93 Mazda B2600-I

The mechanic should do a compression test both wet and dry to rule out the new engine. The difference between cylinders shouldn't be more than 20 lbs or a rough idle will occur. Very rough idle will be 40 lbs or more.

The most usual reason for a rough idle in these conditions is a vacuum leak. The easy way to determine a vacuum leak is to spray the intake with carb cleaner. If the RPM goes up you found it. Check all hoses especially the brake booster.

If the shop has a smoke machine have them use it, it's the best way to isolate a vacuum leak. Hopefully the lifters are are OK. On a Mazda they are very small and prone to problems especially if the engine has been sitting a long time. It's an easy but pricy change.

Guess I should had elaborated a bit more. The original engine had the same problem, but not quite as bad. The reason for getting another engine is because the old one blew the head for the 4th time. I got the engine from ASAP and had a recomended shop install it. The mechanic had to use parts from the old engine. This makes me believe that some part from the old engine is making the new one do the same thing. Might it be a sensor or maybe the distributor itself causing the problem. The mechanic tried new plugs, wires, and distributor cap.

Thanks for the quick response and your previous suggestions. By the way, this engine is fuel injection.