C2 L79 Exhaust gas temp

[Rich G.]

I've noticed the exhaust temp is higher on the left side vs. the right side during warm up. In fact, even after the temp gauge is up to normal the left side feels warmer at the tail pipe. Very scientific measurement by holding my hand near the tailpipe outlet. The left side is too hot to stand when real close, but the right side is just "toasty" warm. The engine runs fine, no oil or coolant consumption.

I know EGT is a function of mixture. What else could do this? Different Comp ratio due to head milling?

The exhaust system is 6 month old stock under car exhaust.

Rich Giannotti

[Duke W.]

Re: C2 L79 Exhaust gas temp

The heat riser valve on the RH side should be closed during cold start and warm up forcing all the exhaust gas through the inlet manifold heat riser passage to the RH side. The purpose of this is to warm the manifold under the carb to promote rapid fuel evaporation for good "drive away" when the engine is cold. When the valve is closed the exhaust flow on the RH side and temperature are noticeably lower than the LH side. As the engine warms up the valve should open.

These heat riser valves become problematic with age and can stick. Also, it is my opinion that the thermostatic spring goes out of calibration with many cycles.

Check that the heat riser operates freely when the engine is cold by moving it with the counterweight. and also check that it opens fully when the engine is warm. Many owners block it open, which should not cause any problems if you only drive your car in mild weather. Others have replaced the valve with a fuel injection spacer, which is just a semi machined valve body without the valve and control mechanism. Still others have removed the valve while leaving the shaft and spring intact so as not to loose judging points on visual inspection.

The heat riser valve on my SWC caused the RH muffler to rot out in 1966 after only three years of service. After I replaced the muffler I wired the valve open and ultimately replaced it with the FI spacer.

Duke

[Jim T.]

Re: C2 L79 Exhaust gas temp

My 68 L79 is wired open as well as my 70. Muffler rotted in first two years of ownership with my 70 when new.

[Rich G.]

Re: C2 L79 Exhaust gas temp

Thanks guys...This forum is REALLY useful thanks to people like you.

Rich Giannotti

[Lawrence C.]

Re: C2 L79 Exhaust gas temp

Rich,
I have a recently restored '66 L79 which experiences the same thing during warmup, and still a slight imbalance at idle after it is completely warmed up. My observation of the manifold valves (and I have now tried two of them with exactly the same results)is that when you observe them at idle when warmed up they actually do not fully open, but are about half open and can be pushed very easily in either direction. I think that when driven and the volume of exhaust increases, the valve is probably forced more fully open but returns to that partially open position at idle, and that gives the right exhaust a little less force and temperature.

Again I have tried two valves, the second due to thinking the first must have been defective, but the second behaved the same as the first.

[Joe C.]

Re: C2 L79 Exhaust gas temp

Larry:

You are EXACTLY right. That is how the heat riser should work, but you must check it often to make sure that it works properly. As you probably know, they have a tendency to seize in the closed position, which will seriously affect your engine's performance, and limit the service life of your exhaust system.

Joe