Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
It could also be a plugged muffler or exhaust pipe, or a frozen heat riser valve.
The heat riser valve is between the exhaust manifold and the exhaust pipe on one side. If it sticks in the closed position, it forces almost all of the exhaust from that head to squeeze through a little passage under the intake to the other side of the engine. With the engine cold you should be able to rotate it freely. If it's stuck, work on it with penetrant and patience until it's completely free.