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Why does my car hesitate?

(1994 Mazda Protege)
in Aurora, CO on April 08, 2009
My car started hesistating in October 2008. I had just about everything replaced or repaired, except the engine itself. It has a new radiator, battery, alternator, water pump, distributor cap and rotor, fuel filter, spark plug wires, and ignitor. The engine would cut out while I'm driving 75 mph on the interstate. What else could possibly be wrong?
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Popular Answer
Hesitation can be caused by several things, without driving the car it is hard to form a firm opinion. The radiator, battery, alternator, water pump would not contribute to a misfire, but certainly the distributor cap and rotor, fuel filter, spark plug wires, and ignitor would.
Does the misfire occur on hard acceleration or only when you have maintained long steady moderately hard acceleration?
If the engine has an EGR valve the passage ways on many Mazda's plug up with carbon on engines with over 80,000 miles but it is most noticeable as a stumble on acceleration and I don't think I have ever seen blocked EGR passageways cause a car to stall at 75 mph. What restricted EGR passageways do is allow all the EGR to be pushed in to one or two cylinders rather than be distributed to all cylinders.
If you were at steady moderate hard load on a freeway and the fuel pump was failing perhaps it could cause fuel starvation. It is not hard to verify fuel pressure and volume. I did on my own car put in a set of spark plugs and only a week later notice on hard acceleration I had a misfire. I replaced the spark plug wires and started to test everything except what I had just replaced only to find I had a defective spark plug. I put back in my old worn out set of plugs and the car drove great.
If it is regular occurrence that the car cuts out I would investigate the wiring and attach test equipment and voltmeters to see what is loosing power or a sensor signal when the car actually fails.
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