e30tech.com Forums


Go Back   e30tech.com Forums > Technical > Engine > M20
Register FAQ Members List Sponsors Advertise Shop Search Today's Posts Mark Forums Read







Reply
 
Thread Tools Display Modes
Old 11-11-2012, 09:50 AM   #1
ahlem
Senior Member
 
ahlem's Avatar
 
E30: 1987 325is 1989 325isa 1990 m3 1990 535im 1976 2002 1991 318is
Location: Grand Rapids, MI
Join Date: Apr 2008
Posts: 560
ahlem has a little shameless behaviour in the past
Default piston slap vs. rod knock

The car is a 1989 325is. It broke it's timing belt 22,000 miles ago at 120k. It got a used freshened head and had valve to piston contact in hole#2. The car runs well otherwise and has good compression but a slight knock/slap at idle that minimizes some when it warms up or at slightly higher RPM. Nothing was replaced in the lower end when the head was done. Thoughts, comments or what to do next, if anything? Anything else make that sound? Oil pump?
ahlem is offline   Reply With Quote
Guest Message
Tire Rack - Pure Control
Old 11-11-2012, 04:07 PM   #2
DEG
Senior Member
 
DEG's Avatar
 
E30: 325is
Location: Sequim Wa
Join Date: Mar 2010
Posts: 527
DEG is on a distinguished road
Default

sounds like piston slap if it happens when cold and disappears once warm.

Get a stethoscope [aka screwdriver] and try to listen where its coming from
__________________
DEG is offline   Reply With Quote
Old 11-12-2012, 08:43 AM   #3
vvurdsmyth
Member
 
vvurdsmyth's Avatar
 
E30: '86 325e
Location: NYC
Join Date: May 2009
Posts: 56
vvurdsmyth can only hope to improve
Default

to check for a possible rod bearing gone bad, disconnect the spark plug wire when running (wear rubber gloves to prevent shock); if the knock is worse, chances are it is the bearing; a knock can also happen if the mixture is running too rich; I sometimes get a bit of a knock when the engine is cold.. until the cold start valve shuts off; if it were a rod bearing or piston slap it would happen all the time when cold.
vvurdsmyth is offline   Reply With Quote
Old 11-14-2012, 12:04 PM   #4
ahlem
Senior Member
 
ahlem's Avatar
 
E30: 1987 325is 1989 325isa 1990 m3 1990 535im 1976 2002 1991 318is
Location: Grand Rapids, MI
Join Date: Apr 2008
Posts: 560
ahlem has a little shameless behaviour in the past
Default

So what would the solution be? A rebuild?
ahlem is offline   Reply With Quote
Old 11-14-2012, 01:03 PM   #5
Me+SPEED=
Senior Member
 
Me+SPEED='s Avatar
 
E30: 1989 325i turbo, 1988 325i
Location: York, PA
Join Date: Feb 2012
Posts: 449
Me+SPEED= will become famous soon enough
Default

I slightly over adjusted my valves and it sounded like a bearing knock.
Me+SPEED= is offline   Reply With Quote
Old 11-15-2012, 05:58 AM   #6
ahlem
Senior Member
 
ahlem's Avatar
 
E30: 1987 325is 1989 325isa 1990 m3 1990 535im 1976 2002 1991 318is
Location: Grand Rapids, MI
Join Date: Apr 2008
Posts: 560
ahlem has a little shameless behaviour in the past
Default

that's what I'm hoping
ahlem is offline   Reply With Quote
Old 11-20-2012, 10:14 PM   #7
Quick99Si
Member
 
E30: Wheels
Location: Undisclosed
Join Date: Jul 2011
Posts: 29
Quick99Si will become famous soon enough
Default

Quote:
Originally Posted by ahlem View Post
that's what I'm hoping
Get under the car: if the sound gets louder, it's in the block, and you're looking at a full rebuild. If it sounds like it's coming from high up, it's likely localized to the head and should be quite a bit cheaper to repair.
Quick99Si is offline   Reply With Quote
Old 02-28-2013, 01:18 PM   #8
ahlem
Senior Member
 
ahlem's Avatar
 
E30: 1987 325is 1989 325isa 1990 m3 1990 535im 1976 2002 1991 318is
Location: Grand Rapids, MI
Join Date: Apr 2008
Posts: 560
ahlem has a little shameless behaviour in the past
Default

Well the solution to the problem was a rocker going bad. At Christmas it broke and that defined it very clearly. Fortunately the car wasn't far from home when it happened. When I replaced the broken rocker, I checked and found that 3 more had worn pads with slightly concave surfaces. I swapped those and the car sounds like a sewing machine again. I'm planning a valve adjustment double check this weekend hopefully now that it has been running well for a month. No rod knock or piston slap.
ahlem is offline   Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -4. The time now is 12:02 PM.






Powered by vBulletin® Version 3.6.8
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.