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How to diagnose a plugged cat. converter?


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11 replies to this topic

#1 Monte

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Posted 10 March 2006 - 08:43 PM

So, how do you diagnose a plugged catalytic converter? I've heard of them being plugged, but how do know that's the problem? I'm trying to figure out why my '86 carbed GL Wagon is so low on power. I had a leaky vacuum advance actuator, fixed that, now it pings a little but doesn't really accelerate much better.

The car sat in the back yard for 3 years, so the carb might be in bad shape, but it runs smooth, just not much power.

Monte

#2 Brumby Boy

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Posted 10 March 2006 - 08:54 PM

well if you suspect the cat remove the y pipe wit the cat in it and go for a drive and see if it has power if it has you know you need it replaced if it still hasnt got power theres somethin else

thas the only way i know of withont special gadets

PS is the fuel the stuff thats been sitting for 3years?

#3 ShawnW

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Posted 10 March 2006 - 09:41 PM

Theres a vac gauge test you can do. Do a search for posts by me that have vac gauge/cat converter in them.

#4 hooziewhatsit

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Posted 10 March 2006 - 09:48 PM

in my case, for a completely plugged cat, I couldn't get over 35-40ish on flat ground, and had a whistle from somewhere in the engine compartment. This was a car with SPFI.

I tapped on the cat with a rubber mallet and heard the little pieces of the element bouncing around.

Ended up cutting the cat open, removing the element, and welding it shut again.

#5 Monte

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Posted 10 March 2006 - 09:49 PM

well if you suspect the cat remove the y pipe wit the cat in it and go for a drive and see if it has power if it has you know you need it replaced if it still hasnt got power theres somethin else

thas the only way i know of withont special gadets

PS is the fuel the stuff thats been sitting for 3years?


The old fuel is gone by now, plus a can of Seafoam has been run through the fuel tank. I suppose I should probably replace the fuel filters too, the more I read about them/it, I have no idea when those have been changed.

Monte

#6 Monte

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Posted 10 March 2006 - 09:50 PM

Theres a vac gauge test you can do. Do a search for posts by me that have vac gauge/cat converter in them.


Will do.

Monte

#7 jeffast

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Posted 10 March 2006 - 09:51 PM

ask any one who was at the last colorado meet my car sounded like a hoover it blew out the exhaust donut

#8 Monte

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Posted 10 March 2006 - 09:51 PM

in my case, for a completely plugged cat, I couldn't get over 35-40ish on flat ground, and had a whistle from somewhere in the engine compartment. This was a car with SPFI.

I tapped on the cat with a rubber mallet and heard the little pieces of the element bouncing around.

Ended up cutting the cat open, removing the element, and welding it shut again.


Mine will still get it up to 80, but it takes awhile. Can't pull I-5 hills in 5th, that's the part that bugs me.

Monte

#9 Monte

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Posted 10 March 2006 - 10:01 PM

Theres a vac gauge test you can do. Do a search for posts by me that have vac gauge/cat converter in them.


This one?

http://www.ultimates...t=vac gauge cat

I'll give it a go tomorrow, thanks.

Monte

#10 leadfootracin

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Posted 10 March 2006 - 10:30 PM

you can test with vacum or pressure, either uses a low cost vacum/fuel pressure gauge. DO NOT use this gauge for testing fuel pressure on a fuel injected car. gauge normaly reads between 25" of vacum and 10# of fuel pressure, to test using vacum hook the gauge to manifold vacum, below the carb, or throttle body, engine warm, idle check vacum, should be more than 15" if the vacum drops as the engine is idling steady than there might be a restriction in the exhaust, to test using pressure you remove an o2 sensor or drill a small hole in the pipe ahead of the cat, for the o2 hole drill a hole in a small superball hook your tester hose to the superball and hold it against the bung, have a friend rev the engine and let off, the pressure should not go over 2#, the exhaust is very hot, dont get burnt. converters can meltdown, sending converter parts down the exhaust system so replacing a bad converter may not fix a prob w/ restricted exhaust, you can test the other parts the same way by drilling holes after the cat after bends, etc, drill or use self tapping screws so that you can seal the holes. sorry so longwinded but good luck:eek:

#11 dxrflyboy

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Posted 11 March 2006 - 07:49 AM

Mine will still get it up to 80, but it takes awhile. Can't pull I-5 hills in 5th, that's the part that bugs me.

Monte

If the cat is plugged, it won't make a difference whether you're in 5th or not. If it has enough power to pull the hill in a lower gear, the cat is probably OK. The feedback systems in these cars are notorious for leaning out the mixture too much and killing power. They lack the sophistication of today's systems and overcompensate to the lean side by creating vacuum leaks rather than precisely meter the fuel mixture. Combine that with an engine that doesn't have a lot of torque at low speed and you can understand why you have to downshift on a big hill. If you can find a steel ball of the right size, you can put it in the main vacuum hose that leads to the air suction devices and fool the smog police. Just take the ball back out to pass your emissions test.

#12 Spiffy

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Posted 11 March 2006 - 02:50 PM

whatever you do, don't take the muffler shop's word for it... I had 2 shops, one Mineke and one small local one, both tell me they thought my catalytic converter was clogged... of course it's $250 and plenty of labor for them to put a new one in for you...

after a tune-up (and Superior Import Repair attaching the rest of the exhaust for me) it ran awesome and passed emisions with no problems...

so definately check everything else before replacing it... vacuum and carb...

and it you're going to just remove it to see if that helps make sure you don't drive it too far, or coast fast down a steep hill... you don't want that cold air getting back up in there if you don't have anything coming off the heads...

--John


So, how do you diagnose a plugged catalytic converter?

Monte






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