740 rattle noise on deceleration

Dec 17, 2008 6 Replies
M

Hi all:



Thanks to all who commented a couple of years ago on a flashing oil light. I tried a different oil pressure sensor; and it's been fine since.



Now, the 86 740 turbo has a new rattling noise, mostly on deceleration. Seemed to happen after I drove it harder than normal in a rush to make an appointment. Different than the piston slap I'm told is normal for these engines. Sounds high in the engine. One mechanic thinks it's time for a new (used) engine. (He found one noise in a contact in the alternator, repaired it but that wasn't it.) This mechanic has been good, but is sometimes a bit of a pessimist.



Another mechanic thinks it's valve dampeners, about $200-400 to replace depending if it's top and or bottom (?), but says there is no immediate need to.



I'm not a mechancic by any stretch, just looking to make the right choice.



Thanks!



L
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J

Could it be a loose heat shield or the finger guard on the alternator belt?

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M

I will ask. It sounds more like something in the engine than on it to me though. More like a 'knock' than a 'rattle' really.

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J

That usually makes a pretty distinct gurgling popping noise, my 242 was doing that while I was getting megasquirt tuned up.

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L

Hide quoted text -

Sometimes engines that are running rich will misfire on deceleration, as the extra gas detonates at irregular intervals. Older hotrods and race cars did it all the time. Check your fuel system.

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M

Thanks for posting. The rattle/knock is very regular; every deceleration, same sound, rate seems matched to rpm; ie slower as rpm drops.

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L

belt?- Hide quoted text -

That probably rules out both misfiring and a loose heat shield. It does indeed sound mechanical.

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