1994 C4 - Exhaust Leak Sound Only While Cold

[spinnerhowland]

I have recently purchased a 1994 Corvette (My first after wanting one for years) - Over the last 2 weeks it has started to make a sound very similar to an exhaust manifold leak until it gets warm. The sound is only noticable when I accelerate and once the engine gets to operating temp the sound stops. As a reference sounds as if exhaust manifold gasket has a leak of the manifold has a crack.
Is the EGR valve (or piping) able to create this sound until it moved to the closed position? The sound appears to be coming from the Left Side of the engine but it is hard to get an accurate diagnosis without a car lift (I do not have access to one) - Is this a known problem on this vehicle?

PS. I have plans to take the car with me when I return to Australia as I qualify for the exemption due to being a non resident for more than 5 years. It is going to be a gift for my father who has had a lifelong attraction to the American Sports Car. Any advice on international shippers would also be helpful.

 

[boomdriver]

There is no common problem with exhaust leaks. Nothing that is common to a Corvette anyway. It IS a high-performance car and issues like this will appear from time to time. I seriously doubt that the EGR valve has anything to do with it, that works only when necessary, more often when throttle is applied when hot, not cold.

Doubt its a manifold crack. It would do it all the time. Or a gasket...that would leak hot or cold.

What is very likely is one of the emissions tubes check valves is sticking. The system sends fresh air to the cats or dumps the fresh air depending on the engine temp. If you had a FSM you could trace the route of fresh air in the cold cycle of the air mngt system and isolate the possible points of trouble.
I have one doing this exact thing right now and have not been able to pin down which check valve is leaking back. Makes that spitting sound under accelleration,...sometimes. Next start up it may be smooth as silk. Next start may spit like there was a hole in the exhaust. The air mngt system diverter valve directs where and when fresh air moves thru the system and how and when exhaust can possibly enter the system.

 

[spinnerhowland]

There is no common problem with exhaust leaks. Nothing that is common to a Corvette anyway. It IS a high-performance car and issues like this will appear from time to time. I seriously doubt that the EGR valve has anything to do with it, that works only when necessary, more often when throttle is applied when hot, not cold.

Doubt its a manifold crack. It would do it all the time. Or a gasket...that would leak hot or cold.

What is very likely is one of the emissions tubes check valves is sticking. The system sends fresh air to the cats or dumps the fresh air depending on the engine temp. If you had a FSM you could trace the route of fresh air in the cold cycle of the air mngt system and isolate the possible points of trouble.
I have one doing this exact thing right now and have not been able to pin down which check valve is leaking back. Makes that spitting sound under accelleration,...sometimes. Next start up it may be smooth as silk. Next start may spit like there was a hole in the exhaust. The air mngt system diverter valve directs where and when fresh air moves thru the system and how and when exhaust can possibly enter the system.

Thanks mate. I assume "FSM" is F? Shop Manual? - I am a diesel mechanic so pretty clued in on most stuff but my background is on BIG Earthmoving Equipment like Caterpillar D11N Dozers and 792 Dump Trucks, so the amount of room to work on this thing is a challenge. I have the original shop manual and will take a look at the system and see what I can find. It is very frustrating not having a car lift to get it on. Cheers for the advice mate

 

[boomdriver]

Yessir...shop manual.

I hear ya....I spent my life on large equipment working with hydraulics and getting comfortable under the hood of a Corvette just isn't that easy...
Most of what you need to do is top side anyway. The lift won;t help much.

I'd locate the air diverter valve system and air pump and trace the pipes and hoses thru the check valves. It may be as simple as a dirty elect connection to the air diverter valve or pump. The clue is the noise going away when it gets hot...thats also when the ECM tells the air system to direct air to the cats, not until then. Lots of things change when a specific temp is reached.
Good Luck

 

[toptechx6]

I will respectfully disagree with boomdriver based on personal experience and suggest at least checking the exhaust manifold bolts for tightness. I have seen loose bolts there cause such a leak when cold, as the parts expand during engine warm up it can diminish or stop the sound you hear.
The gasket may also be damaged if the bolts have been loose for a long time, but I would at least attempt to snug them up first, hope it is something that simple, good luck.
I have a mate in Victoria who is shipping a Harley back that he purchased when visiting, I will ask him for the name of the shipper and post it here assuming he is satisfied with their service.

