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.
Friday, March 31st, 2006 AT 8:12 AM