90 L-98 Knocks for 20 seconds after start?
Thread Starter
Instructor
Joined: Mar 2002
Posts: 110
Likes: 0
From: youngstown Ohio
90 L-98 Knocks for 20 seconds after start?
First of all I want to say hello to all. Although I am a new guy on the forum I have been lurking in the background for a while getting A LOT of great tips from the masters!
Here goes: My 1990 L-98 has a loud knocking at start-up. It ONLY happens when cold outside, (not to be confused with just "cold" engine) Hard to tell where the knock is coming from, specifically where in the engine compartment but it is loud enough to hear inside.
Strange thing is, after approximately 20 seconds, no knock at all and it purrs like a kitten.
just in case anyone needs this info to diagnose (spelling) I cant identify any engine mods it may have had in the past, IE heads, cam valves etc. I bought it used 8 months ago and it's been too cold to really get into it. :chevy
Here goes: My 1990 L-98 has a loud knocking at start-up. It ONLY happens when cold outside, (not to be confused with just "cold" engine) Hard to tell where the knock is coming from, specifically where in the engine compartment but it is loud enough to hear inside.
Strange thing is, after approximately 20 seconds, no knock at all and it purrs like a kitten.
just in case anyone needs this info to diagnose (spelling) I cant identify any engine mods it may have had in the past, IE heads, cam valves etc. I bought it used 8 months ago and it's been too cold to really get into it. :chevy
Race Director
Joined: Aug 2000
Posts: 14,130
Likes: 7
Cruise-In II Veteran
Re: (scorp508)
Thinking about piston slap,and you havent had the car long,any idea if the motor was ever rebuilt?Sometimes a rebuilt gets forged pistons and can have that slap on cold start up for a little bit then quiets down.
Anmyways hope it isnt anything to worry abut.May help to describe the sound more and or possibly pinpoint exactly where its coming from when its warmer out to check it.
Btw,welcome to the forum.
:)
Anmyways hope it isnt anything to worry abut.May help to describe the sound more and or possibly pinpoint exactly where its coming from when its warmer out to check it.
Btw,welcome to the forum.
:)
Team Owner
Joined: Nov 1999
Posts: 26,679
Likes: 17
From: This vette goes to 11 Milford MA
St. Jude Donor '03-'05
Re: (Bill's86Coupe)
Piston slap from a forged psiton isn't that loud usually and won't go away so fast. Might be a rod rap, which means the rods are hammering the bearings. When you drain the oil, is there any trace of silver in it? Motors with bad bearings usually show signs of wasted bearing, looking almost like silver paint was mixed in the oil.
Burning Brakes
Joined: Aug 2000
Posts: 984
Likes: 1
From: Cincinnati OH
Re: 90 L-98 Knocks for 20 seconds after start? (thnbluline50)
Is it really knocking or is it more of a ticking/tapping noise? May be as simple as running the wrong weight oil, causing the lifters to run dry for half a minute upon cold startup when the oil is too thick to flow. Also oil filters without anti drainback valves could cause the same problem. Just some ideas, hope it's as simple as an oil and filter change.
Burning Brakes
Joined: May 1999
Posts: 760
Likes: 1
From: Greenwich, CT
Re: 90 L-98 Knocks for 20 seconds after start? (thnbluline50)
Take a look at the serpentine belt tensioner. If it's too close to it's upward limit stop, a cold belt will make it clank on start up - usually because an aftermarket belt was installed that's a little too long.
Burning Brakes
Joined: Nov 2000
Posts: 1,040
Likes: 2
From: BowlingGreen KY
Re: 90 L-98 Knocks for 20 seconds after start? (DOCTOR J)
Check your harmonic balancer. The outer metal balancer has a tendancy to slide back towards the timing chain cover and hit the cover. If its not that, I'd say you have a bad bearing and the knock is a rod. My buddy had the same problem and a week later the knock was constant. Time for a rebuild.
Thread Starter
Instructor
Joined: Mar 2002
Posts: 110
Likes: 0
From: youngstown Ohio
Re: (scorp508)
About 40 degrees. Also I dont know hhat oil is in it. The guy I bought it from had just changed the oil right before I bought it. I am thinking of going to that Royal Purple stuff. Any thoughts?
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Thread Starter
Instructor
Joined: Mar 2002
Posts: 110
Likes: 0
From: youngstown Ohio
Re: (Bill's86Coupe)
Have no ideas if the engine has been worked on. Got it off a guy who caught his wife cheating and he wanted to dump it before he filed for divorce, and she took it. He bought from a private owner in California one year prior.
I took it out today and again, it knocked for 20 seconds and then nothing. It does run excellant and there is difenately no loss of power on the road.
I took it out today and again, it knocked for 20 seconds and then nothing. It does run excellant and there is difenately no loss of power on the road.
Le Mans Master
Joined: Jul 2001
Posts: 8,183
Likes: 3
From: Santa Maria, CA
Re: (thnbluline50)
first off, Welcome! Glad ya decided to join. Wave at other Vettes on the road :seeya :chevy
Can ya get like a mechanic's stethoscope or put a piece of pipe to your ear and probe around? I once had an EGR valve that would rattle(but that was only with lots of throttle since it doesn't operate at idle). Pre-cats not rattling? I wonder if the knock sensor is picking it up and retarding. How does motor run if you un-plug the EST connector?
Can ya get like a mechanic's stethoscope or put a piece of pipe to your ear and probe around? I once had an EGR valve that would rattle(but that was only with lots of throttle since it doesn't operate at idle). Pre-cats not rattling? I wonder if the knock sensor is picking it up and retarding. How does motor run if you un-plug the EST connector?
Thread Starter
Instructor
Joined: Mar 2002
Posts: 110
Likes: 0
From: youngstown Ohio
Re: (89 Paul in cal)
I decided what I am going to do! Since the Semi comes in a few days to take it away (I am moving back to Ohio from Colorado) I am going to call my good friend back home in ohio to look at it. He builds engines for NHRA racers and has had built/rebuilt many ZR1s. I DO thank ALL of you who replied and I am really enjoying this site. :crazy:
