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Old 07-30-2020, 10:58 AM   #1
Codum
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Default Engine knock

Hey gang, I'm eyeing a 740 turbo to purchase and the seller was forthcoming enough to share that there's engine knock. Initial guess by them is something loose in the rockers. Sounds like a quick valve adjustment job to me, but would like anyone else's input or experience with this issue.
Thanks!
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Old 07-30-2020, 11:39 AM   #2
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Knock or tick?

A lower frequency knock from deep within the engine is likely a major issue, unless it's only piston slap.

A tick from the upper valve train probably is not as serious, but could be.

Might want to take it to a Volvo tech for an informed opinion before pulling the trigger, as the cost of repair likely exceeds the value of the car.
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Old 07-30-2020, 11:51 AM   #3
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Excellent, I asked for a video of the sound so maybe that will be enough to scare me off. But for sure will be inclined to ask a local shop to have a look.
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Old 07-30-2020, 12:57 PM   #4
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No rockers on a B230. Cam acting on shim in bucket on top of valve. A worn shim (or cam lobe) could cause a tick / clatter which could be pretty loud if the clearances opened up. Excessive valve clearance definitely does not sound like a knock and would be coming from the very top of the engine. I had my 1987 745 T for 25 years and the clearances never changed to the point that the shims had to be changed.

I presume that this knock is not detonation that can occur under load? Detonation will occur if the knock sensor has failed and the owner is running 87 octane fuel.

What vintage 740? Around 1987 Volvo changed the pistons, at least on the B230 FT, to ones that were closer to a slipper skirt profile in an effort to reduce friction and improve fuel consumption. The slipper pistons would develop piston slap as the clearances opened up. My 745 T developed it somewhere around 50 - 70,000 km. I talked to a Volvo tech when I was having the car in for some other service and he just made a comment kind of under his breath "they all do it, don't sweat it". I presume that at some point Volvo fixed the problem; but, I don't know what year piston slap ceased to be a problem. Perhaps when they installed the piston squirters.

The piston noise was most noticeable when the engine was cold and generally became unnoticeable once the engine was up to operating temperatures. For some reason the #4 piston was the most prone to develop slap.

If the knock does not go away when hot and is coming from the bottom of the engine - that is an expensive fix.
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Old 07-30-2020, 01:44 PM   #5
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Ah that's a good stuff, I'll have the seller describe the behaviour a bit more and ask for video/audio from the top vs under the engine (if they'll climb under).

This particular 740 is a '91
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Old 07-30-2020, 01:48 PM   #6
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@142 guy, I'm also poised to crack open the engine of my current 740'90 churbo since I've never worked in that part of it plus the casing on everything is really crusty (oxidized)
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Old 08-04-2020, 11:47 AM   #7
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I’m posting not so much to give guidance to OP but more of an example. We had a, I think 87’, 740 jurbo. Bought it with around 150K and sold/gave to a friend at close to 300K. There was always a knock. Most of the time it would get worse or at least more apparent after warming up. Sometimes quite and other times quite loud. Valves were adjusted/checked twice and the knocking would go away when stopping I think #2 injector from spraying.

Maybe an idea for OP is to investigate for a tricky ticky exhaust leak.
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Old 08-04-2020, 02:18 PM   #8
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Disable each cylinder one at a time and if the knock goes away you probably have a bad rod bearing on that cyl.
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Old 08-04-2020, 02:25 PM   #9
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Carbon Knock? = Squish too tight
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Old 08-04-2020, 07:03 PM   #10
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Thanks gang, super good info but the fella selling hasn't got back to me for a few days so I may not need to. On the bright side I know how to check local cars and learned how to diagnose knock!
Much appreciated
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