Gang, my Manifold Pressure gauge started to act up a few months ago. At first it worked sluggishly. I would preset the MP at say 25 inches; and I then would reduce MP. It would take a huge throttle change before the needle would come down. It did that for a few hours; then, the gauge got “stuck” at 30 inches (ambient pressure ), and just did not move anymore. I got the gauge rebuilt last week ($250, thank you), and yesterday the AI re-installed it. The gauge still does not work. The AI told me that they checked the line to the engine, and that this all looked fine (before they took the gauge out). Any ideas??
Harry Roussard
Harry, my bet is that the fitting hole has crusted over inside, in the wall of the intake port, where the fitting screws into the cylinder head. I can’t believe that the mechanic just had the gauge overhauled without a couple of simple tests.
You should be able to easily move the gauge needle by sucking (like on a straw) on the end of the MP line. Just disconnect it from the engine fitting and suck on it. If the needle moves freely, and reads ambient (30″) when you quit sucking on it, your line and gauge are fine. Take a piece of safety wire and work it in and out of the hole in the MP fitting in the cylinder head, until you can blow through the hole. It is small, so you can’t blow a lot. This will probably dislodge some small carbon granules that will easily pass through the engine. If you prefer, you can carefully remove the fitting from the cylinder head, and examine the bored port in the head. It connects into the intake port, near the intake valve. If you dislodge a lot of carbon material, you can draw some or most of it out with a vacuum. Please let me know what you find.