Knock when engine warms up
Thread Starter
Instructor
Joined: Sep 2007
Posts: 163
Likes: 0
From: Highland Village Texas
Knock when engine warms up
I need some advice please. Test drove '73 with what I believe is crate motor with lots of add ons;ie Edelbrock Intake with Holley ,valve covers , Hooker headers, elestronic ignition many other goodies. The problem is on start up engine is quiet and strong sounding good response from throttle. however when operating temp. is achieved there is a knock on the fuel pump side. The car seems to perform without any other issues. Any ideas ? 
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
The quickest way is to warm the engine,shut it off,remove the 2 bolts holding the pump,pull the pump out a little until the rod falls down,drop the pump down and let it lay there,start the engine back up and see if the knock is gone.(engine will run with the remaining fuel in the carb.)
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
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Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
If someone changed the fuel pump and used the bolt method to hold up the rod they could have "REBENT" the rod.
A local shop had a vette in and was sure the noise was coming from the fuel pump.They took the pump off and it was broke so they replaced it.Engine still had the noise.They pulled it apart and found the rod was bent so they replaced the rod.Engine still had the noise.They pulled it apart and found the rod bent again and replaced the rod and fuel pump thinking the other pump bent the rod.Guess what -engine still had the noise.I was called and I went over and ask I ask the mechanic to explain his procedure. He was running the bolt in the front of the block to hold up the rod-but because the threads were full of crud he wasnt using his fingers so he could feel the bolt just touch the rod he was using a wrench to get the bolt to go in and bending the rod each time.
The original problem was a broken pump,the mechanic created the bent rod problem.
Op.Tech3- Not saying this the problem with your car but it does happen.
Thread Starter
Instructor
Joined: Sep 2007
Posts: 163
Likes: 0
From: Highland Village Texas
Thanks very much for your input, this is my first purchase of a Vette and so far it's not been a good experience. Other than this issue the car is very clean and straight,but to me this knock is a big deal breaker.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Personally, I think you are hearing a rod knocking...the oil is cold and thick when it starts up, so the knock is dampened. When the oil warms, and thins out, the rod bearing has lots of clearance and is knocking away. Maybe this sounds pesimistic, but as a 'buyer' you have to think the worst....otherwise, you end up with a lemon. Tell them that if they are able to FIX the knock [not just mask it or subdue it], you'll seriously consider the purchase. They might just drop the pan and find/fix the problem. But, as long as you are just 'dangling on the hook', they won't do anything else.
Le Mans Master
Joined: May 2006
Posts: 7,708
Likes: 266
From: indy indiana
ull the plug wires,one at a time.If the knock goes away,you have found the cylinder and the rod.Many times,as things get warm and expand,those clearances will open up,causing a knock.If it is a double knock,its the piston pin.Replacing a rod bearing(s) is no big deal.Use plasti gauge in case you need a bearing 001 overized.
Le Mans Master
Joined: May 2006
Posts: 7,708
Likes: 266
From: indy indiana
pull the plug wires,one at a time.If the knock goes away,you have found the cylinder and the rod.Many times,as things get warm and expand,those clearances will open up,causing a knock.If it is a double knock,its the piston pin.Replacing a rod bearing(s) is no big deal.Use plasti gauge in case you need a bearing 001 overized.
Thread Starter
Instructor
Joined: Sep 2007
Posts: 163
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From: Highland Village Texas
Got some more info,the car can sit in the garage and run at idle and never start to knock but take it out for a drive and the knock will start. Any ideas why this happens? Help.
Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
That doesn't change anything. A rod knock can exhibit the same symptom if it's not too bad yet.
If they have any kind of decent shop or have an on-call mechanic have them use an auto stethoscope which will help locate the knocking. A long wooden dowel will do the same thing. If the oil pan is making all the noise or that bottom rail of the block then it's a rod. If it's just the fuel pump it's probably a bent rod. Listen to all the accessories trying to find the source of the knocking.
Thread Starter
Instructor
Joined: Sep 2007
Posts: 163
Likes: 0
From: Highland Village Texas
That doesn't change anything. A rod knock can exhibit the same symptom if it's not too bad yet.
If they have any kind of decent shop or have an on-call mechanic have them use an auto stethoscope which will help locate the knocking. A long wooden dowel will do the same thing. If the oil pan is making all the noise or that bottom rail of the block then it's a rod. If it's just the fuel pump it's probably a bent rod. Listen to all the accessories trying to find the source of the knocking.
If they have any kind of decent shop or have an on-call mechanic have them use an auto stethoscope which will help locate the knocking. A long wooden dowel will do the same thing. If the oil pan is making all the noise or that bottom rail of the block then it's a rod. If it's just the fuel pump it's probably a bent rod. Listen to all the accessories trying to find the source of the knocking.
Gonna meet with them Monday and go through the accessories.Could it be in the timing chain?I even heard a crack in the flyweel
or possibly oil pump.The oil pressure stays at 35 until warm-up then drops to about 25 and flucuates with acceleration back up to 35. Just more thoughts.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
You can talk yourself into believing that there is something 'simple' that is causing the knock... But, if that were the case, the dealer would have already fixed it. He hasn't, so there is something significant that is wrong with the engine. Now, if you intend on pulling the engine/tranny and going through them anyway, you have the option of 'playing dumb' and low-balling the dealer on his/her price. If that's not what you are planning to do, take a walk from this car. [just my 2-cents]