Along those lines; there's a thermostatic diverter in the heatriser on the exhaust manifold. If it's stuck in the "cold"
position (closed). That will also (obviously) cause exhaust restriction(s).
HTH
--Chris
At the risk of being "that guy", the heat riser does not restrict exhaust in any position. The flap simply allows exhaust to heat the intake manifold, or it redirects the exhaust directly out of the exhaust manifold, depending on what the bi-metallic spring dictates.
Having a malfunctioning heat riser can cause driveability issues.
1) During cold weather and a cold engine, with the flap in the "warmed up" position, the engine will fall on its face upon acceleration due to the accumulation of liquid fuel pooled up on the cold floor of the intake runner.
2) After the engine is thoroughly warmed up with the flap stuck in the "cold" position, the intake manifold will become too hot, causing the air/fuel mixture to be out of specifications and will not run well.
Carl
"That guy"
