock was always there, idling, accelerating, coasting, no plug lead on it etc. As a last resort you may want to try the same thing I did by removing all compression load on a cylinder.

This knock was initially thought to be a faulty brake pump, customer had previously rebuilt the pumps himself then with no change actually installed two brand new pumps but the same knock was still there. I pulled the pumps and checked pump to pushrod clearances, which were within limits. A stethoscope pointed to the knock coming from the back of the engine on the right (A) side. I eventually ran the engine with both brake pumps and associated pushrods removed and knock was still the same. Running the engine with a spark plug lead shorted out one by one made no difference to the knock. I next removed the A bank valve cover and while the engine was running pushed an .015 thou feeler gauge under the rocker arms one by one. When feeler was pushed under A4 exhaust rocker arm the knock stopped. Assumed it was a bad lifter then so stripped off the intake and valley cover and replaced the A4 exhaust lifter with a known good one. Got it all back together but knock was still there with no change. When lifter was replaced I did check cam lobe for damage and there was none and cam lift was in specs. So next I removed A bank rocker shaft and both A4 pushrods, put shaft back on and ran engine with no pushrods in A4. Knock is still the same. Then using a pry bar under the rocker shaft I open the A4 exhaust valve while engine is running, finally a result. With no compression load on the piston the knock disappears. So the result of diagnosis is that the A4 piston has likely broken or the wrist pin in that piston is badly worn. I'm leaning towards a broken piston as block corrosion issues in Shadows are known to squeeze the A4 cylinder liner until the piston seizes breaking the skirt or lands. Next step is head and sump off and pull out A4 piston and rod and inspect.
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I listened to your video again in post 55, I think the noise is coming from the wrist pin area _ bad bushing, bad wrist pin or the wrist pin is worn in the piston, one or a combination of the three.

Could be a connecting rod bearing as well.
Looks like Bert may be correct, and at this point since pretty much all things have been eliminated, there's not much left.

Jim, I would assume that you opened the exhaust valve very little as it's an interference engine, or is it ?
How much did you open the exhaust valve while the engine was running ?
If it only took a 15 thou feeler gauge, I guess just enough to take the compression off of A4.
This is fascinating (in a scary way). Are you thinking there's a break in the skirt below the rings so there's no obvious loss of compression?
I opened it at the most an eighth of an inch, very carefully. At no time was there an indication that the piston was hitting the valve, I was very aware that it could hit if opened too much. It was definite that without compression in that cylinder there was no knocking. I got exactly the same result opening the intake valve too. The usual break is the skirt, but could also be a ring land broken. I checked inside with the borescope but could not make out any seize marks on the cylinder wall, and the top of the piston wasn't broken. However, there is excess oil getting into that cylinder. It shows as carbon on the plug, and when the engine was last shut off the day before yesterday the A4 piston stopped at the top of the stroke. When I went back to it yesterday with the borescope and cranked it over until the A4 stopped at the bottom of the bore, there were several drips of oil running down the cylinder wall from the top, much more than one would expect to see so I think rings are broken too. Haven't checked compression yet but that will be on the schedule today.
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Wonderful diagnostics Jim! Hopefully with all the labor involved going forward your diagnostics will pan out.
Thanks Jim and others for your valuable input.
Interestingly I did go through the process of removing push rods to try and identify the cylinder, but the noise was still present as in your example. All evidence points to my issue being engine load (and engine speed)related. So I am homing in on the piston, piston ring, wrist pin, little end bearing, big end bearing, as those are really the only load taking items.
I dont think I can do the feeler/pry bar test though. My noise is only present under engine load and not at idle. Although I can load the engine in drive with the brakes applied to induce the rattle, just need to be careful though and this only replicates the fault when all 8 cylinders are firing. I could do the feeler gauge test perhaps.
The key thing here, as with your example Jim, is diagnosis. If I am to pull the heads/engine that is a massive job for me especially if I can’t remove the head(s) in situ. This will be done on my drive at home.

So to get as much information as possible I repeated my compression test. Looks like my compression tester was sticking at 129psi. So I corrected that and got the following results. Warm engine, fuel pump disabled, throttle fully open, 5 cranks (which allowed max compression reading)
Psi readings
A1 160
A2 140
A3. 158
A4. 137
B1. 144
B2. 149
B3. 128
B4. 138

B3 was low at 128 so I did a wet test and got 147, which is a 15% increase. All others look pretty good.
So if I was to do the feeler gauge test that’s the cylinder I would target. I need to cover all and every test I can to diagnose this before diving into the engine!
thanks again.
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Dripping oil from above can also be caused by bad valve seals.
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Do the feeler gauge test (15 thou) with the engine in drive with the break on _ I don't understand why you can't ?
All 8 cylinders will be firing _ do one bank at a time, you may get lucky so you don't have to remove both valve covers.
It's done while the engine is running, Jim's quote:
"I next removed the A bank valve cover and while the engine was running pushed an .015 thou feeler gauge under the rocker arms one by one".

Pulling the push rods does not relieve the compression on the engine.
To do the feeler gauge test is possible but not desirable. It means heavily loading the engine and stalling the torque converter whilst trying to work on it. So 200 bhp trying to launch the car, so not desirable.
An alternative method would be to remove individual spark plugs, I tried that but the noise of the escaping air scared the neighbours. So what I could do is loosen the spark plug enough to simulate the 15 thou valve open scenario but not remove it completely, to get pressure off the piston and then run the car. This may identify if it’s an individual cylinder issue.
I may then get the sump off and check for big bearing play/wear and little end play with the big end caps removed. So that may be my next test before attempting the feeler gauge method.
Loosening the plugs is not a good idea due to the fact if they blow out it will will take the treads with it further complicating things.
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Understood. I may need to to run it with the plug removed then. Ill warn the neighbours.
Isn't there a place where you could take the car that's not going to bother anyone ?
Here in Canada there's all sorts of space for that, maybe not so much in the UK ???
Canada is 100 times the size of England, a vast wilderness of open space. Here in South Manchester it’s a bit more cramped. There is a good spot about 3 miles away, but I really need to sort out my rear shock absorbers before venturing there. One pushed through the bottom cup and on removal has a bent shaft. I took the other one off and that is practically seized and spewed its oil out when extended. So two new shocks and spring cups are needed.
I did manage to get my stethoscope out and had a prolonged listen at idle, when cold there is a very faint knocking around B3 and B4 area, when hot this is more pronounced and if the revs are increased to say 1500rpm you can definitely hear a knocking with the stethoscope, but not without it. So whilst I have the car in the air to do the rear shocks I will probably take the sump off and investigate further from the bottom end.
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