SIMPLE algorithm implementation, pressure correction error
 

I have been implementing this SIMPLE algorithm to a 2D tunnel flow. I use inlet constant velocity boundary condition and outlet constant pressure boundary condition.

I can get very good flow velocity distribution, which have the max velocity in the center and zero on both tunnel wall. The max velocity magnitude also seems reasonable.

But I cannot get good pressure distribution, the pressure inside the tunnel could be high up to 7 times of boundary pressure. And the strange part is the pressure distirbution seems didn't influence the velocity momentum calculation a lot, I can still get good velocity contour.

Is there some one can give me some hint about this problem, which part I should look into? Thanks.

HIs problem is really like me
 

I am also implementing versteeg's book, the pressure correction got similar problem. The velocity distribution profile, pattern are all ok, but the magnitude value, if compared with slit flow theoretical value

u(y)=(-dp/dx)H^2/8Miu*(1-(y/0.5H)^2)

the max velocity is always 50% higher than the theoretical number, I believe the problem is at pressure correction and don't know how to solve it.