Split in Manifold Pressure.....Need Help

[cessna02337]

Well.....went to take the aircraft (1967 O-2A) for a 25 Hour Oil Change, noticed while taxing over at normal idle I had a manifold split (Front engine reading 16, Rear 14). Briefed mechanic, during oil change he found a small crack on front engine manifold, left side. Removed and replaced along with the correct gasket. Taxing out, for maintenance run, same issue, no change, still had the split.

I performed various engined runs at different power setting and it seemed to follow at all ranges (about 2 inch split, front engine always being higher). I have a Shadin fuel / air data computer and fuel flow to RPM seems to be within .08 per engine fuel flow in gallons per hour. when RPM matched.

Looked at a video I shot last flight to confirm previous matching of manifold pressure and noticed my Manifold pressure as I remembered were always EXACTLY matched to include during taxi/ground ops as well flight.......any info regarding next steps, haven't flown it yet, planning on a test flight Thursday.

From what I was told, this is within the maintenance limits, just don't like the split as I have never seen before.

Thanks as always in advance, T.

[billsheila]

I just had this same problem. It was a bear to get to the bottom of. Easy way to test the gauge is to swap the lines at the gauge, see if the problem follows the swap in which case the gauge is probably OK.

Remember that a leak in manifold is a higher reading since the gauge works on vacuum.

I ended up tracking mine to a small leak in one of the copper lines (there are like three sections front and rear). The leak was really hard to find because it was under a fitting. Sure didn't take much of a leak to create the MP split.

I agree with you it is worth getting to the bottom of. In my case the MP was off by about 3 inches at anything under about 20" (above that it didn't seem to make much difference). I found it very distracting.

[JeffAxel]

could just be a gauge issue, what does it read with the engines off?? Both sides should read atmospheric pressure. If they read different, you have found your problem. If they are the same with the engines off, then while it could still be a gauge problem, I would keep looking for induction leaks. Just my .02, keep us informed.......

[stackj]

I chased a manifold pressure problem on my front engine for about two years... Finally found a split nut on the manifold pressure copper line where it went through the firewall.

[Paul462]

Is that plane turbocharged?