Cat Testing. Am I doing it wrong?

mgbeda

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Hi folks,

I fairly recently got an '87 Fifth Avenue with about 115,000 miles on it. A variety of symptoms made me think that I had one or more clogged cats. Long story short, I dropped the exhaust system, and I "tested" the cats by taking my shop vac, setting it up to blow, and sticking the hose in the top of the Y pipe (it's almost a tight fit). Well, to my surprise, when I blow down one end of the Y-pipe I get a nice breeze coming out the other end. And when I blow down either end of the Y-pipe I also get a nice breeze coming out of the tailpipe. At no point does the vacuum sound like it's straining against pressure.

So did I really just prove that my cats aren't clogged, or is there something more subtle I'm missing, like just a little bit of cloggedness causes a big hit in performance?

If the cats really aren't clogged that's halfway great because I didn't know how I would replace them (and I live in CA so they would have to be replaced, unless I sell it to my brother in Cleveland). But it also halfway sucks, because it means I wasted a LOT of time and effort.

Thoughts?
 

Mikes5thAve

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The telltale of clogged cats is usually that the exhaust is much hotter before them then after and extreme cases the clogged cat can actually start to glow from the extreme heat.
But it's probably better to know what your symptoms are and go from there.
Cat converter problems are generally the result of running problems not the cause.
 

XfbodyX

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Go run the interstate at 70 at night for 20 minutes and pull off and see if they glow, you can also try to run low gear out, a bad plugged cat system will choke a 318 out and make it stutter and miss about 3500 rpm in low.

A pyro gun can give you a reading although I forget the upper temp limits.

I tried your method of blowing in one end of the cat and all I got was scratched to hell and a cat that wont come near me.

Wouldnt your smog test sheet tell you if things were amiss in your emissions system?
 

Aspen500

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Wrong kind of cat X!:D

As mentioned, we need to know the symptoms. It is possible for the cat(s) to be plugged enough to choke the exhaust flow but seem OK with a vac set to blow. There's way more cfm of exhaust than a shop vac can blow. Also, I've run into cats that are fine when cold but when they get up to operating temp, they start to restrict the flow. You could try driving it with the y-pipe disconnected and see if it seems OK. Easier said than done because of the noise, I know. A pressure test with a pressure transducer tapped into the exhaust before and after the cat is the best way to test but not feasible at home. A good cat will generally have a max case temp of around 600 degrees. Over that indicates a problem, either the cat is restricted or the engine is dumping excessive fuel into the exhaust and it's burning in the cat itself.
In most cases, a cat doesn't get restricted for no reason. An over rich condition, misfires, etc can cause the substrate to melt down. Sometimes they do fail for no reason, usually the substrate comes apart and then you have a restriction. Age can cause that to happen as well as a defective cat, although if defective the problem shows up when the car is still fairly new, not after 40 years.

If you have, or can get a borescope, look down the pipes and see what the substrate looks like. Nothing beats a visual inspection.
 

mgbeda

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Some good info here. It does sound like my shop vac test proves nothing. Also sounds like I really should have done more testing BEFORE I dropped the exhaust system. Oh well.

So, symptoms. 1) The car idles rough. I did the usual maintenancey things which didn't help. I first thought vacuum leak, but I went through every hose and could find nothing. Weirdly the only thing that made the car run better was disconnecting the PCV system. Not usually a problem, and I replaced the PCV valve and hose. After a few weeks, even with the PCV valve disconnected, it still runs lousy; in other words it's getting worse.

2) Struggles to get uphill. My mom had a Gran Fury in '87. It wasn't that fast, but it wasn't that slow either. This thing slows down to 50-ish on a steep hill. No way that's right.

3) The cruise control only works sometimes. Other times it seems to maintain 10 mph below the set speed, or not at all. Maybe not related, but it made me think low vacuum. And if there aren't vacuum leaks, what else causes low vacuum?

4) A bit of a stretch here, but last time I started it I just pulled it around back and put it up on ramps, just a few minutes. Then I got under and started messing with the exhaust. The main cat was merely warm, so I felt safe. Then I touched the passenger side pre-cat. Ouch, big time. So...much hotter at the pre cat than the main cat...(Not quite as scientific as a pyrometer of course).

I do have a cheap boroscope that hooks up to a laptop. I'll try that tomorrow.

Thanks,

-mB
 

Mikes5thAve

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New plugs and cap/rotor? timing? EGR openining or gummed up and stuck partially open at idle...
Can very easily need carburetor work or the computer or one of it's sensors took a dump. If you don't have a factory service manual get one. it'll save you aggrevation and money on not replacing needless parts in the long run. They have step-by-step trouble shooting charts and info on how to check the different systems.
 

Aspen500

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Might want to pull the egr valve off and visually check if it's stuck open, will cause a rough idle.
The pre cats will get hot within a minute or two, while the main (downstream) cat won't. The pre cats are designed to work that way, which is why they're as close as possible to the manifolds. Get them hot and working quick, until the main cat comes up to temp. They don't "convert" until a certain temp. Threy would have to be really plugged bad to affect idle and the car wouldn't be able to accelerate hardly at all.

Compresion rest wouldn't be a bad idea either. Possible the timing chain jumped a couple teeth. That would cause a rough idle and lack of power also. A quick test would be to check the ignition timing. If it's way off, and the ditributor hasn't turned, that would point to the timing chain.

