what are the symptoms of a plugged cat?
my symptoms are high engine temps, something under the car smells hot hot hot, and the acceleration is very poor. it acts like an enormous vacuum leak.
this suddenly happened.
the idle surges, and engine temps do not recover fast enough, and now the car seems a lot louder for no reason.

 

Here's an easy way to check it. You could probably rent one


http://www.sjdiscounttools.com/ta33600.html

 

Sounds like it could very well be your problem. Have any rattleing before? You can drop the cat back and see if you have a cat in there and shine a light on it and see if it's plugged. Usually the front ones go first though and plug the rear one.

 

I have had them glow red hot. You can check in the dark. I guess this is your main cat since its a 1986?

 

yank it, knock out the contents, put a test pipe thru it, reinstall. you'll be as good as gold.

oh, and get rid of the pre-cats if you haven't already, with a mandrel-bent front Y pipe.....unless of course you're subject to a visual when it comes to state inspection time --- in which case you simply install the original front Y w/cats, just for the inspection man's eyes.

 

Everything you describe sounds like a bad CAT, except for the "suddenly happened" part. CAT's usually clog over time. With that said however, a chunk of catalyst may have suddenly broke off and lodged in such a way to cause "sudden" severe blockage.

Upon hard acceleration, does it feel like you are trying to push a 3500 pound car uphill with a 4 cylinder engine? If yes, sure sounds like your CAT.....

 

In Arizona, we have inspections, although once, a newbie at the inspection station wanted to have a look, and the senior tech said "close the hood, it passed the emissions test and that's all we are here to enforce. We don't care how it gets done, just that it's clean." The younger guy asked me to step out and close it for him.
Two weeks ago, I changed the fuel pressure regulator, and when I drove it home, it seemed smoother. Then the next day, I took it to the alignment shop, and the car wouldn't start. I thought it was the vats, turned out to be the starter.
I changed the starter, and now, it has a hard time starting. I thought the fuel was the problem, but today, I put a gauge on it again today, and the fuel was at 40 PSI and it was still cranking. I kept cranking till it caught, and open loop, the engine just didn't want to accelerate. Acted like a huge vacuum leak. after the thing goes closed loop, theres this big nothing when you give it gas.
when I get it going, maybe 40, I give it gas, then nothing happens, and then there's a rapid increase in acceleration, and it's like a big effort to keep the speed up.
Yeah, kind of like it suddenly has a 4 cylinder engine.
I think I will disconnect the cat, and drive it up and down the street to see if there's a difference.
I know that you can disconnect the oxygen sensor and put a gauge on the pipe to check for back pressure. Probably should check that first before I get my neighbors to call the police.
The biggest problem is that it's 115 degrees outside, and it's miserable to do any undercar work.
I do have an 85 front Y pipe on my engine run stand......hmmmm

 

it was a weak fuel pump. at idle, it had 42 psi, pulled the vacuum hose, and it jumps to 46-47.
yesterday, it just felt like it was running out of gas.... I give it gas, then nothing, and it just sort of catches up like my old truck did when the float needle unscrewed itself.
anyway, my car is workin fine with it's new pump.

 

Well I am stumped! How doe's pulling a vacuum line effect the FP pressure at the fuel rail ?

 

The fuel pressure regulator is on the fuel rail and it's vauum operated. It should actually be 35-7 at idle, and jump to 44-7 when the vacuum drops under acceleration (manifold vacuum) The FPR bleeds off excess pressure to the fuel tank.

 

the diaphragm in the regulator has a valve that is being pushed on it's seat against fuel pressure coming into the rail by a spring. the fuel pressure overpowers the spring, flows through the open valve and bleeds off it's excess pressure. That's the acceleration fuel pressure. It's set at 45 PSI
The easy way for Chevy to drop the fuel pressure, is to adjust the fuel pressure with the vacuum. The vacuum and diaphragm will be fighting the spring inside the FPR, and reduce the fuel pressure.
when the vacuum is no longer fighting the spring, the valve goes back to it's preset pressure, and more fuel pressure is introduced.
ie every time you move the throttle towards the firewall.