When im on the hwy and the engine is cruising at around 3500rpms or higher, I get a slight, what feels like, an engine misfire. It was getting to the point where it was always "jumping" while cruising.

The car has 189,000 on it.
I changed the factory timing belt.
I changed the spark plugs
I changed the wires. (2 ignition wires)
I cleaned the MAF sensor
I cleaned the throttle body
I cleaned the EGR tube and valve
No change in the hesitation

The engine light was on for catalytic converter PO421.
I changed the catalytic converter.
Engine light went out
The hesitation went almost away but there is still a slight "jump" while cruising.

Why would it get better after I changed the converter but not go away totally?
Any ideas?

 

when you say, "while cruising", does that mean while on Cruise Control?

If yes, does the problem happen primarily when descending a hill?

 

No cruise control on.
It does it intermittently, some times twice in a mile or nothing for a few miles.

 

Have you had the OBD-II codes read?

 

Codes are clear.

I just read this on a post- "Turns out that the ignition coil and wire failed, and it turns out the jerking motion was a sign of the coil's life coming to end. After replacing the coil and wire the jerking went away and the car is now running very smooth."

I was thinking fuel pump or a coil. Ill buy a new coil, install it and if still does it ill swap it over to the other coil.

 

did the coil resolve the problem? I have a 2000 lx with almost the exact problem, and have done almost exactly as you. However the previous owner stated he had recently replaced both coils. I am ready to try, new plugs, wires, and coil but wondered if you solved your issue and how?

 

i have a 03 mazda speed protege that does that only when it warms up. changed out both coils, plugs and wires. any idea on that issue??

 

Never changed the coils. The condition has not changed either. Still no code and hesitates the same.

 

if it only does it when it warms up, it's your tune. The speed protege stock tune is pig rich so what you are feeling is hesitation as the ECU dumps excess fuel that isn't needed. Install a piggyback air/fuel controller like the split second afc and you won't regret it.
change the coils, the problem should go away. If not, try cleaning your MAF with some cleaner from any auto parts store. If that doesn't fix it, maybe your o2 sensor needs to be replaced or your egr valve needs cleaning.