Why below sentence is wrong?

"We test-drove a car having engine trouble."

I have taken above sentence from Manhattan Sentence correction guide. The guide says that the construction is awkward. I wanted to know exact error in the sentence?
Thanks

well, this sentence is grammatically fine, because the modifier lacks a comma and therefore attaches to the nearest noun (which in this case is 'a car'). however, the mere addition of a comma changes the sentence to 'we test-drove a car, having engine trouble', which would imply that WE 'had engine trouble' as we test-drove the car (a notion that's as amusing as it is absurd).

based on patterns evident in the og's preferred answers, this sentence is to be considered more 'awkward' than 'a car that had engine trouble'. both are fine, but the original version teeters on the edge of confusion while the latter's meaning is unmistakable.

bottom line: their preferences, not ours. learn 'em!

Hi Ron!

Could you please explain more on "comma"'s job?

For example:

A study of Ritalin consumption in Michigan, which has a high percentage of
A study of Ritalin consumption in Michigan [NO COMMA] which has a high percentage of

Thanks
Nir

first one is correct as "which" in almost all cases requires a coma to be preceded where as the same is not required by "that".
second lacks the coma and is wrong.

on the gmat, 'which' is always preceded by a comma (unless, of course, the answer choice is wrong!).

i think that answers your question; if it doesn't, please re-post and let me know.