misfire in boost after driven for about half an hour. Options

[Geddes]

My car (94 GT4) is misfiring when in boost between about 11-14 psi, but only does it if its been driven for about half an hour. Works fine up to redline at 14psi for the first half an hour then the issue shows itself.

I did have a misfire happening in boost all the time, and discovered that the plastic on the rotor was cracked so I replaced the cap and rotor 2 weeks ago and thought I had rid myslef of this problem until I went on a nice highway run the other day and it started again. Since then i've tried it numerous times, it works perfect for the first half an hour and then all of a sudden, misfire. If you let the car sit for about an hour and go again, it will work fine for another 20 min or so, then start missing again. Kind of a weird issue I know, it seems something must be getting too hot.. Anyway, I'm going to change out the wires and put new plugs and order a fuel filter shortly but was wondering if anyone else had ever run into this issue before? Its quite a sharp misfire, definitly feels ignition related more than fuel, as my first thought was that maybe the fuel pump was getting hot after driving and was dropping off volume, but it feels too sharp to be a lack of fuel issue. No CEL codes either.

Thanks for reading

[lagos]

I have a same problem but mine disappeared for some reason. Its reallly common to change plugs on a 3s almost every 3-5k miles. Might as well change them every oil change.

People only have that problem with copper ngk plugs. Some people who switched to oem or iridium plugs seem to have better luck.

One thing I personally noticed about the copper plugs is that they tend to quickly loose their gap setting. If you take them out and check the gap after a few miles, it will often be all over the place from one cylinder to the next. could be the source of the problem.