997.1 (2007) Knock from engine

[hollany]

Guy's I need help.

 

I have recently replaced the pistons and sleeved cylinders 4-6 on a 3.8 Carrera S for a customer of mine. The car came into my workshop with the usual symptoms of bore score, a heavy knocking sound which gets worse as the car warms up and the left hand tail pipe much darker than the right. The car was making a terrible noise but I've done a few of these before without any problems and I was sure this one would be the same. Sure enough cyl 5 & 6 were very bad so I sent that side of the block off to my machine shop and they put steel liners in all 3 cylinders. I replaced the pistons and rings (Genuine Parts) and rebuilt the engine. All looked good and before I gave it back to the customer I gave the car a run home. The car drives really well but with one small problem. If I blip the throttle quickly there is a heavy knock. It knocks once as I press the throttle, not on over run but everytime I blip the **** thing. The thing is I'm not sure if the noise was there before I stripped the engine. It was making so much noise I doubt I would have noticed it anyway.

I have listened to it with a stethoscope and it sounds like its coming from the intermediate shaft but I have never heard this noise before and I was wondering if any of you had. I have removed the tensioner and it seems fine. In a moment of self doubt I convinced myself that it was an assembly issue so I removed and stripped the engine again. I can't see anything wrong but I don't want to stick it back together before I've heard from you guys.

 

Hope you can help.

 

Kev

 

 

[JFP in PA]

Welcome to RennTech

This is a rough one to try and diagnose remotely, plus you have not identified the year of the engine, but I do see a couple of problems.  The original cylinders were aluminum, and the factory pistons are designed to be used on aluminum liners, not iron or steel.  Usually, using pistons and rings meant for aluminum in iron liners does not end up well.

 

Before you disassembled the engine the second time, did you check the cam deviation values?

[RFM]

A little thought about this, the flywheel is however fixed according to the indicated values? What JFP in PA quotes is definitely true.

[hollany]

Thanks for replying.

 

RMF, could you explain your answer a little more? I'm not sure what you are saying.

JFP in PA the camshaft deviation was around 3.5 degrees at idle which I thought was ok. I thought it might change when I revved the engine but I'd heard that 6 degrees was the top end of tolerance.

Regarding the steel liners, its quite a common repair over here in the UK and I've never heard of a problem. (There's a first tme for everything though.)

 

Thank you again for your time and the welcome.

 

Kev

[hollany]

Sorry, I meant to add it's a 2007 engine.

[RFM]

The question is whether the flywheel is not loose relative to crankshaft flange, defective dual-mass flywheel, loose torque converter/broken mounting plate (starter ring) if Tiptronic.

[hollany]

OK Thanks. It is a tiptronic and there's no play that I can see. I should add that it sounds like it's coming from the center of the engine. Possibly the intermediate shaft chain area but the chain and tensioner look ok.

[JFP in PA]

7 hours ago, hollany said:

Thanks for replying.

 

RMF, could you explain your answer a little more? I'm not sure what you are saying.

JFP in PA the camshaft deviation was around 3.5 degrees at idle which I thought was ok. I thought it might change when I revved the engine but I'd heard that 6 degrees was the top end of tolerance.

Regarding the steel liners, its quite a common repair over here in the UK and I've never heard of a problem. (There's a first tme for everything though.)

 

Thank you again for your time and the welcome.

 

Kev

 

+/- 6 degrees is the limits on the cam deviation values at idle, but what is also worth looking at when check the cam deviation is if the values remain steady at an idle, or are jumping around.  They should be relatively steady as jumping values indicate excessive "slop" somewhere in the cam drive assembly.

 

The piston skirt shape and coatings are designed to work with the characteristics of the surface they ride on, as are the ring alloy composition.  As the factory liners were an aluminum alloy, they would have different thermal expansion characteristics than iron or steel liners, so at temperature, the piston skirts and/or rings can be subjected to significantly different wear than they would see in an alloy liner, which is why we use high quality alloy replacement liners here.

[hollany]

Thanks for your help. I'll Let you know how I get on.