Catalytic Converter Diagnosis

Catalytic Converter Diagnosis

Ed Ruiz Replied to a post about poor performance under load above 4k:

>> It sounds like you've done a very thorough test of nearly
>> all the systems that could cause the problem. I can only
>> think of one more; The Cat. If it is clogged, or partially
>> melted, the engine will not rev freely as the back pressure
>> builds up. YMMV.

This is a problem that afflicts cars that have been run underexcessive load, where the mixture is excessively rich and lots of load/RPMs. There's also an air pump issue that's a possible root cause too.

Simple diagnosis for a plugged cat: A common vacuum gauge will help, attached to the intake manifold. Manifold vacuum will normally reduce when you first tip in on the throttle. On a steady throttle position, the vacuum will rise as the engine rpms rise and the load is reduced. With the cat plugged, the vacuum will stay low. In your case, you can watch the vacuum drop to almost atmospheric pressure as you run out of power under load at 4k rpm, even at the partial throttle opening. FWIW, this is the same phenomena you'll see when the engine is running perfectly at full throttle, full load, full RPM. No manifold vacuum means that the engine can't draw any more air/fuel charge.

For those with supercharged cars this method does not apply perfectly, but turbo car owners may also find it handy.

Most engineers (and pilots...) will relate to intake pressure instead of vacuum. Substitute "manifold pressure is high" for "vacuum is low" and you'll be right at home with the above method. ;-)

Hope this helps!

dr bob

A partially plugged cat or exhaust can cause some interesting problems. The easiest test is to put a vacuum gauge on a manifold port. Check the vacuum on a warm engine at idle (probably 15" to 18" mercury). Pop the throttle, revving the engine to about 3000 rpm, and hold the revs steady there. The vacuum should drop momentarily, then climb to a higher number (probably 20" to 25"). If the vacuum stays steady, the exhaust probably isn't plugged. If the vacuum starts dropping after a few seconds, the exhaust is probably partially plugged.

Wally Plumley
928 Specialists

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