Liquid Filled vs. Dry
Indeed, an "oil filled" gauge is tougher and more durable than a "dry" gauge, with the oil protecting the precision internals from shock and vibration, which explains why fewer and fewer dry gauges are offered for performance engines today. And there's no arguing that a liquid filled gauge looks like a higher quality instrument. Unfortunately, there's a hidden "gotcha" that users of this type of gauge experience, a reported lack of accuracy, or consistency, is common. In fact "oil filled" fuel pressure gauges do exhibit seemingly random pressure swings from 1-9 PSI. This is a fuel pressure "yellow-flag" for EFI engines running 40-60 PSI of fuel pressure, and a "black-flag" for carbureted engines running as little as 2-10 PSI.
Bottom line, a common liquid filled pressure gauge is very susceptible to heat, changing its reading with the temperature of the gauge. Imagine that; engine bay temperatures can rise as much as 200-deg. Fahrenheit from cold to hot and we're using a fuel pressure gauge that changes it's reading with temperature... how does this happen? The simple explanation is when the glycerin used to fill the gauge heats up it expands, and being inside a sealed container (the gauge case), the internal case pressure rises. Internal gauge case pressure can increase as much as 1-9 PSI above atmospheric pressure when hot.
What difference does it make? Well, for every 1-PSI of pressure rise inside the case, the needle will fall 1-PSI. Of course, the higher the oil temperature, the higher the internal pressure, and the deeper the reading on the gauge falls. In extreme examples the result is like entering the "Twilight Zone"; take a carbureted engine with 8-PSI fuel pressure, and a hot gauge with 8 PSI of internal pressure, the pressure gauge went from reading 8 PSI to 0 (zero) PSI... is it any wonder racers get a little tense now and then?
Not every liquid filled gauge will get that hot and drop that far, but experience shows it's common for enough temperature rise to create the illusion of a 3-5 PSI drop. Few can ignore this, and so begin the dance of adjusting the regulator so the gauge reads right. Now, once we start chasing the gauge, well then pressure is all over the map, the engine is getting as angry as we are and we've gone from the frying pan into the proverbial fire.
