Knocking rough idle after engine is warm knocking rattle like if you strike two ratchets

[hsz33]

1990 6 speed base corvette knocking sound at idle after warms up. the idle is not really rough just sounds like two metals hitting. its never consistant it comes and goes. Bad gas mileage, crankcase breather valve (passenger side valve cover) is missing so I just shoved the hose in it directly from the throttle body until I get one. ( it looks like the pcv valve on the driver side but its hallow!) which i replaced along with spark plugs. Also when gunning in 2nd 3rd I hear a wining sound from the passenger side under hood. At shut off I hear a rattle from under legs on driver side below the gas pedal. Its not the clutch or tranny for the noise does NOT go away when I press the clutch and it does not slip. could the crankcase breather valve missing cause a vac leak for the bad gas mileage and possible rough idle? Thank you, I hate to start replacing parts if not needed. No engine lights on.ad

 

[Rob]

Moving out of the Site Help and Feedback forum and into the C4 Tech Forum...

 

[hsz33]

Moving out of the Site Help and Feedback forum and into the C4 Tech Forum...

Thank You Sir! I was not sure at first how to post properly.

 

[Rob]

Thank You Sir! I was not sure at first how to post properly.

Not a problem.

 

[tmkassin]

You need the vacuum line from the throttle body to the passenger valve cover to be installed correctly or the pcv wont perform correctly. If the pcv is not there then get one and install it. This can cause poor idle issues. The knocking might be the innards of your catalytic converter. If they become loose they will beat around inside the chamber until they turn to dust.

 

[hsz33]

You need the vacuum line from the throttle body to the passenger valve cover to be installed correctly or the pcv wont perform correctly. If the pcv is not there then get one and install it. This can cause poor idle issues. The knocking might be the innards of your catalytic converter. If they become loose they will beat around inside the chamber until they turn to dust.

ok great thanks I will install the vacum line properly, though I called the dealer to get the valve itself that connects to the hose and into the valve cover and they said it was discontinued. Any ideas where I can get one. Autozone and pepboys only have a cap that just plugs it closed.

 

[John Robinson]

You might want to contact this company. I have used them and they have always been very helpful . What I have purchased has been sent immediatly and the quality was always good for what I needed.

Taber Corvette Parts

Also you can search for corvette salvage yards closer to you.

 

[mr.beachcomber]

ok great thanks I will install the vacum line properly, though I called the dealer to get the valve itself that connects to the hose and into the valve cover and they said it was discontinued. Any ideas where I can get one. Autozone and pepboys only have a cap that just plugs it closed.

If you've installed a new, operable PCV, take the old hollow PCV from the driver's side valve cover and put into the passenger's side valve cover and connect the fresh air hose from the throttle body to it. (The stock piece was essentially just a plastic PCV body w/o the valve & spring assembly installed.)

PS - You might want to check the fresh air hose when doing this to make sure there aren't any cracks or leaks in the hose.

 

[vigman]

Check out the following

1) Cats could be breaking apart.. jack up car.. get a rubber mallet, tap ( starting with light force ) on the pre cats.. then on the mains... if you hear a maraca.. problem solved.....

2) Have you had clutch work done recently...I have heard the dual pass flywheels can make this noise... not sure if your year has that type

 

[Hib Halverson]

1990 6 speed base corvette knocking sound at idle after warms up. the idle is not really rough just sounds like two metals hitting. its never consistant it comes and goes. Bad gas mileage, crankcase breather valve (passenger side valve cover) is missing so I just shoved the hose in it directly from the throttle body until I get one. ( it looks like the pcv valve on the driver side but its hallow!) which i replaced along with spark plugs. Also when gunning in 2nd 3rd I hear a wining sound from the passenger side under hood. At shut off I hear a rattle from under legs on driver side below the gas pedal. Its not the clutch or tranny for the noise does NOT go away when I press the clutch and it does not slip. could the crankcase breather valve missing cause a vac leak for the bad gas mileage and possible rough idle? Thank you, I hate to start replacing parts if not needed. No engine lights on.ad

The advice given earlier on the PCV system is well-taken. Make sure it's operating properly and has the right parts or you'll never pass the Smog Check when the car is due.

As for bad mileage, you didn't post enough information to suggest anything.

Diagnosing noises via the Internet is always tough but that the car is a manual, your "knocking" sound always occurs at idle, doesn't occur until the engine (and trans) warm up, sounds like metal hitting, comes and goes, and you hear a rattle at shut off, has me thinking your dual mass flywheel is faulty.

 

[hsz33]

1) Cats could be breaking apart.. jack up car.. get a rubber mallet, tap ( starting with light force ) on the pre cats.. then on the mains... if you hear a maraca.. problem solved.....

2) Have you had clutch work done recently...I have heard the dual pass flywheels can make this noise... not sure if your year has that type

1) No noise from cats
2) recently bought the car no clutch work by me

 

[hsz33]

If you've installed a new, operable PCV, take the old hollow PCV from the driver's side valve cover and put into the passenger's side valve cover and connect the fresh air hose from the throttle body to it. (The stock piece was essentially just a plastic PCV body w/o the valve & spring assembly installed.)

PS - You might want to check the fresh air hose when doing this to make sure there aren't any cracks or leaks in the hose.

Great thanks will do, I keep posted

 

[tmkassin]

It's been a few years but it seems like Corvette Central has the parts for the pcv system. Don't rule out the cats. It took me a long time to figure out the rattle from mine. It sounded a lot like you describe and was embarrassing sitting at a traffic light. It would go away as soon as I took off.