Hello, first time post. I have a EL Fairmont V8. Purchased a few months back and have added about 5000km to the clock which was at 303,000 km when i got it. I went right over it when i got it everything changed from radiator and hoses through to brake fluid.
I have noticed it has started to develop a ( missfire) at about 2500 rpm. The best way i can explain what it feels like is, if it had a carby on it u would think the jets on the secondarys where around 7 sizes to big.
I can drive the car 1/ 4 throttle and if i keep it under 2400 its not to bad. But push it any closer to the floor it bogs down. Engine will continue to rev to 4500 if i hold it but wouldnt pull the skin off a rice pudding and the sound is horrible. Reciently i have replaced the exhaust new 21/2 inch from cats back. The issue started a few weeks after this though. To fix the issue i have checked the fuel pressure 250 -300 kpa. And the mass air flow sensor replaced. TPS seems to be doing what it should and the o2 sensors although test ok i wasnt happy with the results. Anyone got any ideas.

Have you replaced the fuel filter?

Yeah mate all filters, oils , plugs leads cap button everything changed during the service i did when i first got it home. As i mentioned even changed thermostat, radiator and all coolant hoses

What condition are the cats in?

While changing the exhaust i paid close attention to checking out the cats. They where clean still had a good honeycomb shape to it.
Prior to changing i had planned to gut the cats but they looked that good i thought id leave them alone.
Adding another thing to make it all a little strange is no fault codes. I have done the old L.E.D test light trick and 782 comes up on the eng off test and the k.o.e.on. test comes up 111.

An update on strange engine behaviour. After not being completely happy with the O2 sensor readings i decided to replace them. Removed the cats to change them and decided to introduce the internals if the cats to my large screwdriver and a hammer. Turns out IF THE CATS LOOK GOOD FROM THE REAR they can be blocked to the s**t at the other end.
So now with clear cats opened exhaust it breathes better sounds better and pulls better. Loving my car once again.