How can I tell if an exhaust valve if faulty. [Archive] - Nova Tech

: How can I tell if an exhaust valve if faulty.


troche08
Sep 6th, 09, 01:58 AM
Hi, I have been told that the rough, irregular idling could be cause by a burned or faulty exhaust valve. I have pretty much replaced everything from plugs, rotor, cap, deactivated EGR, ign. coil will soon be replaced, now this new theory. The irregular idling improves a bit as the engine warms up on my 75 chev. nova. Thanks for any help.

Philip
Sep 6th, 09, 02:26 AM
A bad valve can be diagnoised with a compression test or if you have the equipment a leak down test.

The rough, irregular idling can also be caused by a worn timing chain and gears. My 400 did the same thing, had a miss of sorts but not related to any one cylinder. I could not find the cause until the engine was disassembled.

The worn timing set will allow the cam to run way behind the crank and all valve events will be late. This causes a loss of power and can affect the idle and engine vacuum.

You can check the slack by removing the distributor cap and watching the rotor as you turn the crank shaft.Turn clockwise until you see rotor movement. Mark the balancer, then turn the crankshaft counter clockwise until the rotor moves. Place another mark on the balancer. If the marks are more than 1/4" apart the timing set has a lot of wear and should be replaced.

Big Dave
Sep 6th, 09, 03:45 AM
First question I would ask is did you used to have a flat tappet cam in your motor now? A cam going bad also has symptoms you exhibit.

Second is do you have access to an air compressor and an old spark plug with an air chuck welded into one end. Fill each cylinder and listen for leaks. Air escaping from the exhaust pipe indicates bad exhaust valve if it is supposed to be closed (bottom of intake stroke). Air leaking back out the carb when piston is at the bottom of the power stroke indicates a bad intake valve As does constant back fires from intake charge being light off by fire escaping the adjacent cylinder). Air leaking out of the PCV indicates bad rings (as does smoke from burnt oil).

Big Dave

troche08
Sep 6th, 09, 09:10 PM
First question I would ask is did you used to have a flat tappet cam in your motor now? A cam going bad also has symptoms you exhibit.

Second is do you have access to an air compressor and an old spark plug with an air chuck welded into one end. Fill each cylinder and listen for leaks. Air escaping from the exhaust pipe indicates bad exhaust valve if it is supposed to be closed (bottom of intake stroke). Air leaking back out the carb when piston is at the bottom of the power stroke indicates a bad intake valve As does constant back fires from intake charge being light off by fire escaping the adjacent cylinder). Air leaking out of the PCV indicates bad rings (as does smoke from burnt oil).

Big Dave
Hi Big Dave; My Nova is as original as they come, not many changes made for the past 33 years. As always very good advise. By the way I conducted a compression test on all cylinders and they seem to be more than good readings. Readings around 185 on all cylinder except for one 180, is that to high for a a 75 chev. Nova. Thanks.

troche08
Sep 6th, 09, 09:17 PM
A bad valve can be diagnoised with a compression test or if you have the equipment a leak down test.

The rough, irregular idling can also be caused by a worn timing chain and gears. My 400 did the same thing, had a miss of sorts but not related to any one cylinder. I could not find the cause until the engine was disassembled.

The worn timing set will allow the cam to run way behind the crank and all valve events will be late. This causes a loss of power and can affect the idle and engine vacuum.

You can check the slack by removing the distributor cap and watching the rotor as you turn the crank shaft.Turn clockwise until you see rotor movement. Mark the balancer, then turn the crankshaft counter clockwise until the rotor moves. Place another mark on the balancer. If the marks are more than 1/4" apart the timing set has a lot of wear and should be replaced.
Hi, you could rest assure that this test will be conducted to check for worn timing chain. No one had thought of this as a possible cause for my troubles. Compression test seems to be fine. Thanks very much.

Philip
Sep 7th, 09, 12:01 AM
Your compression numbers look fine. What shows as a problem is a big variation from one cylinder to another.

BTW what size engine are you working on? In your profile your piston factor is 6. If this is a 250 cubic inch motor it does not have a timing chain. However these engines have a reputation for the head cracking. Depending on where it happens it can cause coolant leaks or vacuum leaks. I am sorry that I assumed everyone has a V8.

troche08
Sep 10th, 09, 11:40 AM
Your compression numbers look fine. What shows as a problem is a big variation from one cylinder to another.

BTW what size engine are you working on? In your profile your piston factor is 6. If this is a 250 cubic inch motor it does not have a timing chain. However these engines have a reputation for the head cracking. Depending on where it happens it can cause coolant leaks or vacuum leaks. I am sorry that I assumed everyone has a V8.
Hi, the more I look into it the more it appears it is a burned exhaust valve, this all started after some overheating due to a clogged radiator. I will have that cylinder head taken to a machine shop for fixing lets hope they do a good job. My 75 Nova indeed has a 259 eng. with a 3 speed manual transmission. Thanks again.

Philip
Sep 10th, 09, 12:13 PM
If it has over heated then chances are the head is cracked. A burned valve will cause a loss of compression on the cylinder with a bad valve.