'They were driving more faster'At this sentence,what was the problem of more?

The "more" is redundant. 

more faster(X)

比較級的比較級要用much
much faster (O)

examples:
They were driving much faster.
He is much poorer before.
I was much younger than her.

It should be "They were driving faster" because they are no such "more faster". You only use more when the word have three or more constances.

more is contrasted with less and fewer---> many, most, much, adj.
More  is independent comp. means greater in number, quantity, quality, degree, size,etc additional.
You don't need the word "more".
eg:-They were driving faster.
Or eg:-
They were driving "hardly any more faster"---(adj.phr. independent comp.)---than you.---(the comparative degree)
Hence:-eg:-much ,adv, much faster     than you
----------------Driving is much faster =is very fast=use "much" with adj made from passive form  of verbs

......................

It should be "They were driving faster."

They are driving even more faster than before.

♦ more：[語法] 形容詞と和副詞的比較級；-er型和more型。用法：形容詞(或副詞)-er，more + 形容詞或(副詞)；並無 “more + 形容詞-er” (❌例. more faster、……)的用法。
# They were driving faster (than you).