【Answer】Solution: As analyzed above, the maximum sum is $8497+6503=15000$; the minimum sum is $3496+1502=4998$. Therefore, the answers are: 15000,4998.
18. (3 points) Given the arithmetic expression $\overline{\mathrm{abcd}}-\overline{\mathrm{efgh}}=1994$, where $\overline{\mathrm{abcd}}$ and $\overline{\mathrm{efgh}}$ are both four-digit numbers; $a, b, c, d, e, f, g, h$ are 8 different integers from $0, 1, 2, \cdots, 9$, and $a \neq 0, e \neq 0$. Then the maximum value of the sum of $\overline{\mathrm{abcd}}$ and $\overline{\mathrm{efgh}}$ is $\qquad$, and the minimum value is $\qquad$.