okay...i dont know what it is...i initially thought catalytic converter or exhaust. But here's my symptoms, just throw me some ideas

- Car smells like exhaust fumes
- Bad gas mileage
- Loud hissing sound when throttling
- Between 3k-4k RPM's the car shudders and lurches a lot and I either have to keep the RPM's a little higher or a little lower.
- Loss of power/torque

GO!

wanna give us a video??? but anyways, sounds like a huge vaccum leak or a melted catalyst, u may or may not have an exhaust leak, check underneoth and look along the ehaust pipe for any black marks, and there is your leak, and check both cats! if they melt, they will cause insaine amounts of backpressure, the one that fails first is the one on your header, start there

do you have a bad exhaust leak or a bad cat?

could be an ignition problem.. giving you that rich exhaust fume smell, loss of power/torque, and shudders at high rpms

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this is also possible, check the coil, and ignitor

could this be related to the header?

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If it's kicking its timing/Ignition for sure, could be other problems:
Header leak, cat leak, check your plugs

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I'm on the same boat, except my RPM shudder range is 2-3k.

Perhaps this can offer you some insight:
I bought the car with the issue, so I don't know what it was like before. The problem seems somehow linked to how much the car backfires when I shift - sometimes it'll backfire with most shifts, sometimes with few... the shudders hasten and slowen respectively.

Running lower octane gas (87) seems to help the issue - but the car runs stronger otherwise on 93.

Changed my plugs/wires and it helped but didn't cure.

When I bought the car it had NO exhaust after the flex pipe AND the flex pipe was broken. Back then it backfired like all hell and shuddered way more. Now it has a straight pipe because at the time I couldn't afford to do a muffler/resonator or fix the flex pipe.

A good seafoaming of the intake really smoothed the car out at idle and higher RPM's and helped some with bringing in power, but didn't help at all with the shudders.

The cat doesn't seem out of the question but the car doesn't feel now like it has excessive backpressure.

First, I think I'll gut the cat and see what happens.. but before that I HAVE to get at least a resonator - this thing is way too loud already

If not cat it'll have to be a vacuum leak or distributor/coil. The whistle inclines me to belive its a vacuum leak... that coupled with my inability to turn the car off with pouring sea foam into the brake booster line makes me think it was sucking air in somewhere else just enough to not get completely choked by the liquid I was literally dumping in there.

I have a 94 GT manual, btw.

did you ever find a fix?

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The week after next I'll be doing the timing belt and header to delete the cat.

Hopefully that will take care of it.

It seems to be consistently worse with the car warmed up - especially on hotter days... and that cat is getting really hot really fast - while leaving the rest of the exhaust relatively cool.

I'm running now just a resonator (no muffler) with 2.25" piping and its just slightly louder than stock. It doesn't "feel" very free flowing at all...

If the manifold/cat wasn't such a rusty nightmare I'd gut it out - but IMO its worth swapping for the ebay header since its not very expensive anyway.

plugged up cat

its gotten worse for me, can't go over 50mph which is a very bad thing considering my drive to work on the interstate

Pulled off the hood insulation and I can definitely hear a vacuum leak loud and clear.

Now to find it...

You may have the same issue, a random vacuum leak... though the cat may also be the problem for both of us.

Use carb or intake cleaner -- spray it on the vacuum points you suspect of being the source of the leak. Hear a difference, you've identified the leak.

I'm having this problem with my celica now too. I've got a '96 GT, just swapped in a 5sfe from a '94. Getting the hesitations/bucking and zero torque at times when I press on the gas. Any one find out the cause of this yet and fix it? Please let me know!

wow i have the same problem, except i feel like there may be a vacuum leak, but where the downpipe connects to the flex pipe is lose and i think my rear cat is bad, there is black on it, what exactly does that mean besides a leak, burnt out?

installing a header gets rid of the cat correct? While talking to my mechanic after recently receiving my car back he told me that I had no cat, so for me it's def not the cat. I put new plugs in but it is still kicking/hesitating. I might go to the parts store and buy all new vacuum hosing since the ones on from the previous owner are all real crappy and see if anything changes. I really hope I can fix this easy so I can get on to figuring why my car tries to overheat while idling but is fine while driving....

Did you replace your spark plug wires and your distributor cap? Plugs alone won't solve the problem if the others are toast, too. You should ALWAYS replace these things (along with fluids and the air filter) when you get a new-to-you car.

I should replace the distributor cap as well? I can try this, I haven't yet confirmed that there are no leaks in the vacuum hoses yet. I just noticed something that I saw in another person's post as well. When I started up my car just now, ALOT of thick, fumy exhaust came pouring out.