'81 intermittent engine knock at idle
Thread Starter
Burning Brakes
Joined: Feb 1999
Posts: 1,165
Likes: 1
From: San Jose CA
'81 intermittent engine knock at idle
I need some help.
My automatic '81 developped an intermittent knocking sound during idle and only when warmed up.. It seems to come from the rear of the engine block.
The car idles fine at 650rpm, oil pressure is about 50 psi.
The knocking sound has an irregular pattern , sometimes disappears alltogether for approx. 5 secs and it always disappears as soon I start to drive.
I noticed that I have a small engine oil leak at the rear of the block. (oil drip through the torque converter cover plate)
I'm not that worried about the leak but I'm concerned about the knock.
What can it be?
I purchased the car almost 3 years ago with 59000 miles and has now 77K on odometer but I have no idea about the real mileage.
The engine runs great and does not burn any oil. Also all smog equipment is installed and functional.
The transmission is a new 4 speed TH400 with approx 5000 miles
Any advice is appreciated.
Roger
:cheers:
My automatic '81 developped an intermittent knocking sound during idle and only when warmed up.. It seems to come from the rear of the engine block.
The car idles fine at 650rpm, oil pressure is about 50 psi.
The knocking sound has an irregular pattern , sometimes disappears alltogether for approx. 5 secs and it always disappears as soon I start to drive.
I noticed that I have a small engine oil leak at the rear of the block. (oil drip through the torque converter cover plate)
I'm not that worried about the leak but I'm concerned about the knock.
What can it be?
I purchased the car almost 3 years ago with 59000 miles and has now 77K on odometer but I have no idea about the real mileage.
The engine runs great and does not burn any oil. Also all smog equipment is installed and functional.
The transmission is a new 4 speed TH400 with approx 5000 miles
Any advice is appreciated.
Roger
:cheers:
Le Mans Master
Joined: Sep 2000
Posts: 5,635
Likes: 4
From: Orlando Floriduh
Re: '81 intermittent engine knock at idle (rhoeven)
What you are hearing sounds very much like a worn rear main bearing. I know that's not what you wanted to hear but... the noise and the leak sure make a good case for it. Try running a higher weight of oil like 20W50. If thicker oil helps or corrects it then that is most likely the culprit.
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
Re: '81 intermittent engine knock at idle (rhoeven)
Before you start tearing the engine apart make sure it's the engine that's causing the problem. It could also be something loose in the exhaust system. The pipes go through the crossmember not too far behind the rear of the engine so a loose pipe or a pipe that's hitting the floorboard or something else can sound very much like an engine knock in the rear of the engine. Might be worth taking a look. Also, about the leak. I thought I had a bad rear main seal because of oil that dripped from the flywheel cover on my 4 speed. It turned out to be a leak from the valve cover gasket that worked it's way down the back of the engine. Take a look here too before major surgery.
[Modified by vettfixr, 9:39 AM 8/1/2003]
[Modified by vettfixr, 9:39 AM 8/1/2003]
Burning Brakes
Joined: Sep 1999
Posts: 931
Likes: 0
From: MI
Re: '81 intermittent engine knock at idle (rhoeven)
Sounds like your heat riser valve! Disconnect and plug the vacuum hose feeds to the heat riser valve on the pass. side exhaust manifold to see if the noise goes away.
Thread Starter
Burning Brakes
Joined: Feb 1999
Posts: 1,165
Likes: 1
From: San Jose CA
Re: '81 intermittent engine knock at idle (Dalannex)
Is it a knock, or more of a tick????? Could be a lifter, or an exhaust leak if it's kind of a tick sound.
-Justin
-Justin
If it were a crankshaft bearing, I would think that the knock is very regular and dependent of the engine rpm. The noise I have is very irregular and only during idling on hot engine.
Oh, I must correct my initial post regarding the transmission. It is a 700R4 instead of TH400 but I think this is not related to my problem.
Bluevetteman: I already run 10W40 synthetic.
