Hi all, new to site. I've got a 2000 A6 4.2 with 200K that has a knocking noise coming from the engine which just started. When I start the engine it sounds fine, then when it starts to idle in 20 seconds the knocking noise starts. It also seems to go away when I drive at higher RPM. I took it to local Euro shop and mechanic said I need to pull engine and replace hydraulic lifts (among other things as well). He seemed unsure about his diagnosis, and my jaw dropped when he said it would cost between 3.5K and 6K. Yikes. Should I get the engine rebuilt instead? Who and how much? Any guidance is appreciated.
Thanks for your help.

 

if it kind of rattles like a diesel motor, you need your timing belt replaced. if it has 200k and you've not yet done that, then you might want to get that done. if it's more of a rhythmic, sharp rap, lifters might be the problem, but i'd take it somewhere to get a more confident diagnosis before you invest that much. i'd think a collapsed lifter might set off a knock sensor, is your check engine light on?

 

Hmm.. look and see if the 4.2's have 'oil pressure valves' in the heads like the 2.8. If these are clogged or bad, it will cause lifter noise while idling that goes away at cruise.

If you dont have a book or some way to check, lemme know. Easy fix if you do...

 

Ok.. Looking around, I found a oil system diagram that shows 6 pressure (or 'non-return') valves. In the heads, these help keep oil in the lifters and cam bearings at low RPM.

1 after oil pump, another at oil cooler, 1 in filter, 1 after filter to block, and 1 in each head or at least going to them. There is also an oil pressure regulator. Without a manual I cant seem to find the actual locations.

However, it seems to me that you have a low oil pressure problem, at least to the heads where the pressure sensor wont see it. You could try a quick oil flush and see what that does.

 

i had a similar problem on a 4.2 A6. It would only knock when it got warm and sounded like it was coming from the heads......long story short, for whatever reason, the water pump would start making an awful "knocking" sound but it ran perfect when it was cold. I would check timing components. A roller or the water pump bearing could be causing this noise.

The big questions here are have you had the timing belt service? How often do you change your oil?
Any lights on in the dash?

If you dont change your oil often, the sludge could build up in the oil filter screen after running for a few minutes and not let oil to the top of the motor but this should trigger an Oil light on the dash.

We need some more info.....

 

One guy's motor knocks just after it starts and the other's knocks when it gets warm.

Let's not confuse things here.....

marcodonn - lifters would clack immediatly on start-up and then maybe quiet down as the motor gets more speed, but sometimes they don't quiet down.
I'd get a second and maybe third opinion before spending any money.

as for rebuilding the motor -- not shure what I'd do with 200K on the vehicle - there are lots of things that could wrong soon.