Hooking up vacuum gauge, I donít know which is the manifold port... - JeepForum.com
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post #1 of 12 Old 10-18-2021, 11:09 AM Thread Starter
GA1984Cj7
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Hooking up vacuum gauge, I donít know which is the manifold port...

I know this this a silly question but I donít know where the manifold port to connect the vacuum gauge too. Stock setup has emissions. 1984. There are ports on the side,front, rear of the carburetor and in the corners.



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post #2 of 12 Old 10-18-2021, 11:24 AM
John Strenk
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I actually unscrewed a plug in the intake manifold and put a hose fitting in there to hook up my vacuum gauge.

vacuum-port-intake-manifold.jpg

Usually all the good ports are used up on the BBD.

Look to see if the orange one at an angle is not used but it may respond slowly.
Usually it goes to the vacuum motors on the air cleaner housing so depending upon temperature there may be a leak.
The other orange line goes to the choke pull off so you want to keep that one in place.
bbdvacuumport.jpg

Blue one is ported so you don't want that. No vacuum when the throttle plates are closed.

The Yellow one goes to the PCV valve, that one has a pre-determined leak so it may not be accurate.

That's why I went with the intake manifold port.
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post #3 of 12 Old 10-18-2021, 11:41 AM
Matt1981CJ7
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Generally speaking any vacuum port that is at the base of the carb, or on the intake manifold will be manifold vac. Just make sure there is vacuum at idle.

I think both ports on the rear of the BBD are manifold vacuum. There should also be a manifold vac port on the intake manifold near the EGR. Your brake booster is probably connected to it.

If all manifold ports are being used, you can T into one of the lines to check vacuum. Just make sure to cap the T when you are done.

Edit: As John said, don't T into the PCV purge line, for the reasons he stated. I, too, installed a fitting on the #6 intake runner to check vacuum on my 258

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post #4 of 12 Old 10-19-2021, 05:13 PM Thread Starter
GA1984Cj7
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Hooking up vacuum gauge, I donít know which is the manifold port...

Thank You...just added. When I started the Jeep it went to 30. (Gauge defaults to 10 on left side) I ran it for 5 minutes when the rpms dropped and it was still at 30. Engine really wasnít warm. I will start researching the forum.
Edit- moved the needle now reads 20. I will run the Jeep more this weekend and see what readings I get.



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post #5 of 12 Old 10-19-2021, 05:32 PM
John Strenk
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Ah so the needle isn't at Zero.
Can you pull the cover off and reset it?
Most of them allow you to do that.


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post #6 of 12 Old 10-20-2021, 10:16 AM
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If you look closely at the gauge you'll see a red and a green line. You want to tune it to keep it in the green area. I always used a rule of thumb to keep it in the 20-10 range. At idle it should read at a higher number and the number gets smaller with acceleration. I have a vacuum gauge in my camper and if I can keep the vacuum less than 10 and closer to 20 I get better gas mileage. JEEPFELLER is the only one I know who has vacuum gauges in his CJs.

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post #7 of 12 Old 10-20-2021, 11:43 AM
John Strenk
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Quote:
Originally Posted by bob4703 View Post
If you look closely at the gauge you'll see a red and a green line. You want to tune it to keep it in the green area. I always used a rule of thumb to keep it in the 20-10 range. At idle it should read at a higher number and the number gets smaller with acceleration. I have a vacuum gauge in my camper and if I can keep the vacuum less than 10 and closer to 20 I get better gas mileage. JEEPFELLER is the only one I know who has vacuum gauges in his CJs.
Ahem...

I've had one on my jeep for a couple of decades now...


But I usually like to keep it above 10 to get good gas mileage

This is how it looks today
be142691-fbd3-4df3-9486-e865e3b2a937.jpg

I designed a holder for it to fit the steering column.
9c70ed07-8bf1-4d5d-80d9-cef15c19b103.jpg


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post #8 of 12 Old 10-20-2021, 05:30 PM
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Quote:
Originally Posted by John Strenk View Post
Ahem...

I've had one on my jeep for a couple of decades now...

https://youtu.be/izzWgcnMgRU

But I usually like to keep it above 10 to get good gas mileage
I apologize John, but I've never met your Jeep before. I've read a lot about it and have been impressed with the snow removal equipment you have installed. If I ever end up in your neighborhood I want to stop and see it. I want to make a CJ road trip someday.

I do have a question. In that the measurement of vacuum is a negative measurement which is more 10 or 20" Hg? Or is it which is less? I have the the multi purpose gauge--it measures vacuum or boost in my RV mounted with Zero at 12:00. My thinking is that Zero is the high mark and 29.92" Hg is the low point or perfect vacuum. When you say above 10 you mean at a higher number.

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post #9 of 12 Old 10-20-2021, 05:58 PM
John Strenk
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20” mean’s a better vacuum than 10”.
The higher the vacuum number the better.


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post #10 of 12 Old 10-20-2021, 07:37 PM
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Quote:
Originally Posted by John Strenk View Post
20” mean’s a better vacuum than 10”.
The higher the vacuum number the better.
We are paddling in the same direction only just expressing it differently.
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post #11 of 12 Old 10-25-2021, 01:04 PM
Charleston82CJ7
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Quote:
Originally Posted by John Strenk View Post

The Yellow one goes to the PCV valve, that one has a pre-determined leak so it may not be accurate.

That's why I went with the intake manifold port.
John,

Can you explain the predetermined leak?
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post #12 of 12 Old 10-25-2021, 08:37 PM
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Quote:
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John,

Can you explain the predetermined leak?
Even at idle, the PCV valve is sucking fumes out of the valve vover so there is basically a small vacuum leak there.

The 'leak' gets bigger as the manifold vacuum drops and it pulls more fumes from the crank case.
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