Ford Repair: Ford Ranger 97 6cyl 4.0 L automatic, competent mechanic, vacuum leak

Question
I have a 97 Ford Ranger with a 6cyl 4.0 L engine automatic with about 130,000 miles on it. A problem I have been having for a while is with starting and cold idle.

It takes a few tries to start, and when it does start it shakes and rattles and the rpm surges to 2500 rpm then stays at a constant rate of 1100 rpm. It hasn't died or stalled yet, but it sure is rough when I take off.

I took it into a mechanic to have it tuned and to put in new plugs and wires, and that didn't work. I then took it into my auto-mechanics class at my highschool, and they connected it to a computer thing (forgot what it was called) and the auto teacher saw that one of the pistons wasn't firing.

He took off the air tube thingie and had a student hold down the accelerator as he checked inside the engine. He stated that it was full of black gunk and it should be cleaned out. He also checked out the air filter and he said it looked fine.

Prior to all of this, I used Seafoam fuel line cleaner to see if it would work, and it didn't. I told the teacher that and he said to use another bottle of Seafoam and to take a toothbrush and dip it in the solution to scrub out the black gunk near the air intake thingie.

I did, and it still shakes and rattles the same as before. I hesitate to bring it back to the mechanic, because they would just make me buy things that I didn't need.

Answer
Did the mechanic know the problems you were having before he replaced parts? I hope so, because there is not much to "TUNE" on a '97 Escort. The term tune is an old one from the days when we could adjust nearly everything on an engine. If he just replaced parts without looking into the cause of your problems, then shame on him.

What you are describing could be due to a number of things. Low compression, a vacuum leak, a bad injector, low fuel pressure, a stuck idle control valve, PCV problems.

Your best bet is to find a competent mechanic to diagnose it for you. The dirty throttle plate can cause problems, but not usually a fast idle or rough idle.

When the high school hooked up to it, did they find any codes? That would be a good starting point to know any codes.