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

[BriinumsBo]

plugs/wires should fix that. mine was misfiring because one of the wires had come loose a bit, but i couldnt see it under the intercooler. the spark was going from the wire to valve cover

[Geddes]

plugs/wires should fix that. mine was misfiring because one of the wires had come loose a bit, but i couldnt see it under the intercooler. the spark was going from the wire to valve cover

heres hoping, i've got my doubts though. But hey, gotta eliminate the simple things first before I dig any deeper. I'll have to drive it to work one of these days and pull the intercooler off and change em. Ill post here if it made any difference or not . In the mean time, I've turned the boost down to 11 so I dont have to think about it haha.

[Geddes]

well took me forever to get around to doing it but I changed the plugs and wires a few weeks ago. I squeezed the gap down to .028". Seems to have eliminated the issue entirely. Also found a boost leak while I was at it (i was suspecting there was one anyway as my vacuum seemed slightly lower than normal to me at idle). Car works great now!, except I lost my lowbeams last night.. Now I get to figure that problem out.. It just never ends hahaha

The plugs i pulled out only had about 5k km's on them. I'm guessing either they were gapped too far apart for the amount of boost i was running, or were just simply too hot of a plug (6, stock heat range). They looked completely fine visually.

[Stambo]

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.

[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.