Luke Thomas had put Rovers ahead in the first half and it appeared they were heading for an opening day victory in League One before Yengi's late strike.
The enthusiastic welcome the players received was not matched by either side in the opening period, with constant downpours making the pitch difficult to play on.
Rovers forced the early pressure and took the lead in the 24th minute. A quick break saw Jevani Brown go clear on the left and a low cross fell to the feet of Thomas, who confidently drove home.
Pompey produced two good efforts from Colby Bishop and Gavin Whyte as they pressed for an equaliser.
Finley was booked in the eight minute for a late tackle before, moments later, having a shot deflected behind for another Rovers corner. 
Wilson and Connolly combined to prevent a Portsmouth opener in the 11th minute as the home side looked to get themselves going.
Rovers deservedly took the lead in the 23rd minute as great work from Finley and Brown set up Thomas, who planted his shot beyond Norris. 
The home side went desperately close to an immediate equaliser but Whyte’s shot flashed wide of Cox’s far post.
Pompey came close to breaking the deadlock on 12 minutes, when Rafferty found Scully at the back post.
He laid the ball off for Whyte just a few yards out, but the winger’s effort was deflected over the crossbar.
Jevani Brown collected Sam Finley’s pass and found Thomas in acres of space to slam home.
The hosts improved after that and looked the more dominant side for the remainder of the opening half.
A series of corners suggested a breakthrough might be near and when Pack found Whyte in the box, he fired inches past the post.
Rovers forced the early pressure and took the lead in the 24th minute.
Pompey lost the ball in midfield and a quick break and a cross from Jevani Brown saw Thomas sweep the ball home.
His strike looked like it would be the winner but substitute Yengi saved the day for Pompey two minutes into added time.
Bristol Rovers coach Andy Mangan felt that a draw was probably a fair result.
He said: “Ours was a fantastic goal. We’d already had a couple of counters before that.
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