85 Chevy Caprice. Restricted Exhaust ?

Hi,

I just did a vacuum test on my 85 Chevy Caprice V8 305. At idle I
get a reading of 20, when I rev the engine, it drops right down to 15
or 10, and builds back up gradually to 20 in a couple seconds, but
never over 20, until I let go of the throttle, at which time it pops up
to 25, then settles back to 20.

Exhaust restriction ? Could it be something else ?

When I had a new tailpipe put on, they took it down from a 2 1/4
inch to a 2 inch. Would that have caused it ? After they put the
smaller pipe on, I noticed a jet engine sound on acceleration. The
noise is coming out of an inlet pipe to the cat. Exhaust gas is
rushihng out there, making the noise. Before the smaller pipe, the
noise wasn't happenning.

The car runs OK, but on hills, it isn't too strong. Even before the
new smaller tailpipe, it was not very strong on hills. Once in a while
I have gotten the rotten egg smell on long steep hills. At first I
thought the engine was just getting old, but maybe it's a clogged cat
or something ?

Most people I ask say a 2 inch pipe should be fine, so maybe it's a
combo of smaller pipe plus a partially clogged cat ?

In any event, do the vacuum numbers above indicate exhaust
restriction ?



Thanks

Last tune up was new spark plugs about 10K miles ago. Air filter is
pretty new ( less than 20K on it). Oil change every 2 to 4 K. Timing
set 30K ago. EGR tested OK last month at inspection.

> wrote in message
oups.com...
> Hi,
>
> I just did a vacuum test on my 85 Chevy Caprice V8 305. At idle I
> get a reading of 20, when I rev the engine, it drops right down to 15
> or 10, and builds back up gradually to 20 in a couple seconds, but
> never over 20, until I let go of the throttle, at which time it pops up
> to 25, then settles back to 20.
>
> Exhaust restriction ? Could it be something else ?

normal <Excellent> operation!

> When I had a new tailpipe put on, they took it down from a 2 1/4
> inch to a 2 inch. Would that have caused it ? After they put the
> smaller pipe on, I noticed a jet engine sound on acceleration. The
> noise is coming out of an inlet pipe to the cat. Exhaust gas is
> rushihng out there, making the noise. Before the smaller pipe, the
> noise wasn't happenning.
>
> The car runs OK, but on hills, it isn't too strong. Even before the
> new smaller tailpipe, it was not very strong on hills. Once in a while
> I have gotten the rotten egg smell on long steep hills. At first I
> thought the engine was just getting old, but maybe it's a clogged cat
> or something ?

perhaps or perhaps not!

> Most people I ask say a 2 inch pipe should be fine, so maybe it's a
> combo of smaller pipe plus a partially clogged cat ?
>
> In any event, do the vacuum numbers above indicate exhaust
> restriction ?

when was the last service done >Tune-Up<

BBA

> wrote in message
oups.com...
> Hi,
>
> I just did a vacuum test on my 85 Chevy Caprice V8 305. At idle I
> get a reading of 20, when I rev the engine, it drops right down to 15
> or 10, and builds back up gradually to 20 in a couple seconds, but
> never over 20, until I let go of the throttle, at which time it pops up
> to 25, then settles back to 20.
>
> Exhaust restriction ? Could it be something else ?

I'm a little confused why you think those readings indicate a problem,
particularly an exhaust restriction. Do you understand why vacuum drops
when you rev the engine? Why would it go up to 25 when you let go of the
throttle?

Bruce,

The manual that came with the vacuum guage said that if the engine
is revved and the guage reading heads toward zero then there could be
an exhaust retriction or an overactive egr valve.
When I rev the engine the guage reading does indeed head toward zero
(it instantly drops down 5 or 10 points , then rises again to 20, as I
maintain the rev ).
Perhaps the manual means to say that if the guage reading heads
toward zero and ** does not return to normal ** as you maintain the
rev, then there is a restriction.
Why it goes up to 25 when I let go of the throttle, I don't know.

No, I do not understand why vacuum drops when I rev the engine, but I
have read somewhere that it is normal.

On 19 Feb 2005 16:51:27 -0800, "
> wrote:

>Bruce,
>
> The manual that came with the vacuum guage said that if the engine
>is revved and the guage reading heads toward zero then there could be
>an exhaust retriction or an overactive egr valve.
> When I rev the engine the guage reading does indeed head toward zero
>(it instantly drops down 5 or 10 points , then rises again to 20, as I
>maintain the rev ).
> Perhaps the manual means to say that if the guage reading heads
>toward zero and ** does not return to normal ** as you maintain the
>rev, then there is a restriction.
> Why it goes up to 25 when I let go of the throttle, I don't know.
>
>No, I do not understand why vacuum drops when I rev the engine, but I
>have read somewhere that it is normal.
of course it is. where are you getting the vacuum
reading from? the carb? right. i suggest a little
study of how a carb works and the light will shine.
then you'll be able to use the vacuum gauge
correctly.
....thehick

Sounds like you're over reacting <grin>



Still think there is an issue -- perhaps there is! -- you're the one that drives
the vehicle in question, right! -- Your the person that would know if there was
an issue! -- You seem to be a bit persistent! >> Okay 1st how many total miles
on that (possibly rebuilt) engine?! If it is @ or around 150k then, do a wet/dry
compression test -- Next, post the wet and the dry readings!



