ok my truck has been doing this for awhile. it'll start shuttering around 55 and up, but mostly around 75 or 80. and i don't know whats wring. this isn't the first time its done this. i went to florida last xmas and it started doing this right as i got there. the guy at the ford place said a piston was miss fireing, which i don't doubt cause somehow two wires got burt on the trip down. but to the point what could be the problem now, the wires are failry new and the plugs i changed back in january. any help would be appreciated, i kinda want to get some ideals of what i do or check my self before paying someone else to fix it again.

 

What year and what engine? Lots of things can cause something like this. I would pull the plugs again and check them to see how they look. It could be a fuel problem, (filter, carb, injector, etc.), it could be wires, dist cap or rotor (if you have one), low compression, bad coil, vacuum leak. We need a little more info. Thanks.

 

sorry thats the most obvious thing i shoulda said, its a 1998 mazda 4.0. i'll check the plugs. i run 93 for my gas cause i was told it was better for the engine. what would the plugs look like if their ok, like new right?

 

The plugs should look clean and probably discolored. There should be no soot or carbon, or any foreign material on the plugs. Also check the ceramic to make sure none are cracked or broken. If you pull all the plugs, you can put 1 at a time into the plug wire and ground the threaded part to the engine and crank the engine to see how the spark looks. It should be a real crisp blue spark. Just be careful hanging on to the plug, it will bite hard if you grab it in the wrong place. If all the plugs look good, check for a vacuum leak or maybe fuel pressure. Keep us posted.

 

Forgot to ask the obvious. Is the check engine light on? Have you tried to pull any codes?

 

nope no engine check light yet, how do i pull codes, what would i have to half?

 

You need a scanner, a local parts store can pull the codes, call around to find one.