If only one area of the exhaust manifold is glowing, I might suspect an
exhaust manifold leak. As many of you already know, there are areas of
negative as well as positive pressure in an operating exhaust system, due to
complex flow and pressure wave dynamics.
If you happen to have a leak in an area of low pressure, it could draw in
outside air, which could then add enough oxygen to unburned fuel in the
exhaust manifold to allow it to burn quite readily.
This effect is reminscent of the use of an "air pump" type emission control
system in newer vehicles that injects air into the exhaust manifold to
initiate and/or modify oxidation of unburned hydrocarbons, both in the
exhaust manifold and in a catalytic converter.
The unwanted combustion of fuel in the exhaust manifold may be exacerbated
by an overly rich mixture that allows more unburned fuel to enter the
exhaust manifold, or by a leaky valve that contributes to less complete
combustion (also allowing more unburned fuel to enter the exhaust manifold).
And, even if the overall fuel-air mixture is OK, rarely (read "never") do
all cylinders in a carbureted system receive exactly the same fuel-air
mixture, especially at idle speeds (and a six-cylinder, single-carb intake
manifold setup is one of the worst combinations for uneven mixture
distribution).
The manifold glow may change (or go away completely) with changes in engine
speed, load, and throttle position, partly because of changes in fuel
mixture/unburned gases, and partly because the areas of low pressure in an
exhaust manifold will increase, decrease, move, disappear, and reappear with
changing operating conditions.
On the other hand, some engines may "naturallly" develop hot spots like you
describe, although I've only had it happen on one of the forty-odd vehicles
I've owned, and in that case it was cured by repairing an exhaust manifold
leak. (On newer vehicles, I've seen exhaust manifolds on engines with air
injection glow brightly as the result of an overly rich fuel-air mixture.)
So, it may be that you have some combination of:
- A rich mixture on #4 at fast idle.
- A generally over-rich mixture.
- A leaky valve.
- An air leak in the exhaust manifold.
- An idiosyncrasy of engine design.
- Or ?
Some reading on such exhaust topics as extraction, anti-reversion, and
header tuning may be of interest.
Alan Bowes
Juan Gonzalez wrote:
> I have got the #4 area of the exhaust manifold glowing during fast idle.
> Any suggestions what it might be?
>
> Juan Carlos Gonzalez
> owner@wgn.net
> www.wwiiimpressions.com
> WWII Impressions, Owner
Juan Gonzalez wrote:
> I have got the #4 area of the exhaust manifold glowing during fast idle.
> Any suggestions what it might be?
>
> Juan Carlos Gonzalez
> owner@wgn.net
> www.wwiiimpressions.com
> WWII Impressions, Owner
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