Afternoon Gents

A friend is in the process of restoring a Renault 5 GT Turbo. It was it bits when purchased, and he has got to the stage of starting it for the first time over the last few days.

All seems fine, except when coming off the throttle, as the boost drops, there is an audible knocking as it settles back to idle.

First concerns were bottom end - can anyone advise if that's correct, or if there are any other indicators that would point towards bottom end bearing failure?

I know nothing about these........



[Edited on 24/7/14 by Norfolkluegojnr]

Detonation from the mixture being too lean??

Is it audible at idle - goes when throttle blipped
Could be worn piston skirts,the piston rocks on the rings

Hmm, my engine (S50B32) did this after an over-rev which trashed the main bearings - they knocked, but only when it was rev'd to about 3.5k, as the revs were falling again. In my case I could identify the cylinder by using a stethoscope on the primaries. It didn't knock at idle.

I guess under power, the crank is always being pushed against the upper bearing, whether moving down or up. When unloaded, the crank pulls the con-rod down by pushing on the lower bearing, so I suppose it must be bouncing between upper and lower bearing, hence the knock under no load.

What speed is the knocking relative to the engine speed? I believe if double-speed it's top-end, if single-speed it's bottom end.

[Edited on 24/7/14 by rodgling]

Fresh oil will hide knocking ends for a few hundred miles, hope he was not sold a duff one.

A mate bought one many years ago and it developed the same noise after a few hundred miles , sump sealant looked fresh and shells had been fitted to a scored crank to get rid of it .