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How to tell if there is a restriction in the exhaust using a vacuum gauge?


Bgunner

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I have read the "How To" on vacuum but it does not mention what the vacuum gauge will read and do if there is a restriction in the exhaust, like a plugged cat. What should I be looking for to tell if I have an issue with exhaust restriction? :icon_confused:

current test results:

Idle: 18 Hg but fluctuates 1 Hg over and under, Not rapidly so I don't believe there is an issue there.

Throttle blip: starts at 18 Hg drops to 0-1 Hg swings up to 23-24 Hg then goes to back to 18 Hg quickly. According to the How To this is fine also. Add in the compression tests I know the motor is physically fine internally.

The purpose of all the testing is I'm getting a drive train vibration under heavy loads, kinda acts like a miss but it isn't missing. My instinct says injectors causing a week cylinder but I want to be sure it isn't something else first. Any help and advice you may have is greatly appreciated, Thank you.
 


adsm08

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Exhaust restriction will show as low vacuum. Yours is low, but I think its a bit higher than is typical of a fully plugged cat. That doesn't mean there isn't a partially plugged one.

Try removing the oxygen sensors and see if it comes up.
 

Bgunner

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I'll give that a try when the weather is better, not raining. Thanks adsm08, I'll report back on the results of pulling the O2 sensor.

EDIT: how big of a jump should I be looking for on the 3.0? Basically what should it run for vacuum because the "how to" says 17-20 for average?
 
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I hold throttle steady at about 2k rpms, if the guage holds steady your exhaust is ok. If the vacuum slowly drops you have a restriction. The faster the drop the worse the restriction. Simple low vac at idle speed can be multiple issues, the steady throttle test is more accurate for exhaust issues.
 
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Put your hand in front of the exhaust pipe and have someone operate the throttle the air flow coming out should get greater as the rpm increases it it stays the same there is blockage in the system!
 

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I'll give that a try when the weather is better, not raining. Thanks adsm08, I'll report back on the results of pulling the O2 sensor.

EDIT: how big of a jump should I be looking for on the 3.0? Basically what should it run for vacuum because the "how to" says 17-20 for average?
Engine wear figures into it too, but at idle it really should be 20. A plugged exhaust usually lands you into the 13-15 range.
 

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Restricted exhaust in my experience shows up more under load then idle. Try the off idle testing dirtman described. If you don't pull an acceptable level of vacuum... seperate the exhaust in front of the cat and test again. What you describe sounds like it could be an exhaust restriction... but could also be the ignition system breaking down under load. Typically a bad injector is just bad and will be noticed through all rpm ranges.
 

Bgunner

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Uncle Grump, what exactly are you referring to for ignition system break down? The reason I ask is so far, due to other issues, I have replaced the coil, distributor, cap, rotor, wires and plugs and the issue remains.
 

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On another vehicle I made a quick disconnect adapter to replace the O2 sensor. I then hooked up a low pressure PSI gauge and ran the engine to see ig any back pressure.

It took a little work to make the setup but I can know for sure if I have back pressure.
 

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They sell simple adapter kits for a basic vac guage for less than 5 bucks at HF. Basically just a set of big triangular rubber plugs you jam in an o2 sensor hole or whatever. If the o2 sensor is a bitch to get to you can also just drill a #10 hole before the cat, test there and then plug the hole with a SS self tapping screw.
 

Bgunner

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My vacuum kit has 2 or 3 cone shaped adapters, the ones from HF, I'll see if they fit the o2 whole, maybe I'll get lucky and one will be the right size.

I did run the vacuum test with the engine running at 2k RPM but haven't gotten to the o2 sensor yet because it's chilly here with no garage to work in.


results:

Idle vacuum today: 19 bouncing to 20
2K RPM : 22 steady.

Any thoughts?
 

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Uncle Grump, what exactly are you referring to for ignition system break down? The reason I ask is so far, due to other issues, I have replaced the coil, distributor, cap, rotor, wires and plugs and the issue remains.
Did you replace the entire ignition system before or after your original question?

A restricted exhaust will exhibit a symptom that the vehicle will just not have power. you can't just drive around the problem or symptom. If the exhaust can't clear the combustion chamber and expel it out the tail pipe.. there is no way it can pull in a fresh air fuel charge to burn. You would be lucky if it even reaches cruising speeds. If you can reach highway speeds... chances are the exhaust isn't restricted.
 

Bgunner

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Did you replace the entire ignition system before or after your original question?
I replaced the Coil with a Accell super coil for hotter spark, Distributor: due to the bushings being bad in it, Cap: standard tune up, Wires: standard tune up Rotor: standard tune up and plugs: standard tune up in august of last year but just replaced the plugs again last month due to breaking some of them while doing the compression test. This shake has been there since I purchased the truck at the end of last July.

I replaced them before the original question but the issue was there before I got the truck.

(Edit: ) I am replacing the radius arm bushings tomorrow so if I don't hurt to much after that I will pull the o2 sensor and check the back pressure. What is the normal range of back pressure these things should have?
 
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Bgunner

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Anyone know how much back pressure the 3.0 has stock? I will be doing the work in a few hours and would like to know before the work is done. I know there is back pressure because of the cat and muffler but do not have any clue what would be normal and what isn't.

My issue would be, if any, a partially plugged cat because acceleration and getting up to speed seems fine but the issue has been there since I got it so I would not know what normal would be for this truck.
 

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Exhaust restriction will show as low vacuum. Yours is low, but I think its a bit higher than is typical of a fully plugged cat. That doesn't mean there isn't a partially plugged one.

Try removing the oxygen sensors and see if it comes up.
Try this^^^. Take vacuum reading before removing o2, then do it again after. Do both at idle and at 2000 rpm. Make sure to do the reading for 20 or 30 seconds. Post results.
 

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