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[eddioso]

My 98 GT has a misfire at around 1500 to 2000rpm. The car accelerates great but in 5th gear at around 55mph it's missing all the time. I replaced the plugs with new platunims and a set of Ford Racing wires. I also replaced the coils and had a fuel injection service done. The problem persists. Any help much appreciated.

[tomlg2003]

Only while under slight acceleration in 5th gear? or while maintaing a speed that may be not up hill? 

If it is while accelerating, then I would still look for a bad plug or wire?

Since you stated you replaced them, check connections, and make sure they have the high Voltage conductive grease compound installed on both ends of the wires.

Otherwise you may have a valve train problem on a cylinder ( broken spring, burnt valve/seat, rocker arm loose)?

[eddioso]

Thanks for replying. It's in any gear at lower rpm. No problems during acceleration or idle. Once I get above 2000rpm it's fine.

[Soaring]

If it isn't missing at all rpm's my guess that it isn't the ignition or the valve train, because those problems would cause a miss at all rpms.  It's possible you have some water in your fuel from condensation.  Put in a can or two of water dryer stuff to see if that helps. 

[tomlg2003]

It could be the TPS (throttle position sensor)!  They have a tendency to wear the positions mostly used, like idle and slightly above.  I don't think they are real expensive, $30?

With 10 years and many miles it is possible that this moving potentiometor (variable resistor) has gone past it's life.  It is located on the side of the throttle body and connects to the end of the main throttle flapper thing.

[rich96cobra]

Hello there!

First let me give you my opinion of platinum plugs.....there junk.
people who say I am wrong, need to own a mustang longer than a week.

My brother has had trouble, I have had trouble with platinum plugs doing what....causing a miss.  It clear right up after I removed them.

I have had my brother go purchase NEW platinum plugs, she deveolp a miss.....  He changed coil, wires, etc.  As soon as the new platinum plugs came out.... The miss was gone.

Personally, my cobra had a slight miss.  I removed the platinum plugs, it clear right up.

Now, You may have other problems.  TPS normally throws a code.

EGR valvue will cause a miss also around lower rpms.  How I know?  I have been thru two nissan sentras.  Everytime I develop a miss, I thought OH NO!  My mechanic unplugged the EGR.....the miss went away.

Keep us up todate.....
 
Rich