Catayltic Converter

What is the probablility that my no start, sometimes start...run rough...shut of in 1 minute, problems are due to a boogered up cat?

If there is a good probablility (since I changed dang near everything else) I am considering sawzalling the exhaust to see if it will start.

That could be a can of worms though, as if it isn't the problem, I am stuck with no exhaust and a non-running XJ.

Any advice?

Any way to see if the cat is bad without hacking it apart?

Thanks..

Cottontail said:

What is the probablility that my no start, sometimes start...run rough...shut of in 1 minute, problems are due to a boogered up cat?

If there is a good probablility (since I changed dang near everything else) I am considering sawzalling the exhaust to see if it will start.

That could be a can of worms though, as if it isn't the problem, I am stuck with no exhaust and a non-running XJ.

Any advice?

Any way to see if the cat is bad without hacking it apart?

Thanks..

Click to expand...

Drill a 3/8 hole just ahead of the cat, attach a 0-10 psi gauge, you need 3# or less.

When you say, "just ahead of the cat" you mean between the cat and the manifold pipe correct.

If the guage reads 3# or less, does that mean it is good or bad?

How will this prove that the cat is bad or good?

A lot of times, the engine will cough on itself while it tries to start. I've determined that there isn't enough air coming in, and that leads me to wonder if there is nowhere for the air that is already in the system to get out. That is why is suspect my cat is clogged.

Pardon my ignorance, but I am not sure I follow the whole drill a hole test completely.

Thanks

Cottontail said:

When you say, "just ahead of the cat" you mean between the cat and the manifold pipe correct.

If the guage reads 3# or less, does that mean it is good or bad?

How will this prove that the cat is bad or good?

A lot of times, the engine will cough on itself while it tries to start. I've determined that there isn't enough air coming in, and that leads me to wonder if there is nowhere for the air that is already in the system to get out. That is why is suspect my cat is clogged.

Pardon my ignorance, but I am not sure I follow the whole drill a hole test completely.

Thanks

Click to expand...

3# or less means the cat's not plugged or the exaust system causing your problem. Hooking a presure gauge between the engine and the Cat will tell in it's plugged.
I have seen wasps/muddaubers plug the exaust system when a car is left sitting.
In my other thread I said it sounds like the potato trick where your jam a potato in someones exaust.

The idea is to see if the cat is creating a significant restriction in the exhaust - if you get more than three pounds on the pressure gage, the cat is clogged (or the matrix is collapsing) and should be replaced.

I tend to use a "universal fit" cat when I do a replacement, getting one that's spec'd for a BBChevvy. I've not had any trouble passing it by the Smog Nazis out here, and I cut the bolt flange off the OEM and have a pipe stub welded on to preserve the bolt joint (which comes in REALLY handy sometimes.)

"just ahead" of the cat can be pretty much anywhere in front of the catalytic converter but behind the collector - but it's preferred to do it within six inches of the cat inlet.

Performing "component pressure tests" is a common practise when diagnosing or tuning a full exhaust system - in this case, a small (usually 3/8") hole is drilled in the pipe a few inches forward of each component under test, and pressure readings are taken. The holes are plugged when not used. This can be used to select mufflers, resonators, catalytic converters, and even to test the results of changes in pipe section from one size to another. Desired pressure readings are low, but depend largely upon the results you're trying to achieve.

5-90

Now this test assumes that I can get the truck started...

More often than not, the truck won't start, it will just cough like it wants to start but is not getting air.

Is there a way to test if the cat is clogged, if the truck won't stay running?

Discuss...

Cottontail said:

Now this test assumes that I can get the truck started...

More often than not, the truck won't start, it will just cough like it wants to start but is not getting air.

Is there a way to test if the cat is clogged, if the truck won't stay running?

Discuss...

Click to expand...

If you see more that 3# when cranking it won't start.

Well, I bought a guage, but it was a tire guage and wouldn't register anything when I put it up to the hole in the exhaust.

There was air coming out through that hole, but, again, it wouldn't register. Is there another type of PSI guage that I need to use?

Just for fun, I decided to plug that first hole and drill another one between the cat and the muffler. There was air coming out of that one as well.

I suppose that means that there is good airflow through the cat right?

Now I am at a complete loss. I can't figure out why I am not getting this dang truck to start!

One more update....I just checked the airflow from the tailpipe and it is really minimal...just a bit more than non existant...

THis is on the crank, as it wouldn't start.

If you still have the bolt flange at the catalytic converter inlet, unbolt it and separate the pipes.

If it's a "no start" (sorry - I didn't read the whole thing - I just explained the test you were getting stuck on...) I typically diagnose exhaust problems by disconnecting the system in various places to remove a component.

If you can start it with the downpipe disconnected from the catalytic converter, you've probably isolated the problem.

If you don't have the 4-bolt flange anymore, you can disconnect at the collector - but don't let that run for too long. Those pipes are pretty short, and you might get enough reversion to suck cold air back into the pipe and warp a valve. As long as you keep it under a minute or so, you should be fine.

5-90

I still have the 4 bolt flange, but it is too rusty to do anything with. I tried to disconnect it there the other day. I'll just start at the muffler and see what I can do from there.


Thanks

It really is sounding like something in the exhaust system is plugged. If you read my post about the backpressure test. I said you need to use a gauge with a 0-10 psi rating. You should get yourself a Automotive compound test gauge so you can test both vacuum and pressure.

Thanks for clarifying the tool I needed. I guess I just wasn't too clear, and am not real knowledgable on what goes into these tests.

I think I'll tear into the exhaust tomorrow!