I’ve noticed that my car has been “lagging” to reach high RPMs. Once 4K rolls around, it’s like power just diess out. However, I’ve been experimenting and forcing my car to 5, 6K (slowly, like a steady climb) and sometimes I’m noticing that the line that links the EGR valve to the EGR modulator (the one with the filter) keeps popping off like it’s under pressure when reaching higher RPMs. When it pops off, I can tell because I can hear a hissing noise when accelerating and sort of a “throbbing noise” at idle.

Is this a bad EGR valve? I’ve done the test with the hand vacuum pump connected to it to see if I could kill the engine and it took quite a bit to kill it. It just bogged and ran erratically until I kept pumping it and it eventually died. Haven’t bothered to remove it and see if carbon was caked in it.

Another thing to note is that the car runs a million times better when that hose pops off. Car doesn’t struggle in higher RPMs and runs beautifully. (I’m sure it’s not the brightest idea to run with it off, kind of hard to pull off on the freeway and reconnect it) I had another engine in this car that suffered from the same issue. Completely different EGR valve and all. Still did it. Where do I start...?

Just extra info in case needed:
1994 Celica GT 5SFE manual trans

Your exhaust is clogged, possibly a melted or broken cat converter which was possibly caused by the engine running poorly at some point in the past.

^That. Drop the cats and check them out. If they're just clogged with carbon and aren't melted or broken you can clean them out and put them back on. If the pre-cat is fubared then you could just get an aftermarket header to remove it and call it a day, if the main cat needs replaced then a universal high flow welded in will be the cheapest way of remedying it aside from doing a test pipe.

I was skeptical about that and what sort of confirmed it for me was the fact that when I swapped the engine, it sill did it. What I really wanna do is get everything going redone from the headers all the way back. However, I’m not really sold on an exhaust. Don’t want anything raspy and garbage. Rather throaty (or as much as you can get from a 4cyl). I appreciate you confirming it. As far as the car cleaning process goes... from what I’ve seen its a soaking process in a bucket to breaks down carbon.

I’m gonna go with not well maintained. The car was purchased with about 294K the only real selling point was the body quality for me.

Yeah it's not hard to clean, just soak in hot water with degreaser and flush it out. Rinse and repeat until clean. If you don't want raspy then keeping cats is pretty much a must, a tube header will make more rasp but if the rest of the exhaust is done right you'll be ok. So you could do a header and pre-cat delete, then clean the main cat or replace with a high flow, then do a baffled muffler of some sort.

Say I do the pre-cat delete and do a header. I won't pass emissions, correct?

It will not pass emissions if they use a tail pipe emissions test or have a visual check.

"Nevada uses On-Board Diagnostics (OBDII) Testing for 1996 and newer vehicles. The operator will connect an analyzer to the OBD computer in your vehicle. Data from your vehicle will indicate whether its emissions systems are operating properly and within emissions limits.

Older vehicles are tested with a Two-Speed Idle Test. The operator will test your vehicle once at idle speed, then test it again with the engine running at approximately 2500 rpm. Older diesel vehicles are tested on a dynamometer."

Welp. I'm assuming they do the tail pipe emissions test since they check at idle and at 2.5k testing for pollutants.

Thanks. I'm probably asking obvious questions. I appreciate the patience.

It won't pass visual but it should still pass tailpipe if you get it hot enough before testing, all the pre-cat is for is to heat up the exhaust faster and get emissions as clean as possible as fast as possible. Domestics of the time used smog pumps that served the same purpose, but they injected air into the exhaust for more complete combustion of the exhaust gasses to speed up heating the cat(s) and also helped the cat(s) to work better. Though if both can be cleaned you could just switch to the pre-cat before testing just to make sure you pass.

You'll probably want to also replace the main cat, it may be partly blocked by debris from the first cat if it's broken apart any. And to pass emissions you'll need that main cat working as well as possible.

Thanks guys. I think I'm gonna go with the header and thermal wrap it. I know it doesn't add a performance boost or barely noticeable if that but for the aesthetic I guess. Found a Magnaflow High Flow Catalytic Converter for my '94 for $137. I'm sure that will resolve the problem.

Besides that, I'm probably better off just replacing them. I'm a bit of replace-everything-with-new-parts freak that way I won't have to worry about it. 24 years probably hasn't done the cats well.

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Yeah, I think I'll try the header and keep it as sort of a test pipe. Either clean the old one or replace it down the line in time for next year's renewal.