checking engine vacuum
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Postmaster
Joined: Dec 2001
Posts: 3,742
Likes: 2
From: UT
checking engine vacuum
'92 Explorer 4.0 L V6 OHV engine
Yesterday, I put a vacuum gauge on my engine to see what it would do. At idle, I had 15 in Hg, (Normal seems to be around 20 in Hg, but, assuming a constant manifold absolute pressure, I'd lose about 5 in Hg jsut from altitude) so that seems normal. At increased engine speed (no load, transmission in park) I got different results. Twice, the engine vacuum just dropped off to almost no vacuum at 2500 rpm. Three times I got a more normal response -- the vacuum would stay high.
I don't think there are any vacuum leaks, so I'm thinking there's a problem in the exhaust system
My questions:
1) Losing vacuum is often considered a sign of a plugged cat, but wouldn't that be more constant? Can you have a cat that is intermittently plugged?
2) I already know the muffler needs to be replaced -- it's leaking. Could the muffler cause similar results, if it's rusted out inside and restricting flow?
Thanks in advance for your input.
Yesterday, I put a vacuum gauge on my engine to see what it would do. At idle, I had 15 in Hg, (Normal seems to be around 20 in Hg, but, assuming a constant manifold absolute pressure, I'd lose about 5 in Hg jsut from altitude) so that seems normal. At increased engine speed (no load, transmission in park) I got different results. Twice, the engine vacuum just dropped off to almost no vacuum at 2500 rpm. Three times I got a more normal response -- the vacuum would stay high.
I don't think there are any vacuum leaks, so I'm thinking there's a problem in the exhaust system
My questions:
1) Losing vacuum is often considered a sign of a plugged cat, but wouldn't that be more constant? Can you have a cat that is intermittently plugged?
2) I already know the muffler needs to be replaced -- it's leaking. Could the muffler cause similar results, if it's rusted out inside and restricting flow?
Thanks in advance for your input.
Freshman User
Joined: Jul 2002
Posts: 44
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From: Memphis USA
checking engine vacuum
mrshorty, any more going on here?
15" seems a little low - I see 20" on a well-tuned ride at 300' but I don't know what happens at altitude. I don't think that an exhaust problem would cause manifold vacuum to drop to zero. How long does this last?
15" seems a little low - I see 20" on a well-tuned ride at 300' but I don't know what happens at altitude. I don't think that an exhaust problem would cause manifold vacuum to drop to zero. How long does this last?
Thread Starter
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Postmaster
Joined: Dec 2001
Posts: 3,742
Likes: 2
From: UT
checking engine vacuum
I've been watching it off an on since then, and vacuum only went to zero those couple of times. The muffler has been changed this month. My engine vacuum is still ~15 in Hg at idle. Maybe I should get a longer piece of vacuum hose so I can bring the gauge into the driver's seat with me and watch it a little while driving.
