Re: RE: Pressure Test

From: Bob Tom (tigers@bserv.com)
Date: Wed Jul 12 2006 - 15:30:41 EDT


At 11:58 AM 7/12/06, Ray wrote:

>-----Original Message-----
>From: Jamie Calder
> My '96 318 won't go above 2000 rpm. I did a vacuum test and the cat seems
> fine.

>I'm curious how one determines the condition of a cat through a vacuum test?

I haven't tried it myself but it involves diagnosing the effect of a blocked exhaust
on intake vacuum.
a) Connect a vacuum gauge to an intake manifold vacuum source.
b) Warm the engine to operating temperature, place the transmission in Park
     and apply the parking brake.
c) Note and record the vacuum reading at idle.
d) Open the throttle until the engine speed is about 2000-3000 rpm.
e) Release the throttle quickly and record the vacuum reading.
f) Perform the test three more times, recording the reading after each test.

If the reading after the fourth test is more than one in-Hg lower than the reading recorded
at idle, the catalytic converter, muffler or exhaust pipes may be plugged or restricted.

Once you've determined that there is a restriction, then the next step would be to drill
some holes in front & back of the cat & take readings from those :-( If no or little difference
in readings then it's the muffler. The main reason why I've never tried it :-)

Bob

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