How do you check the Cat ??

Mike Mike · Jul 13, 2010

Got a 94 4.0 5spd
Over like a week's time lost lots of power on hills and when pulling. lots of blowby out the dip stick on the climbs and pulls. No blowby out the front breather tube. Runs great around town. very little blowby. Cat looked like a new one when I bought the Jeep a year ago.
So going to check compression and plugs tommorow. Saw someone say about checking Cat with a vacum test. How is that done? Anyother things to check?

Banditt007 · Jul 13, 2010

here is a link about reading vacuum gauges. http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

Other things to look for, for a faulty cat are the following.

Cat is glowing and or piping in front of it glowing hot
when you hit it with a closed fist/mallet it rattles like there is rocks in it
your exhaust smells like rotten eggs
when you turn the truck on or off you hear the cat rattle around.

CaliXJ · Jul 13, 2010

Where is the best place to hook a vac guage to for this test?

Andy Steiner · Jul 13, 2010

Whack it with a dead blow hammer. If it rattles, it's dead.

cruiser54 · Jul 13, 2010

Long sleeve shirt. Two latex gloves. Lift his tail and work quick.

Talyn · Jul 13, 2010

CaliXJ said:
Where is the best place to hook a vac guage to for this test?

In front of the cat to check for back pressure. Maybe behind it as well for comparison.

cruiser54 · Jul 13, 2010

Talyn said:
In front of the cat to check for back pressure. Maybe behind it as well for comparison.

Connect to the intake manifold, perhaps the vac hose for your CCV system. Note the vacuum reading at idle. Rev the engine to 2000 RPM and hold steady. That reading should be the same as the idle reading. If it's lower, exhaust restriction. As an additional test, after holding at 2000 RPM, let the throttle snap shut. The vacuum should quickly rise and then very soon after drop right down to the idle reading. If the needle comes down slowly and jerkily, exhaust restriction.

Talyn · Jul 13, 2010

cruiser54 said:
Connect to the intake manifold, perhaps the vac hose for your CCV system. Note the vacuum reading at idle. Rev the engine to 2000 RPM and hold steady. That reading should be the same as the idle reading. If it's lower, exhaust restriction. As an additional test, after holding at 2000 RPM, let the throttle snap shut. The vacuum should quickly rise and then very soon after drop right down to the idle reading. If the needle comes down slowly and jerkily, exhaust restriction.

I was referring to the back pressure check test where you drill a hole in the exhaust and put a pressure gauge on it. http://www.aa1car.com/library/converter.htm

Mike Mike · Jul 13, 2010

Thanks for all the info. don't think Cat wore out more pluged up.

cruiser54 · Jul 14, 2010

Talyn said:
I was referring to the back pressure check test where you drill a hole in the exhaust and put a pressure gauge on it. http://www.aa1car.com/library/converter.htm

I see that now. He was talking vac gauge and you were talking pressure gauge. The old vac gauge test works quite well. I found that technique quite by accident years ago when I read the instruction manual that came with my vac gauge. There are some pretty cool tests you can do with that simple old thing.

How do you check the Cat ??

Mike Mike

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Banditt007

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CaliXJ

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Andy Steiner

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cruiser54

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Talyn

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Mike Mike

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