04-07-2003, 10:07 PM
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Senior Club Cobra Member
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Join Date: Jul 2002
Cobra Make, Engine:
Posts: 15,712
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RPM sounds a to low for plugged cats, the symptoms are correct for the suggested diagnostic however.
Heres a simple way to test the theory, a vacuum gauge. Very surprising the Ford mechs didn't use one. Hmmmm,,,could be the "new mechs" are relying to much on electonics and forgetting about "basic" diganostic skills.
Anyway, if you don't have one, buy one, they don't cost that much at NAPA. Also you will need a vacuum line "tee" connector. Find a good source of MANIFOLD vaccumm, any fitting will do as long as it IS direct manifold vacuum.
With engine idling it should read about 18 Inches, maybe 20 on a REALLY GOOD new motor with timing on the money! As you rev the engine it will fall slowly but hold steady around 14-15 Inches. As the rpm increases it may fall a little more but will still hold steady. But when you hit the "magic" rpm number, and IF the cats are plugged, it will drop like a rock!
If you got an old Chilton or Motors manual they should have a write up about using a vacuum gauge in diagnostics. The newer books have more on the "new" diagnostic procedures and less for the "back yard" guy. The guague will tell you a LOT about your motor, incorrect timing, valve leaks, blown head gaskets, it's great "old school tool". Best used with lots of "field experience" with YOUR motor, but worth the effort. There are no hard and fast rules in using one, thats the problem. It's an "art" not a science.
Ernie
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