Posted on: March 9, 2009

by Tom Ehrhardt in the July 1997 RootesReview

Sunbeams with matured 1725 engines, CD 150 carbs and PCV valves, tend to have unstable and rough idle, requiring idle speeds in the 1,200 rpm or higher range. I offer this suggestion to improve idle characteristics and overall driveability.

Remove PCV valve from intake manifold and plug hole with standard 3/4 inch pipe plug. Redirect hose down towards road on right side of the engine. The action will greatly improve low idle rpm stability and smoothness. It will also allow lower (600-900) idle rpm. The hose will likely drip oil, but not on car. Let’s face it, the engine is going to drip on our environment – no sense in letting it drip on the car too.

Rationale
Engine idle requires a delicate blending of air and fuel. Unfortunately, the PCV valve allows an erratic excess amount of air in at idle, which upsets this balance. The effect requires higher idle speed to keep the engine running. In the process the idle is usually unstable and rough, varying from 900 to 1,500 in extreme cases. In many cases the engine will sound like it’s misfiring or not running on all four.

Removal of the PCV valve is justified for two reasons. The first is the benefits above to improve idle. The second is that the valve does not accomplish its intended design functions on engines that are not new or freshly rebuilt to tight tolerances. Therefore, this tip applies to most of us. The intent of the PCV valve is based on environmental considerations that allow crankcase vapors, fumes, etc. to be burned in the engine, thus reducing pollution. The concept is that the PCV valve regulates a portion of the intake manifold vacuum towards the crankcase. This in effect sucks the vapors into the intake to mix with the new fuel vapors and burn in the combustion process. While leaving the PCV valve in place continues this process, it doesn’t fill its intended function for other reasons.

Primarily because of age and loose tolerances, our engines tend to have a lot of combustion blow-by which fills the crankcase at a greater rate than the PCV valve can suck it out. The end result is that oil vapor and eventually oil droplets are forced out openings like the front and rear crankcase seals.

The bottom line is, unless you have a tight new engine, accept this problem and make the best of it. Vent the crankcase to the atmosphere. It’s going there anyway, and you might as well enjoy the benefits of a smooth running Alpine engine.

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