there is no point chasing problems if the basics aren't up to scratch.
Air filter
fuel filter
plugs
leads
coil
O2 sensor
then start chasing lack of power problem.

If you can't afford it, you're wasting your time.

Hey guys, I've replaced a lot of of things since and the problems are still occurring. I've gained a bit of response and a tiny bit of power but that's about it, all my worst problems still exist.

I've replaced:

Fuel filter
air filter
oil
plugs
leads
oxygen sensor
cleaned TB and ISC
Also going to replace the fuel pump this week.

The main issue is the excessive fuel consumption, which hasn't improved at all. Also when stationary (sitting at the lights in drive) the idle always drops to 500-600RPM and the car runs really rough at this RPM when it never used to, in neutral it always idle normally/smoothly at 700-800RPM. It only happens when stationary with it in drive. This problem doesn't seem to occur when the car is cold/first drive of the day, only when the engine is warm.

Also I've tried a few times to locate this vacuum leak but have failed, it seems to be actually coming from behind the big rubber vacuum plug (all the hoses connect to it) that sits behind the glovebox, I've taken the screw out of the plug and I've looked as far behind it as I can possibly get and it sounds like it's coming from behind that area near when the hoses plug into the firewall. The vacuum leak is only there when the climate control unit is off.

The main thing bugging me is the fuel issue, if I fix that i'll be extremely happy, where should I start to fix that?

Cheers

Vacuum leak can cause higher fuel consumption. I had fuel consumption issues and power issues with my leaking vacuum in the cabin, but I also had other little bits that contributed as well.

Unplug the vacuum hose that goes through the firewall, from the engine bay side. You can trace it back to a small vacuum distributor block I think. Block it up with something (I taped mine) and see if you can still hear something from inside the cabin. If you don't well then something is broken/cracked with your climate control vacuum box thingy. If there is still that hissing sound then I can't help anymore with this part.

Not sure on the weird idle .

Fuel Consumption--Try a new 91degree Thermostat

Rough Idle in Drive-- Check your Engine mounts especially the drivers side. Your car will have the round hydraulic mounts and they can sag and the metal body sits down on the rubber pad on the bracket. You should be able to see aleast a 5mm gap between the bracket and the metal body of the mount, if not then this will cause rough idle and vibrations. You can also rev the car with brake on in drive(note movement) then put it in reverse brake on rev it(notice movement) Should be the same.

Take your cat off and give it a shake, it could be that it is blocked through the veins as opposed to collapsed. Different issues with the same outcome. it will very easy to spot once its off as to if the veins are blocked, it could also be that it has collasped in such a way that the material has gone to powder and is not shaking round to make a rattle noise. Blocked veins are also known as a burnt cat.

From all your symptoms I would deffinatly look for a blockage in the exhaust system, it will present elsewhere as well, such as increase in engine heat, but changing thermosts will be worthless without correctly finding the cause of the extra heat.

Increase in temps almost always comes from a cooling or exhaust blockage.

Hey Phongus

I disconnected the all the hoses on the firewall and the hissing disappeared completely, I didn't block the hoses though, is it necessary to block them to diagnose the issue or is it more a safety thing? Thanks again for the help!

Cheers.

Hey dude, I forgot to mention I replaced the thermostat sorry.
I'll check the mounts and report back tomorrow, but the actual revs drop down 300RPM or so which is causing the roughness/struggling feeling, it's also fine when cold. Thanks!

Also I forgot to mention that my gearbox is shifting rough, not engaging sometimes and the fluid is burnt/brown. So instead of wasting $200 on fluid flushing it I'm going to chuck a T5 in it. I'm going to remove the dash to fix the vacuum leak, so while it's out I want to put a manual pedal box in. Question is, with the manual pedal box in can I still drive it, until I find a manual conversion?

Well if you pull the vacuum hose off, you have to block the vacuum hose that is connected to the engine side (the one that is sucking), not the one that goes into the cabin. The whole point of this exercise is to find out if there is a vacuum leak...so by blocking off the engine side vacuum hose, you have stopped the leak (hopefully). Since you said there is no more hissing, I assume the vacuum control box thingy behind the dash might be cracked/broken...or maybe even one of the vacuum actuators that control the flaps to the vents. Hopefully you will have a better more stable idle with the car in Drive and have a better fuel economy with the vacuum line blocked off.

In regards to the pedal box. Yes you can still drive the car with the manual pedal box in, you will just have a flopping third pedal. Before you put it in though, I suggest you reinforce the pedal box where the clutch pin is, that way you know it won't break on you later down the track.

I blocked the 2 hoses (engine side) except there was one I couldn't reach, one of the smaller hard plastic ones, it ran straight down from the firewall and looked like it ran under the manifold.
I test drove it, seems like it fixes my power issue but the poor idle and hard start still exist.
I'll find out tomorrow night if my fuel economy has improved. And yeah there was no hissing once the hoses were removed.

Cheers