My friend's got an 88 S-10 Blazer with a 2.8 L. When he starts it up and takes it on the road, it runs good for 10 miles, but then starts running rough and won't go over 45 mph after being driven for 10 miles. Any ideas? Is it maybe a coolant system issue? I don't know all the details on it, cept that it ran like crap after 10 miles out on the road. He just got a new motor recently, but other then that I don't know any other likely causes. Thanks for ur replies.

I have heard of the fuel pump doing this. The pressure will decrease over the first 10 to 20 miles of driving, causing lots of power loss. Just a guess though.