Heat Riser

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Datsun 1200 uses a traditional heat riser in the Exhaust Manifold. This heat stove valve is on the exhaust manifold. It deflects exhaust heat to the underside of the Intake Manifold during the first 30 seconds or more of starting a cold engine. Result: quicker starts, more efficient running. However, the A12 GX Engine does not use one, so it requires extended choking on cold days.

Overview

A heat riser is a mechanism in the exhaust manifold to divert the exhaust up to the bottom of the intake manifold. This is done when the engine is cold for the first minute or so of operation. The hot exhaust causes the cold air/fuel mixture to vaporize for smoother operation and for lower emissions. The heat riser butterfly valve is controlled by a bi-metallic (thermostatic) spring, and as the engine warm, it will move the valve so that no exhaust is hitting the bottom of the manifold. It's totally automatic and works very well.

The intake and exhaust are bolted together in engines with a heat riser:

Inspection

Check for free movement of the Heat Riser. If it is stuck, it can cause the engine to run poorly -- or even not at all! (depending at which position it is stuck).

If it doesn't move freely, lubricate the shaft with free-up oil/penetrating oil. In some cases you may need to replace the manifold.

If the spring is rusted out, a replacement can be fitted.