BBA

I'm a part time auto mechanic with over 10 years experience. Your
vacuum gauge readings seem normal to me. At idle the engine is like an
air pump, pulling in air past throttle and shoving it out exhaust. You
have a fairly high reading (20in) at idle since engine is sucking air
through a nearly closed throttle, creating a vacuum in intake manifold.
As you open throttle you let in alot of air in intake manifold, so
vacuum drops a bit depending on how much you open throttle. But this
increased air and fuel speeds up engine so it sucks air and creates
vacuum in intake manifold even with a throttle that is more open. So
it returns to 20 or sometimes alittle higher. Also when you are
revving it you have no load on engine to slow it down with same
throttle opening, while going up a hill with highly opened throttle
vacuum might be 10 or lower.
The manual means if the gauge reading heads toward zero and stays
there as exhaust restriction will not allow engine to pump air out
exhaust fast enough so air backs up and vacuum can't be maintaned in
intake manifold, also engine will lose power.
When you close throttle the engine is still running fast even
though air and fuel is lowered but momentum takes awhile for engine to
slow down and you have just shut off most of air by closing throttle so
vacuum will go very high for a short period of time (25inches), again
this is normal.

Well, howsabout I reduce the "issue" to the quick plunge in vacuum when
I depress the throttle. The vacuum drops from 20 at idle to 15 or to 10
when I stab the throttle, but quickly, as in about 1 second, rebounds
up to 20. The manuals I've read don't address the temporary plunge. I
just want to know if that's normal. But I see from BBA's reply that
it is normal. Must be the sudden rush of "extra" air entering the
manifold that reduces the vacuum, temporarily, until the engine revs
catch up. Hmm. Sounds like the Venturi effect may be largely a function
of the speed of the air flow, not the volume. The sudden opening of the
throttle allows more volume, but less speed, therefore the vacuum
drops, but when the revs are high enough, the speed is back high enough
to bring the reading back up to 20. Oh, well, just a guess.

BTW the engine has 368,000 miles on it, and has never been rebuilt.
Everything's original except a bunch of bolton things like alternators
and waterpumps, gas pumps, etc.

I spose I may have some wear on the cam, meaning less volume is allowed
to be sucked into the cylinders for burning. I spose I have some valve
wear and ring wear too. Yeah, I know with this mileage, I can expect
some loss of power on hills.

I did a compression test about 100K ago. Wet and dry. It indicated
some ring wear, but the readings were within 10 percent of each other,
so the compression test was OK. Highest cyl was 185, lowest was 174,
rest were about 170, all dry.
Can't find the wet readings, but they were a normal amount higher than
the dry.

> wrote in message
oups.com...
> Well, howsabout I reduce the "issue" to the quick plunge in vacuum when
> I depress the throttle. The vacuum drops from 20 at idle to 15 or to 10
> when I stab the throttle, but quickly, as in about 1 second, rebounds
> up to 20. The manuals I've read don't address the temporary plunge. I
> just want to know if that's normal. But I see from BBA's reply that
> it is normal. Must be the sudden rush of "extra" air entering the
> manifold that reduces the vacuum, temporarily, until the engine revs
> catch up. Hmm. Sounds like the Venturi effect may be largely a function
> of the speed of the air flow, not the volume. The sudden opening of the
> throttle allows more volume, but less speed, therefore the vacuum
> drops, but when the revs are high enough, the speed is back high enough
> to bring the reading back up to 20. Oh, well, just a guess.
>
> BTW the engine has 368,000 miles on it, and has never been rebuilt.

<Gasp> Liar, <smile> that's Bull **** >YOU LIE!!!...



....Well, unless or course your the original owner AND your one of those far and
in-between few that takes supreme care of ones property!!!


> Everything's original except a bunch of bolton things like alternators
> and waterpumps, gas pumps, etc.
>
> I spose I may have some wear on the cam, meaning less volume is allowed
> to be sucked into the cylinders for burning. I spose I have some valve
> wear and ring wear too. Yeah, I know with this mileage, I can expect
> some loss of power on hills.
>
> I did a compression test about 100K ago. Wet and dry. It indicated
> some ring wear, but the readings were within 10 percent of each other,
> so the compression test was OK. Highest cyl was 185, lowest was 174,
> rest were about 170, all dry.



Excellent, I can see that you take (anal) care of your vehicle! <Grin>



BBA