philsmobile

Greetings fellow enthusiasts.I am the proud owner of a 90 s4 (euro model)that i purchased last fall.Before storing it for the winter i replaced the worn motor mounts.From day 1 i noticed a 'knock' from the engine i believe at hot idle.You dont hear it at cold idle or when you increase idle when hot.It is not seriously loud but it is definitely there.Being an automotive technician by trade for many years i believe it is a rod knock as i have never heard a lifter make a noise like this.But i have never worked on a 928 before.I have read about the 2/6 bearing failure but i thought that that was only on 928s that were driven hard.This car has only 76,000 km (not miles) and looks as it was very well taken care of.Just last weekend i replaced the torque tube and converter bearings.I also released tension on the flex plate and measured my thrust bearing play.It was approx .009 inches.Is rod bearing failure common in 928s that were lightly used- as in never been on a track or raced?Thanks in advance.

jon928se

Can't rule out rod knock although in my experience Rod Knock is never quiet.

What does sound like "quiet" Rod Knock is a failing air pump - easy to eliminate of confirm this by removing the belt.

Stan.Shaw@Excell.Net

I would say no.

Of course there is the chance a 928 autocrossed hard or even just driven in an abusive manner could create the situation where rod knock might become a problem. Possibly even if someone ran the car low on oil at some point.

Does it sound like it is coming from closer to the front bottom of the motor than the rear? 2/6 usually is denied properly oiling first, so if there is going to be a problem, then it usually shows up there first.

I guess even running poor octance gas could result in pre-ignition leading to rod bearing failure too.

So, while it isn't common for a normally street driven 928, it is still one of the weak links that could show up for many reasons...

philsmobile

Thanks.I should have mentioned that the airpump is already disconnected.I also did not replace the torque tube or bearings to alleviate this noise but i was quietly hoping for a difference.Also oil pressure is 5 at cold idle and 2-3 at hot idle.Its at 5 all other times.Will double check noise with a stethescope to see if i can pinpoint it closer.By saying the noise is quiet i mean it seems to be louder inside the car.

JHowell37

When you say the air pump is disconnected, do you mean that there is no belt driving it?

mark kibort

remember, rod knock is every revolution, while valve train issues are every other revolution.
click click click click..... (say it as fast as you can) is valve lifters at idle. something twice that fast, and down deep in the engine, not heard to be louder at the valve covers, (even though exhaust leaks follow the same slower frequency and can be heard down low) could be rod knock.

mk

Hilton

That is correct oil pressure behaviour for a 928.

Mrmerlin

Warm up the engine, pull the dipstick and drip some of the oil onto a black piece of cardboard go out in the sun and look at the oil for metallic particles. If the oil is clear color isnt as critical then you might be having a oiling problem in one of the heads and the cam block off pins may need to be replaced.. Anyway one step at a time.

jon928se

Bugger!

I was hoping for you that the airpump would be the issue.

hacker-pschorr

Years ago before I owned the 81 (fathers car) a well known Porsche mechanic mis-took a failed clutch bearing for a rod knock. It too only made noise once everything warmed up.

Needless to say he stopped taking the car there

philsmobile

Yes the belt for the air pump was already removed when i bought the car.

bkingdon

Empty valve lifters make a noise similar to a rod bearing. After I rebuilt my motor I thought a rod was coming out of the side until the lifters pumped up and the noise stopped.
Even now a couple of lifters seem to drain down after about 2 weeks of inactivity and it takes a mile or two until the knocking stops. Must make sure to drive more often.

Brian

RyanPerrella

sounds like something i would investigate.

nice tip

John Welch

Is it possible the t-belt is loose and there's no oil in the tensioner so you're hearing some slapping there?