VR6 Main and/or Big End Bearing Failure - Causes?

[mattwelcer 0]

Well last Saturday the engine started stuttering on the Autobahn while cruising at 130-140km/h after roughly 30mins. Checked oil temps and they were up at 124C so pulled over and heard knocking noises from the engine and then switched it off. Now i am pretty sure its the bearings whether its the crank or conrods i am not sure, opening it up on Friday (oil pan off).

 

What could cause something like this to happen? I rebuilt the engine in April/May and replaced everything on it apart from the oil spray jets. The block was originally a 2.8 which i had bored out to 2.9 and used OEM Mahle 2.9 pistons. The bearings and oil pump were all new from VAG with the associated bolts.

 

How are the conrod bearings fed with oil, through the crank from the main crank bearings? Therefore i assume it is highly likely that the main bearings are also shot if the big end bearings are shot.

 

If i am lucky can i just replace the big end bearings provided the crank and conrods aren't scored? Or does everything need to come out and be inspected to determine the cause of the problem?

 

Any help would be greatly appreciated as after covering 5000km (140km a day to and from work with varied motorway, dual carriage way and B roads) without any issues since it being rebuilt this has me really baffled as to why it has happened now :scratch:

[mattwelcer 0]

Oh and an additional piece of info the engine makes the knocking noise at idle but if revved to 3000rpm and over then the knocking noise goes away. Could this therefore then just be the big end bearings and not the main bearings? And at no point did the oil warning light come on to indicate a pressure or other oil related problem.

[CorradoVR6-Turbo 0]

It sounds like big end bearings,this will fail first as it has the most load from the thrust of the piston,the bearing are fed via the block through crank to big end shells,in most cases if its caught quickly you can get away with just replacing the big end shells and polishing up the crank journal with fine sandpaper.

 

When you rebuilt the motor did you just fit standard shells? did you plastic gauge the shell gaps?

[mattwelcer 0]

Yes just standard shells and no I didn't plastigauge the shell gaps as the machine shop said the crank was within tolerances.

I think that's what I am going to do next try and get hold of some plastigauge and check the tolerances.

 

When you refer to fine sand paper what grade, 1000 or finer?

[CorradoVR6-Turbo 0]

yea its always good to confirm the shell tolerances with plastic gauge when building a motor as you don't truly know the tolerances other wise,as for sand paper 1000 grit is a good start,but it will only clean away shell deposits, for any deeper scoring then machine work is required and that will suck ass!