Wanted to add, the outer case tempof a cat is about 600 degrees max, the internal substrate temp is typically 800-1,600 degrees while driving, depnding on load, rpm, ambient temp, etc.
 
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BudW

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I do see from time to time cat's that are white or cherry red. Sometimes there is a look that doesn't look like normal steel. Either way - DO NOT TOUCH a catalytic converter when car is running or for a good time afterwards (it can burn you to the bone before you feel anything)!

There is only one place to test for exhaust back pressure on an FMJ and that is by removing the drivers side Oxygen Sensor (O2 Sensor), if equipped, and screwing pressure gauge in there. Another thing to do is to loosen all four exhaust manifold to Y-pipe nuts and drop Y-pipe down a quarter inch or so and perform a road test. It will be loud but if you have a pickup in power than you know what to point a finger to. That said, exhaust bolts that are 30-40 years old do not loosen well and may break off because of rust.


I've known for a long time the cats on my '86 fifth Ave were restricted just because I can feel the lack of power when at a constant load (going up a hill, etc.) and because at a certain point car sounds like “a plastic Walmart bag flapping in the wind” all of a sudden – which quits just after the engine load is released.
On a different note, that "restriction" has been recently upgraded to a "stopped-up" converter. Now car will start/run fine and will do so for 10 feet, then die. After car dies, you can here what sounds like a huge balloon with a small leak. After hisses dies, car will start right back up – but not until then. I was planning getting that fixed (IE: with a new Canadian Y-pipe) but been fruitless finding one.

In my area, an exhaust shop will not remove a converter, only replace. Now if a car came into shop missing the converters, then they have no problem building a new system without them. The key is to remove old, um, parts first. You could say a thief took 'em . . . or something.


I'm pretty sure the reason my cats are stopped up is because of excessive oil consumption. The 318 in the Fifth is getting tired. Excess fuel burns for most part clean – when it hits the converters. Oil, not so much, and after a while it starts to clog things. I would bet someone a $100 bill that my valve guide seals are broken/shattered and a big part of the oil consumption (as well it also needs a timing chain and ...).
My problem is I don't want to spend any money on the 318 when the stroked big block is not far from being dropped into it.
BudW
 

Aspen500

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You could cut the cats out and splice in "test pipes" to fill the gap for now. I've been burned by cats so many times in doing my job at work, I don't even feel it anymore.:eek: Well I FEEL it but it doesn't hurt anywhere near what it did 30 years ago.

We don't have this where I work but when I was at the Ford dealer, there was a kit with pressure transducers and a tester. You drilled a hole in the pipe and threaded the probe in for testing and then afterwards, there was a plug that threaded in the hole to plug it again, although we usually MIG welded the hole shut instead (called CYA). That was the ultimate in "it's restricted/not restricted". Haven't seen one of those in years though. In some cases putting a probe before and after a cat, muffler or even double wall pipe if you suspected the inner pipe may have collapsed, was much easier said than done:confused:
 

AJ/FormS

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Pressure testing is too late now, but I do the same thing;
drill a small pre-cat hole,braze in a length of brake-line, install a looooog vacuum line and fuel-pump pressure tester (8 to 10psi max), and roadtest it.
I draw the line at between 4 and 2 psi, depending on the performance level of the engine. Two psi or less is the target.
 

Remow2112

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Do they sniffer test in CA? Block off the EGR. Cut up a soda can and put it between the EGR and intake. It will not burn out have done it for years and on some pretty hot motors. Buy an MSD 6a and convert to standard ignition. If that doesn't help very good chance you have plugged cats. If you get and MSD change plug gap to .40.

Dan...
 

mgbeda

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Hi Again,

Life had gotten in the way of my working on this, but I got some work done this weekend again.

Going back a bit, I have changed the plugs cap and rotor. I also tried blocking off the egr and it made no difference (also the egr is definitely not stuck)

When I first got the car I checked the timing, and it was way advanced, at least 20 degrees. Thing is I don't know who set it last or when. I set it back to spec and that made the car run worse. I compromised on about 10 btc.

That makes it sound like a timing chain, so I did two tests on that. One, measuring how far I can turn the crank before the rotor moves. That is 6 to 7 degrees. Not good, but is that bad enough to make it idle rough? Second there is a test described in the shop manual where you make a tool you can use to measure the location of the cam sprocket by taking the fuel pump out. I did this and it shows the cam sprocket is in the right place, i.e. the chain has NOT jumped a tooth.

So question is, if the timing chain is loose but not slipped, is that enough to cause rough idle? And the real question is should I tackle the huge job of changing the timing chain? Or should I try something else, like a carburetor rebuild (also a huge job, at least time-wise.)?

Lastly, yes, in California anything newer than 19 freakin 75 has to pass smog. Same in Colorado where I used to live.

Thanks all,

-mB
 

AJ/FormS

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Post #5 does point to a power loss, and the go-to test is a compression test. Just one dead cylinder in a V8 is a 12.5% power loss by the math, but in real life is more cuz the 7 remaining cylinders still have to drag that POS dead cylinder around everywhere they go.

The rough running, points to the same thing, a power loss at idle, in one or more cylinders.

You were right to check the cats, and perform the ignition tune up ....... it shouldda just been done in the right order, and after testing.

The Compression test will point you in the direction in which you need to go.
 
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