 

[TedC]

I worked on an engine that had an exhaust leak you could hear at times when cold, but impossible to trace down using our ears. We decided to use some seafoam that produces lots of white smoke and discoverd a hair line crack in the exhaust manifold under one of the pipes.

 

[boomdriver]

I worked on an engine that had an exhaust leak you could hear at times when cold, but impossible to trace down using our ears. We decided to use some seafoam that produces lots of white smoke and discoverd a hair line crack in the exhaust manifold under one of the pipes.

how does the SeaFoam work? Thats interesting....so it makes enough smoke to see exhaust leaks?

In regard to TopTechs advice, it should be taken. It IS possible that there is a crack or gasket leak...I'm just saying that odds are against that IMO. However, all advice here should be taken and utilized to remedy problems and especially that advice of the professionals. I just throw my personal experiences out there (25 yrs of ownership) and sometimes my training with certain engineering challenges comes in handy but I'd certainly not ignore the advice of the experts that do this everyday.

I live to work on my Corvette....TT and other experts here work on Corvettes to live, making thier advice and opinion carry more weight than that of just an enthusiast.

 

[toptechx6]

how does the SeaFoam work? Thats interesting....so it makes enough smoke to see exhaust leaks?

In regard to TopTechs advice, it should be taken. It IS possible that there is a crack or gasket leak...I'm just saying that odds are against that IMO. However, all advice here should be taken and utilized to remedy problems and especially that advice of the professionals. I just throw my personal experiences out there (25 yrs of ownership) and sometimes my training with certain engineering challenges comes in handy but I'd certainly not ignore the advice of the experts that do this everyday.

I live to work on my Corvette....TT and other experts here work on Corvettes to live, making thier advice and opinion carry more weight than that of just an enthusiast.

My post specified respectful disagreement boomdriver because I have noticed your advice is nearly always on target. As a professional I take pride in keeping an open mind simply because I have learned plenty from enthusiasts like yourself.
This old saying seems particularly relevant because of the 100th anniversary just past.
" The Titanic was built by professionals, Noah's ark was built in his backyard, which one survived the maiden voyage?":L

 

[TedC]

how does the SeaFoam work? Thats interesting....so it makes enough smoke to see exhaust leaks?

Seafoam can be used to clean upper intakes and heads of carbon. It produces lots and lots of white smoke. We could see the puffs of smoke coming from the mystery leak in the exhaust manifold. I don't think we would of found this hair line leak any other way. Make sure you do this in an open space with the exhaust pipes down wind.

 

[toptechx6]

Here is a link to the shipper used by my Aussie pal in Victoria

International Car Shipping | Auto Transport Company | Schumacher Cargo

 

[John Robinson]

I will throw this out. On my 93 the air pump which is located on the left side of the engine bay went bad. I have not done anything with it since I found out it only is activated when the the engine first starts. When it gets to running were the ecm can lean it back from what we would call a little choking it goes away. Most of the time the air pump runs less than a minute. You might check to see if the fuse on the air pump is blown. All the other suggestions are good also and mine is only based on my experience. As you are a mechanic you know better then I do about how intricate the new cars and trucks are. Welcome to the CAC

 

[2VettesMike]

Can you narrow down to the right or left side?
Is it near the engine or coming from underneath the driver/passenger seat area?

Just put the exhaust system back up on the 93 after replacing the RH exhaust manifold (wasn't leaking, just broken studs that wouldn't come out)

Anyway the replacement manifold has provisions for an exhaust donut gasket between the manifold and the downpipe/converter assy. This is the GM replacement manifold, in 93 and possibly 94 the flange was part of the manifold. In other words the manifold to downpipe connection is metal-metal no gasket.

Same goes for the LH side and where the exhaust pipe connects to the converter output. Not the same on RH side as that's a slip fit about three inches deep.

Also common for the old style exhaust manifold gaskets between aluminum head and cast iron manifold to fail and leak. New GM service replacement is must stronger and dual layered.

Thanks,

Mike

 

[Tuna]

Don't forget to check that "cats" also.

Many years ago, the driver's side cat on my ol' 92 started breaking up inside and started causing noises.

 

[roger longman]

Here is a link to the shipper used by my Aussie pal in Victoria

International Car Shipping | Auto Transport Company | Schumacher Cargo

I used this company, last year, when I had my 1960 Norton, shipped from England to Los Angeles, by plane. No problem, apart from the cost !......Roger.