strange knock from engine need some help with diagnostic
 

hey guys

i took the 930 out for a drive and while at a stop sign i noticed a strange knock!!

this only occurs when you blip the throttle to about 2k rpms and while its dropping back to idle if you just touch the throttle to stop the rpms from dropping you get a single knock. this only happens at low rpms, the car has good oil pressure at idle and runs 5 bar at 3-4k on the dash gauge. there is no knocking under load and non under .5 bar boost as i was afraid to boost any higher till i find the knock. this knock may have been here a long time because its very heard to here unless you know what your listening to. while behind the car with my chest on the intercooler :D with my left hand operating the throttle i can feel the knock on my chest, it feels like a valve opening and closing when it happens. i cant determine what area of the motor its coming from maybe right side. was thinking chain slap from the chain case area but just not sure.

any ideas?

cheers ed

Before I got my 3.3L Turbo engine, my 911 was a "standard" 911, with a normal 3.0L

One day at the stop light I heard a thump-thump-thump...WTF? I didn't know it, but I spun a bearing at the crank.

I highly recommend not driving it anymore until someone in the know can actually hear what the problem is...it could be something as "small" as the fan housing/alternator going south, or loose, or something much more major.

Good luck!

Ed, are you running the standard "short neck" intercooler configuration? It "may" be the stock recirc valve gaskets. Check here: http://forums.pelicanparts.com/porsche-911-technical-forum/71370-changing-930-dump-gaskets-pictures-galore.html

Hey RSR
Yes standard short neck:(
great link and think that must be what I'm hearing. Do you know if it only happens low in the rpm band ? I went out and played with it somemore and when I first crank it I here the knock one time, then I here it when I step on the throttle and maybe when I lift but the muffler is loud. My car has about 40 k miles and has sat a large part of it's life. I will put a stethescope on that area and hope that's the sound.

This isn't your nomal knock it's a single knock like a valve closing or opening? I think RSR hit it on the head. I'm crossing my fingers on this.

Hello there,
Mine had a similar sound, it would show up every time I blipped the throttle aggressively from idle while in neutral. Kinda a thunk or clack sound.

I bought the new gaskets to fix it from our host (dirt cheap by the way). I went to replace them, only to find that the job is super simple & fast to do.

Pop off the intercooler, pop off the top section of the intake (3 bolts, several hoses, couple of wires). Take off the 8 little nuts on the cover plates(watch out for the spring tension it's pretty high) and there are the offending gaskets.

In the end, I simply changed out the gaskets and the noise went away, and then decided to completely dead end the whole system, but that's another story.

Mark

well ill order the gaskets and swap them out and see what happens,

i have read a little about eliminating this unit from the engine,if so what type of valves are you guys running and what can i expect from a different unit or type. i have a larger intercooler but its the short type to bolt on a factory motor. should i just replace the gaskets and be happy or should i go another route?

cheers ed

Hi Ed. Sorry that I cannot be of any more help regarding the RPMs... I am running a factor C2 turbo intercooler and have upgraded to the Forge Motorsports diverter valve. I bought the car with an Andial longneck intercooler so I cannot offer any comparison between the characteristics between long and shortneck.

You can eliminate the stock recirculation valve assembly completely by converting to a 965 setup with a long-neck intercooler.

The 965 setup uses a tiny plastic Bosch recirculation valve instead of that magnesium housing - although at least some people hold the Bosch part in their hand for less than a minute before they head for the nearest web browser and buy a billet replacement instead (rebuildable, tunable).

(The plastic Bosch valves don't have the best reputation for reliability - but are also factory fitment on SEATs, Audi's and probably some VW's - so the aftermarket is knee deep in better made alternatives.)

It is the stock recirculation valve internal piston slamming against it's stop. Put new gaskets in and be done with it.
Every time you get off the throttle, engine vacuum will pull and hold the piston open until you get back on the throttle and vacuum goes away. Thus, quick throttle punches in neutral will produce this sound.
Fix it, or don't worry about it.

I experienced something similar... I'm going with the BOV valve... nothing worse than a cool car and that stupid "knock"

That's the first thing that came to my mind as well. Common issue. The sound should be a metalic click, and as others have said will be most pronounced during a snap throttle at low rpm.

If you wish to keep your present intercooler you can add a 965 BOV by making an adapter out of your 930 BOV. There are a few threads on that.

http://forums.pelicanparts.com/uploa...1284865750.jpg

brian

i just purchased a larger intercooler so the adapter seems like the way to go. by the pic you posted looks like the stock one has been cut and welded up. ill do a search and see what i can find. im sure know after more diagnostics that it is the stock bov makeing the single clank sound.

thanks guys.

cheers ed

Ed, if you want to do something radically different and still keep a short neck IC yet do away with the large boost recirculation manifold assembly in favor of a BOV-style valve, see my post here. http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/518667-bov-update-short-neck-ic.html

Please don't flame me for the less-than-professional hack and glue job I did on the magnesium parts. It's temporary until I can find someone who welds magnesium. So far, it's holding up just fine though.

Ed,

PM "The Gimp" alias Mark Houghton. He happens to be the man in the know when it comes to this procedure.

Cole

hey guys thanks for all the help. ill p.m. mark and see what i can come up with. should these parts be welded? seam s like heat cycles would eventualy break down the epoxy!


cheers ed

Absolutely they should be welded. This was a prototype and I got too anxious to try it out. We'll see how long the epoxy lasts I guess. Got one season on it so far, so good.
Hey, maybe I can claim some extraordinary glue performance linked to Turbo Porsches and get JB Weld to send me a lifetime supply of their stuff:)