vacuum readings on a 300 I-6?

What's a normal vacuum reading on a Ford 300 I-6? I installed a vacuum
gauge over the weekend and when I drove the truck on Sunday high idle
vacuum was very high, 18 or 19 in. Hg. and idle in drive something like
15. Coasting would hit 21 or 22 in. Hg. Today I drove it again and it
never got about 16 in. Hg. what gives? I don't *think* I have any
vacuum leaks...

nate

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On Tue, 04 Mar 2008 19:43:55 -0500, Nate Nagel > wrote:
>What's a normal vacuum reading on a Ford 300 I-6? I installed a vacuum
>gauge over the weekend and when I drove the truck on Sunday high idle
>vacuum was very high, 18 or 19 in. Hg. and idle in drive something like
>15. Coasting would hit 21 or 22 in. Hg. Today I drove it again and it
>never got about 16 in. Hg. what gives? I don't *think* I have any
>vacuum leaks...

RPMS raise the vacuum. Opening the throttle drops the vacuum.
If you are coasting at high RPM, you'll have the highest vacuum. At that
point you can decide if you have a vacuum leak.

Keep in mind that vacuum guages are **** poor at giving you a sense of
fuel economy. They'd have you driving everywhere in first gear if you
believed the readings. You can have peak vacuum readings most of the time
and still only get half to a third the fuel economy that you should because you are
are accelerating to cruising speed too slowly and in too low a gear for too
long.


And of course, make sure you didn't kink the line while installing the guage.
You might have a vacuum leak in the line to the guage or in its fittings.

Nate Nagel wrote:
>
> What's a normal vacuum reading on a Ford 300 I-6? I installed a vacuum
> gauge over the weekend and when I drove the truck on Sunday high idle
> vacuum was very high, 18 or 19 in. Hg. and idle in drive something like
> 15. Coasting would hit 21 or 22 in. Hg. Today I drove it again and it
> never got about 16 in. Hg. what gives? I don't *think* I have any
> vacuum leaks...
>

Something causing gauge to stick at 16?

Your first readings sounded about right. If the engine had changed that
much you should notice some performance difference.


> nate
>
> --
> replace "roosters" with "cox" to reply.
> http://members.cox.net/njnagel

On Mar 5, 7:15*am, jim > wrote:
> Nate Nagel wrote:
>
> > What's a normal vacuum reading on a Ford 300 I-6? *I installed a vacuum
> > gauge over the weekend and when I drove the truck on Sunday high idle
> > vacuum was very high, 18 or 19 in. Hg. and idle in drive something like
> > 15. *Coasting would hit 21 or 22 in. Hg. *Today I drove it again and it
> > never got about 16 in. Hg. what gives? *I don't *think* I have any
> > vacuum leaks...
>
> Something causing gauge to stick at 16?
>
> * * * * Your first readings sounded about right. If the engine had changed that
> much you should notice some performance difference.

That's kinda what I was thinking too. it's a cheap gauge. only thing
I did between sun. and yesterday was to change the bulb for the
backlighting, that makes me wonder if somehow it's catching on
something in the mechanism. I didn't think of that before, I'll have
to remove it (easy) and see if that changes anything.

nate

N8N wrote:
> On Mar 5, 7:15 am, jim > wrote:
>
>>Nate Nagel wrote:
>>
>>
>>>What's a normal vacuum reading on a Ford 300 I-6? I installed a vacuum
>>>gauge over the weekend and when I drove the truck on Sunday high idle
>>>vacuum was very high, 18 or 19 in. Hg. and idle in drive something like
>>>15. Coasting would hit 21 or 22 in. Hg. Today I drove it again and it
>>>never got about 16 in. Hg. what gives? I don't *think* I have any
>>>vacuum leaks...
>>
>>Something causing gauge to stick at 16?
>>
>> Your first readings sounded about right. If the engine had changed that
>>much you should notice some performance difference.
>
>
> That's kinda what I was thinking too. it's a cheap gauge. only thing
> I did between sun. and yesterday was to change the bulb for the
> backlighting, that makes me wonder if somehow it's catching on
> something in the mechanism. I didn't think of that before, I'll have
> to remove it (easy) and see if that changes anything.
>
> nate

Followup: that's exactly what it was. Since you put the thought into my
head, I stopped as soon as I got home before I even went into the house,
reached behind the gauge and pulled the bulb out, et voila, back to
normal vacuum readings. So for now I will leave the bulbholder
partially out of the gauge and tape it into place, I think the real
"fix" is to replace the gauge. The whole reason I replaced the bulb was
because I was using a cheap "sunpro" parts store gauge, and found that I
couldn't see it at night, so I replaced the bulb (looks like a 53?) with
an 1816 in an attempt to be able to read it. The 1816 is tubular and
apparently longer enough to catch something in the gauge mechanism. I
don't like the backlighting at all in the Sunpro gauges, the one VDO
gauge next to it looks so much better at night even though in the
daytime they are near identical in appearance. Lesson learned, pay the
extra $10-20 for good gauges.

To the other poster, I really didn't expect the vacuum gauge to help me
with economy (economy in a full-sized pickup? heh.) I added it for
primarily two reasons: 1) to keep an eye on engine condition and tune
and 2) because I wanted to add an oil pressure gauge and a trans temp
gauge, and I already had a three hole gauge panel that I wasn't using
for anything and buying a new two hole panel was about the same price as
a cheap vacuum gauge.

nate

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