Slapperbar
Retired Navy NDT Examiner
Could be a vacuum leak.
I know them plates are for the exhaust ports now. But I found out that you don't have to block the rear ones usually because there isn't a full passage in any of the intakes in the rear. I had it explained that by joining the heads back there the coolant can now more easily take the path of least resistance and allow 1 side to heat up more than the other.... Which may explain why my 3+5 cylinder part of my drivers side manifold was smoking so bad..... May be all speculative BS too, but there isnt a crossover passage back there usually for a reason im guessing.Do NOT block off the rear passage, sometime there are block off or restricter plates for the heat riser in the middle of the intake to heat the air charge for emissions.
sure feels it. But both guages are almost the same after setting on an 1100 rpm idle for about 15 minutes. And the look like about 180 degrees. 

