Catalytic Converter plugged up?

[cjnaz]

I haven't needed to post a question for a long time because I usually find the answer already here. I love this site!!

Here's the question: How can I tell if my catalytic converter is getting old and plugged up?
It's in a '90 1500 4X4 Suburban w/350 TBI.

Thanks for your help!!!

[maxwedge]

Do a back pressure test at the o2 sensor port, 1.5 psi or less is ok.

[cjnaz]

Would a compression tester work for this test or do I need another tool.

[klinn]

A compression gauge might not show you that small amount of pressure. But maxwedge is correct, GM usually lists the back pressure at 1.5 to 2 psi with the engine running at 2000 rpms. Some aftermarket tool companys make a backpressure tester, but it will cost you about $50 and who knows when you would ever need it again.

[Chantz]

Well when they're that old, is it safe to assume that they're probably plugged up?

[maxwedge]

You will not know without some kind of test or just put on a replacement.

[Willyswagn]

I have an 89 suburban. In my research I found they seldom plugged up on the trucks but they might not be functional. Do a google search and you will find a lot of info but no practical way to find out.

[klinn]

Are you having any problems or are you looking for information?

[Chantz]

I'm just looking for ways to improve fuel economy. Anything would help right now, and a plugged catalytic converter definitely would NOT be helping

[klinn]

When a converter is plugged up, the vehicle is virtually undrivable.

If you are looking for fuel economy repairs you need to look at the spark plugs, distributor cap, distributor rotor, plug wires, fuel filter, air filter, make sure the timing is correct, run an injector cleaner (like techron), then most important is driving habits. The oxygen sensors can have an affect on fuel economy, but that is an expensive part replacement for a hope of improving economy. Also, don't lose the concept of how long will it take to recoupe the cost of the parts by savings in economy. (ie: spending $300+ in parts, then getting 10% improvment in mileage could take some time to get back the $300+ spent). The biggest killers in fuel economy: short trip driving, city stop and go, poor driving habits, and of course lack of maintaining the vehicle. Hope this helps.

[cjnaz]

Well, I am going to replace the Catalytic Converter and replace the cracked muffler too. I was thinking of going with the typical Flowmaster and a stock converter. I also was going to replace the pipes from the manifold back. Lastnight I measured the outer diameter of the pipes and from the manifold to the converter was 2.3" and from the muffler back was 3.0". Can I/Should I put a 3" pipe from the manifolds back? I just put a rebuilt 5.7L TBI w/modifications back in it. The mods were an RV cam and an Edlebrock Proformer intake manifold. It runs great at RPMs over 900 but won't run at all under that. I am taking it to a performance shop next week to have the ECM reprogrammed and they recommended that I replace the Cat before bringing it in. Your thoughts?

[klinn]

Going with 3" will improve flow and will reduce normal back pressure. I was under the impression you were looking for economy, but after reading your last post, it doesn't sound like you are too worried about economy. There really isn't much room in the modified & performance world for economy. Sounds like a fun project vehicle though. Have fun building it.

[cjnaz]

Klinn - Thanks for the reply. I wasn't really looking for economy although it would be nice at today's gas prices. I am shooting for reliability and longevity more than anything. I bought this Suburban about 4 years ago from an old couple and have been restoring it since. I will be putting pics and a story up on this forum soon.