Tracking an exhaust leak
 

So I have developed what sounds like an exhaust leak on my L98 today. Only when cold for about teh first 10-30 seconds it sounds like there is a substantial leak on my exhaust system but after this time period it stops and the engine returns to a normal sound. If warm I get nothing of the sorts. I cant seem to locate any persistent leak so suggestions are welcomed.

Manifold bolts all check out? On my truck one head popped off and caused a tick when cold for about a minute and then got quiet. A crack in the manifold could also. This is assuming you have stockish exhaust. Then I'd run over the joints. Can you pinpoint were it appears to be coming from.

No by the time I get back out of the car after a start up it's gone. I'll give it another go Tuesday when I get some time off.

Pressurize the exhaust with compressed air and use a soapy water solution to track leaks.

You'll have to rig up an adapter at downstream of the leak to force compressed air in.

Could it be you have a valve lifter leaking down from sitting a while and causing the sound till it's pumped up? If it was something like an exhaust manifold or pipe, I'd think it would happen continuously.

Just a thought.

If you add a chemical that will make exhaust smoke. Then tape off exhaust pipes and start engine. Smoke should show were leak is.

And close all the exhaust valves too?

[left]There is enough back pressure generated in the system to easily find leaks. Been doing this for years in development/validation dyno engines. You’d be surprised how many welds and gaskets fail over thousands of hours of testing.
Recently made an adaptor for my 93’s system to find a couple leaks :thumbs:

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so another cold start today and the noise was louder and much more mechanical than before. I can isolate it to the engine bay, same volume on both sides of the engine. It sounds like its coming from the valve train area. The L98 uses hydraulic roller lifters right?

Yes unless someone changed them out to solid lifters. Not sure if I saw what year car/engine you have. Lifters are flat-tappet through 1986 and went to rollers starting in 87. Again you could have replacement rocker arms that have rollers on them too if changed out.

Just some thoughts.

car is a 90 and the rockers are the factory stamped steel and to my surprise there are pushrod guides. I opened the valve covers about 6 hours after the post above and there is no play or slop in any of the rockers. I have to say that was a chore and a half getting them off. BTW what is the recommended adhesive used to secure the corvette emblem to the driver side cover?

Get yourself a length of rubber hose about 3 feet long and use it as a stethoscope. While it's making the noise hold the hose to your ear and then poke the other end around where you think the noise is coming from. When you get to the right location it'll be very apparent listening through the hose. Obviously be very careful around moving parts while doing this.

I'll just use the stethoscope I have for working on my cars. It was an old cheep one I bought for emt school.

The guides were just there for assy. They aren't actual guides. The valve train has self aligning rocker arms, so using guides in addition to that would cause a "fight" between the two systems.

You're on the right track with the stethoscope. :thumbs:

it can't be the EGR valve???...:cheers:

Other issues have come and gone but this noise is getting worse. Now it will last up to a few minutes and what is starting to concern me is that with the noise I am getting an RPM dependent vibration and lower than usual oil pressure readings. As soon as the noise stops my pressure is back to normal. The sound was isolated to the valley and I am starting to feel like its a lifter issue as suggested above. Would a collapsed lifter cause a drop in oil pressure until it pumps up and what would be the best way to figure out if this is the case?

I have a spare set of good lifters from my LT1 I pulled out during the rebuild because it got a new cam.

yes all emission components are still on the car

The oil pressure issue came without driving the car. I started it to work on it 2 weeks ago without the oil issue, and it sat while I was at work. Today after sitting for almost 2 weeks the pressure is low for the first minute or 2 while the engine is producing a sound similar to an exhaust leak ( this noise has been going on for 2 months, some times better some times worse)

Actually I've had an engine that had low oil pressure; about 2 lbs at idle and maybe 8 pounds at 2k if we were lucky. It was caused by a lifter that had a bad piston/plunger.