Hi Miles,
What do you mean by 'dive' ? Or do you mean backfire?
Does it do it when the alky is on? Or have you tried it without the alky?
You say that the dive happens long before 18psi of boost. Try turning the boost down to about 15 ps, stick in some super unleaded, turn off the alky and see if it does it then. By doing this you are eliminating high power stuff and letting the car run in 'stock' or close to 'stock' tune.
If I remember correctly, the coil pack attaches to the ignition module with torx screws. You could split them and have a look. Check the connectors are okay (be careful pulling the wires off). Check for any signs of insulating material 'melting' or looking 'odd'. Make sure the little captive screw that holds the igntion module cable in place is fairly tight, but not strong armed on there.
TPS - turn the ignition on and watch the scanmaster (multimeter in the old days) while you press the throttle. The voltage rise should look linear and rise nice and gently with no jolts or spikes. Again, make sure that the screws that hold the TPS are tight enough to hold it in place and it is not moving during throttle pressing. Get your wife to press the accelerator and watch the TPS for movement just to be sure.
Check the Y vacuum pipe to the turbo and boost solenoid for splits.
It may sound daft, but if you have a K&N air filter, check it is okay and not excessively dirty. If it looks dirty, whip it off and wash it in the sink with washing up liquid and a paint brush that can get in to the pleats. You will be shocked at how much crap comes off them. Let it drip dry in the sun. If you have K&N oil, all the better, and give it a spray afterwards. If your car has a Kenne Bell type scoop that pushes air up to the K&N and you have been drving in the rain and the K&N gets wet, it can cause funny 'break up' problems. Tom Chou and I worked on a Canadian guys car at the GS Nats at the Motel 6 and it was killing us, until we discovered the K&N was wet. Dried it out and the car ran fine.
If you still have the black concertina pipe between the MAF and the turbo, check it for holes and if you find any, just wrap it with insulating tape. Watch for the stud that runs through the tensioner bracket. It is longer than necessary and can (a) puncture the concertina pipe if you have a K&N or (b) cause a solid pipe to rattle against it and MAF's don't like that.
All being well, we will be at the Nats (with the Regal) and Ian will be there too. If the GN drives okay up there, we could have a look, listen and drive and offer opinions. And, if it is still troublesome, wave it in Steve's face again.
Or, are you bringing the Cougar and sneaking in with the Mustang boys?
It doesn't help that you are at one end of the country, I am at the other, and Ian is in the middle.
Alternatively, we could lowball Arther an offer between us and buy his GN for spares for us all.
ATB
