Another Knock Question
Thread Starter
Burning Brakes
Joined: Mar 2010
Posts: 803
Likes: 4
From: Christchurch, New Zealand
Another Knock Question
My 74 L48 convertible has been back on the road now for nearly 2 months. But over the past 2 weeks, I haven't been driving it because it's developed a knock down low toward the front of the engine. The car has the original engine, but I was led to believe the engine had been recon'd. It's been fitted with an HEI distributor, and a carb from a 78 Vette. It's a TH 400. The knock comes on as the engine warms up, and is loudest when the throttle is let go, ie revs up, and then release. I've pulled the fuel pump out, checked the converter bolts, replaced the harmonic balancer, and am in the process of dropping the pan to check the bearings. A few guys (motorcycle mech, automotive mech, and engineeer) agree that it doesn't sound like a rod knock, and going by what I've found on Youtube, I would agree somewhat. Pulled the leads off the plugs 1 at a time, and no change. I found a vid that sounded similar to the noise mine makes, and that was a stuck lifter. Does anyone have any ideas please?
Grant
Grant
Last edited by mrmagrath; Dec 19, 2011 at 10:50 PM. Reason: Details omitted.
Drifting
Joined: Jun 2010
Posts: 1,781
Likes: 15
From: Oxford Ohio
Put a timing light on it and move the lead from plug wire to plug wire to see when the knock is in time with the light flash. That will narrow down the search a little.
Any change in the oil pressure?
I have a ford 390 that has the same symptoms. Getting ready to pull the bottom end for the guy. I'm suspecting a wrist pin or loose piston bore issue.
You're going to have to pull rod caps to find out if you have a bearing problem anyway.
Try removing all the belts and see if it still makes the noise. Eliminate the other front end sources.
Post what you find, I'd be interested in knowing what the problem is.
Any change in the oil pressure?
I have a ford 390 that has the same symptoms. Getting ready to pull the bottom end for the guy. I'm suspecting a wrist pin or loose piston bore issue.
You're going to have to pull rod caps to find out if you have a bearing problem anyway.
Try removing all the belts and see if it still makes the noise. Eliminate the other front end sources.
Post what you find, I'd be interested in knowing what the problem is.
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
A common cause of "rod knock" is an exhaust leak - especially with headers. Take a minute to check the bolts and if you find any loose take a look at the gaskets.
The fact that it's louder as it warms up and that it's loudest (if I understand your post correctly) under compression braking leads me to think the exhaust check is a good use of your time
The fact that it's louder as it warms up and that it's loudest (if I understand your post correctly) under compression braking leads me to think the exhaust check is a good use of your time
Thread Starter
Burning Brakes
Joined: Mar 2010
Posts: 803
Likes: 4
From: Christchurch, New Zealand
Thanks for the input! The car has the original exhaust manifolds still, and the oil pressure gauge is crook, the problem being the sender. It's another thing on the list to fix!
Billa, you're correct, it is loudest when warm. I'll keep you posted on what I find, as when I've searched for people with the same issue, the thread always dies before a solution is found!
Cheers and Merry Christmas!
Billa, you're correct, it is loudest when warm. I'll keep you posted on what I find, as when I've searched for people with the same issue, the thread always dies before a solution is found!
Cheers and Merry Christmas!
Thread Starter
Burning Brakes
Joined: Mar 2010
Posts: 803
Likes: 4
From: Christchurch, New Zealand
No, I haven't Mel. The pump was brand new when fitted up a few months ago. The knock was still there when the pump was unbolted and pulled back.
I should point out that there's a bit of fuming from the oil filler and PCV holes when their open as well. I put that down to worn valve guides, but it may be important to a diagnoses. (I watch too much House!) I'm just off to pull some bearing caps before work.
Cheers!!
I should point out that there's a bit of fuming from the oil filler and PCV holes when their open as well. I put that down to worn valve guides, but it may be important to a diagnoses. (I watch too much House!) I'm just off to pull some bearing caps before work.
Cheers!!
Thread Starter
Burning Brakes
Joined: Mar 2010
Posts: 803
Likes: 4
From: Christchurch, New Zealand
So who had their money on spun bearings? Pulled No 1 bearing cap and it was in a bad way. Plenty of movement on some of the others as well. Thanks for your help! Looks like I've got some work to do over the Christmas break. I've got 3 weeks off, so at least a week of that will be working on the Vette. Santa, can I pease have new bearings and a crankshaft grind...........?
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rats...sorry to hear that.
Drifting
Joined: Nov 2003
Posts: 1,874
Likes: 7
From: l In
So who had their money on spun bearings? Pulled No 1 bearing cap and it was in a bad way. Plenty of movement on some of the others as well. Thanks for your help! Looks like I've got some work to do over the Christmas break. I've got 3 weeks off, so at least a week of that will be working on the Vette. Santa, can I pease have new bearings and a crankshaft grind...........?
Thread Starter
Burning Brakes
Joined: Mar 2010
Posts: 803
Likes: 4
From: Christchurch, New Zealand
Billla, true to form, the bloody camera batteries were flat! The bearing is a nice copper color with lovely deep grooves! And a groovy journal to match. I'll throw some batteries in tonight so everyone can gasp in horror!
Phil, don't tempt me!!! I admit, the thought has crossed my mind! The whole L48 matching number thing is the stumbling block, so to speak.
Indy, yeah, I was looking for an easy fix. Once I got that sump off and wiggled the rod, I knew the end was near.
On the availability of parts, no worries. American cars are like butts here in NZ. Everyone's got one. I'm in Christchurch, a city of around 400,000 people (and since Sept last year, 8000 earthquakes) and at our annual USA Day last month we had around 500 cars. Here's a pic. BTW, never park your shiny newly restored C3 roadster next to a 2010 ZR-1 if you want people to notice it.
Here's a fun fact. Before 1946, there were more American cars in NZ than from any other country. And pre-war, the 2nd highest cars per capita next to you guys.
Merry Christmas!
Grant
Phil, don't tempt me!!! I admit, the thought has crossed my mind! The whole L48 matching number thing is the stumbling block, so to speak.
Indy, yeah, I was looking for an easy fix. Once I got that sump off and wiggled the rod, I knew the end was near.
On the availability of parts, no worries. American cars are like butts here in NZ. Everyone's got one. I'm in Christchurch, a city of around 400,000 people (and since Sept last year, 8000 earthquakes) and at our annual USA Day last month we had around 500 cars. Here's a pic. BTW, never park your shiny newly restored C3 roadster next to a 2010 ZR-1 if you want people to notice it.
Here's a fun fact. Before 1946, there were more American cars in NZ than from any other country. And pre-war, the 2nd highest cars per capita next to you guys.
Merry Christmas!
Grant