Under hood noise - Chev 366

I have a 1980 GMC straight truck with a GM 366 gas motor. Hits on every cylinder and will sit there and idle nicely. Has the typical dual exhaust with a muffler on each pipe. I did install new donut gaskets between the manifold and the exhaust pipe in 2013. The problem is it seems like you can hear exhaust, especially when the hood is open. Are the manifold to head exhaust gaskets blown out?????? I have owned a 283, 305's, a 327, and a few 350's but never had this problem. But, I seem to think when the head to manifold gasket is blown it will make sort of an exhaust ticking sound?????? Sounds sort of loud. Engine did have the air pump with the pipes into the manifold for pollution control. Has been taken off and the pipes were cut off at the exhaust manifold. I can't feel any air coming out of them. Maybe the exhaust manifolds are cracked??? Any ideas will help. Thanks.

A thin rubber tube used as a stethoscope is useful for finding it. Jim

If you suspect exhaust leak, disconnect one plug wire at a time. The sound will change when that wire is pulled.

If the sound is unchanged, could be valve trane noise. Those were known for dropping valves, so if there is a problem, best catch it now!

What happens is the tip of the rockers will wear a recess where the rocker pushes the valve. When it gets deep enough, it starts bending the valve stem until it breaks the valve off. Also the ball pivot in the rocker will gall and wear through the rocker. It's not a bad job to inspect the rockers, gets real bad if a valve breaks!

At that age check you manifolds over close. They will crack and sometimes burn holes in them. We ran 366 and 427 in some of semi tractor back in late 70's through the 80's. About every 3 to 4 years they put new manifolds on them. At night the manifolds glowed cherry red all the way back to mufflers.

Agreed, I usually use a four foot piece of garden hose.

The big block Chevys as well as fords had problems with manifold exhaust leaks in motorhomes because of airflow problems. I did a lot of reading about this 15 years ago. The Chevy 454 in motorhomes were well known for blowing manifold gaskets and warping manifolds. There were also reports of cracking. Ford 460 had trouble with the end bolt holes breaking out. The recommendation that seemed to come up most on the Chevy was to have your existing manifolds machined true if they were not cracked. This was held up as a better repair than new manifolds as they had seasoned and stabilized. Also said to oversize the bolt holes by 1/16 inch to allow for more expansion and contraction during the heat cool cycle. It was recommended to use no manifold to head gasket. Their was a gm part # anti seiz(sp) they recommended using on the manifold to head connection surfaces. The Idea being that the first heat cycle would bake the anti-seiz in to a very hard and slick surface that would allow movement between the head and manifold as they heated because of the difference in there mass expand and contract at a different rate from each other. We used just good quality silver anti-seiz. I had this done to my 454 in a motorhome and put well over 20,000 miles on with no problems so far. It did a smoke a bit at start up the first time. The shop that did the work was skeptical of the procedure but said they had a lot of returns on using new manifolds as well as just replacing the gaskets so agreed to do this. The no gasket and anti seiz recommendation came from a GM service bulletin if I remember right.

Most likely cracked manifold. They like to crack on the down necks near the exhaust outlet in the back. You can get new ones on ebay reasonable. Don't bother with used ones.

What I would do is-start the engine when it is cold,and if the weather is cold,even better,and quickly feel around the manifolds for hot exhaust blowing on my hands to find a suspected leak.Just let it idle-it takes a bit for the manifolds to get too hot.Mark

your manifolds are cracked. very common on these 366 and 427 engines. stay out of the water, as hot manifolds and water dont mix.