Rear main bearing makes a lot of sense, specially with the leak but I still don't get it why the noise is only there during hot idle.
Do I have to pull and tear apart the engine in order to fix this?
Thanks keep em coming.
Roger
[Modified by rhoeven, 12:51 PM 8/1/2003]
Le Mans Master
Joined: Aug 2001
Posts: 7,804
Likes: 4
From: Standing still with all my might Texas
Cruise-In X Veteran
St. Jude Donor '09-'11
Re: '81 intermittent engine knock at idle (rhoeven)
Oh, I must correct my initial post regarding the transmission. It is a 700R4 instead of TH400 but I think this is not related to my problem.
Thanks
Roger
Thanks
Roger
This did not turn out to be my problem, but maybe that is what is going on with yours.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Burning Brakes
Joined: Feb 1999
Posts: 1,165
Likes: 1
From: San Jose CA
Re: '81 intermittent engine knock at idle (Kevin_73)
Oh, I must correct my initial post regarding the transmission. It is a 700R4 instead of TH400 but I think this is not related to my problem.
Thanks
Roger
Maybe not. While I was investigating a knock in my engine several people suggested that I check the flywheel and torque converter bolts, apparently if they are loose they can cause strange knocking sounds like you are describing.
This did not turn out to be my problem, but maybe that is what is going on with yours.
Thanks
Roger
Maybe not. While I was investigating a knock in my engine several people suggested that I check the flywheel and torque converter bolts, apparently if they are loose they can cause strange knocking sounds like you are describing.
This did not turn out to be my problem, but maybe that is what is going on with yours.
This makes a lot of sense since I had engine and tranny out about 2 years ago :eek:
I'll check it out this weekend.
Thanks for the tip.
Roger
Le Mans Master
Joined: Sep 2000
Posts: 5,635
Likes: 4
From: Orlando Floriduh
Re: '81 intermittent engine knock at idle (rhoeven)
Typically a rear main sounds like a low dull rap and they will do this as soon as the bearing clearance gets more than the oil can fill. They do usually only make noise at warm idle with no load on the engine. (oil is thicker when cold.) Heres's a test to try... When the engine is cold listen very carefully imediately after it starts.. (like the first two or three seconds before the oil starts circulating) if you hear the rapping noise and then it goes away until the engine is warm. .. that is a good indication of the rear main. You can fix it by pulling the pan and rear cap. Not real tough. It just takes a little finnesse. Hopefully the crank is good. BUT.... if it is a bearing and you keep driving it, it can get worse and damage the crank. I do advise checking all of the thing others posted about as a precaution. It's a hard call diagnosing without actually hearing the engine.
Instructor
Joined: Jan 2002
Posts: 150
Likes: 0
From: Springfield PA
Re: '81 intermittent engine knock at idle (Kevin_73)
While I was investigating a knock in my engine several people suggested that I check the flywheel and torque converter bolts, apparently if they are loose they can cause strange knocking sounds like you are describing.
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: '81 intermittent engine knock at idle (rhoeven)
Ill jump on the band wagon to check convertor bolts and for possible cracked flexplate. Ive personally heard this knocking caused by loose and or missing convertor bolts on at least 2 different cars. When I heard it the sound would kinda go away when you increased the rpms, but as you let it slow back down again it would knock knock knock. And then intermitintly knock while idleing.
Le Mans Master
Joined: May 2003
Posts: 5,642
Likes: 1
Re: '81 intermittent engine knock at idle (Jvette73)
You can check easily to see if it is a main bearing, I learned this from some fiat guys, and it works on an inline for so i guess it would work twice as good on a v8 :)
on the inline four you just removed spark plug wires 1 at a time until the noise died down, if it did, it was that main bearing the the cylinders pistion ran too. so i guess on a v8 without too high of compression it would work removing 2 plugs at a time? what do yall think?
on the inline four you just removed spark plug wires 1 at a time until the noise died down, if it did, it was that main bearing the the cylinders pistion ran too. so i guess on a v8 without too high of compression it would work removing 2 plugs at a time? what do yall think?
