Engine tap/knock on start goes away in 15 seconds

[mo968]

Hi Everybody,

 

When I first start my car, the engine has a knock that lasts for about 10 seconds or sometimes longer. It’s intermittent and then goes away. Additionally, if I rev the engine immediately on startup, the knock never appears. 
 

I’m obviously not a mechanic but try to do things myself when I can (mostly maintenance, oil changes, spark plugs, coils, etc.). 
 

I heard a similar knock on a 2005 Carrera that was for sale on eBay in a video the owner posted, but that knock never stopped. Interestingly, the knock won’t return the next day if I start the car in the morning. 
 

Mine is a 1999 Porsche 996 with six speed, 3.4 liter engine, no leaks, burns no oil. 
 

Yhank you for your thoughts on this and how it might be resolved.

 

Monte Olsen

Edited March 30, 2020 by mo968
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[rajk]

I also have a '99 with exact same conditions described.  When I start the car after a couple weeks the sounds is there.  If I use the car consecutive days the sound is much less or not there at all.

 

From all the reading I have done, the IMS chain tensioner is a possible cause and not addressing it can increase wear on the tensioner paddle.  I plan to replace this tensioner soon (pn 99610518059).

[dporto]

I wouldn't ever describe the noise made by timing chains as a "knock"... more like a "rattle", which is quite normal as the hydraulic chain tensioners get old and leak down over time. A  "knock" is all together different and will be timed with the engine turning. Generally speaking these "knocks" are caused by the connecting rods on the crankshaft or the pistons ("piston slap") as they rock in the cylinder at the beginning of the up/out stroke... While these sounds certainly aren't the same as each other, they would be more consistent with a "knocking" sound as opposed to a "chain rattle"... Semantics I know, but it really does make a difference.

[monteolsen1]

That’s what mine is, then. A knock, but not a deep one. It goes away quickly. Thanks. I may check the chain tensioner also, thanks for the link.

 

I was told by one person that the knock could be a result of oil not remaining in a lifter where it should and that once the lifter is lubricated, it goes away. Is there a fix for that, possibly?

[JFP in PA]

8 minutes ago, monteolsen1 said:

That’s what mine is, then. A knock, but not a deep one. It goes away quickly. Thanks. I may check the chain tensioner also, thanks for the link.

 

I was told by one person that the knock could be a result of oil not remaining in a lifter where it should and that once the lifter is lubricated, it goes away. Is there a fix for that, possibly?

 

Yes, cam followers can bleed down, and can be replaced.  Not a small project, however.

[dporto]

Not a small project and not a very common problem with these engines...

[JFP in PA]

59 minutes ago, dporto said:

Not a small project and not a very common problem with these engines...

 

More common than you might imagine, Porsche used a strange design cam follower that is very easily susceptible to varnish build up and particulate debris.

[Uwon]

Been following this thread with interest as I have a 997.1 S with ~120k miles that sometimes does the momentary chain rattle noise on cold startups common to the early air cooled cars.

So, how does one diagnose the issue further?

What would a Porsche shop likely do?

listen with a stethoscope?

Do you get a fault code eventually?

Is it trail and error? First change the chain tensioner (cheaper alternative) and see if that eliminates the rattle?

Johan

[monteolsen1]

I plan on taking mine to a Porsche repair place here (we don't have a dealer, but we do have a few excellent Porsche service and repair folks) to see what they think. I was told that you might be able to clear the particulates by putting half oil and half automatic transmission fluid into the engine, running and flushing and then repeating.

 

Monte

[monteolsen1]

I haven't had a fault code appear. The rattle is likely caused by the chain tensioner (mine is very quick and then gone) and the tick also goes away quickly if I rev the engine. Oil pressure has always been strong.