Baseline vacuum number

[Helirich]

So I've been concerned about the possibility of a plugged muffler. It's easy to confirm the cat is not plugged, but the muffler is much more difficult. If you google it, they tell you to do a vacuum test. The issue is there is a fairly wide span of vacuum at idle in good running motors. They say you should use your experince to tell you if your perticular motor is reading is low or high. Of course, that doesn't help if you have no experince with your perticular motor.

In my case, I checked the vacuum at idle and found it to be about 16 which is pretty low. The motor was not warmed up at the time as it should be, so I don't know if it would go up. I was scared to keep running it because I just got done changing a exhaust manifold gasket and I didn't want to change the other one because of high back pressure.

So I removed the muffler and installed a straight pipe to clear the body. I warmed up the car. (Loud) Now the vacuum is about 18. Im not sure how much, if any, it should go down with the muffler. Do any of you guys know what your vacuum is at idle after warm up?

[dn010]

Have you checked your timing before all of this?


So I've been concerned about the possibility of a plugged muffler. It's easy to confirm the cat is not plugged, but the muffler is much more difficult. If you google it, they tell you to do a vacuum test. The issue is there is a fairly wide span of vacuum at idle in good running motors. They say you should use your experince to tell you if your perticular motor is reading is low or high. Of course, that doesn't help if you have no experince with your perticular motor.

In my case, I checked the vacuum at idle and found it to be about 16 which is pretty low. The motor was not warmed up at the time as it should be, so I don't know if it would go up. I was scared to keep running it because I just got done changing a exhaust manifold gasket and I didn't want to change the other one because of high back pressure.

So I removed the muffler and installed a straight pipe to clear the body. I warmed up the car. (Loud) Now the vacuum is about 18. Im not sure how much, if any, it should go down with the muffler. Do any of you guys know what your vacuum is at idle after warm up?

[Helirich]

Have you checked your timing before all of this?

Yes.

[dn010]

So I am confused about your post. You checked vacuum at idle on a cold engine and got a low number. You didn't warm up the engine and leave the gauge attached to see if the needle dropped to indicate a plugged muffler and instead removed the muffler and then took another vacuum reading? Why are you trying to diagnose a muffler issue with vacuum but dismantled the exhaust? It doesn't matter what your initial value is, if the exhaust system is clogged the gauge will show it. Where are you taking readings from? Are you plugging all other vacuum ports during this? When I was running the original engine, I attached the gauge at the large nipple on the back of the passenger side of the intake and if my memory is fine, I had 21in HG when tested living in upstate NY at an elevation around 1,500ft. Have you sent a bore scope into the muffler?

Also for what it's worth, I blew out every single exhaust gasket on one side of the engine or the other every time I replaced them on my original engine, mostly due to warped manifolds. I now run headers held on by 4 studs per port and I have no problem. If you are changing gaskets and not checking if your manifolds are straight, you are wasting your time.

[David T]

The way you use a vacuum gauge to see if you have a plugged up exhaust is to watch the gauge and if the vacuum starts to get lower as you run the motor it means because the exhaust is causing back pressure the motor can't maintain the level of vacuum it had. Rare on a Delorean for the muffler to clog up but the cat can.

[dn010]

You didn't warm up the engine and leave the gauge attached to see if the needle dropped to indicate a plugged muffler


The way you use a vacuum gauge to see if you have a plugged up exhaust is to watch the gauge and if the vacuum starts to get lower as you run the motor

You're almost at 8,000 posts of constant reposting of what has already been said.

[Helirich]

So I am confused about your post. You checked vacuum at idle on a cold engine and got a low number. You didn't warm up the engine and leave the gauge attached to see if the needle dropped to indicate a plugged muffler and instead removed the muffler and then took another vacuum reading? Why are you trying to diagnose a muffler issue with vacuum but dismantled the exhaust? It doesn't matter what your initial value is, if the exhaust system is clogged the gauge will show it. Where are you taking readings from? Are you plugging all other vacuum ports during this? When I was running the original engine, I attached the gauge at the large nipple on the back of the passenger side of the intake and if my memory is fine, I had 21in HG when tested living in upstate NY at an elevation around 1,500ft. Have you sent a bore scope into the muffler?

Also for what it's worth, I blew out every single exhaust gasket on one side of the engine or the other every time I replaced them on my original engine, mostly due to warped manifolds. I now run headers held on by 4 studs per port and I have no problem. If you are changing gaskets and not checking if your manifolds are straight, you are wasting your time.

In my research, they said a weak vacuum at idle is an indicator of restricted exhaust. If I remove the exhaust and get a reading and then install it and the reading is much weaker, it's plugged. Do you not agree? I attached the same place as you. I did plug the ports. It would appear your motor is in much better condition then mine. (Mine has almost a 100,000 on it) I don't own a bore scope.

I did resurface the manifold when I had it off.

[Josh]

You're almost at 8,000 posts of constant reposting of what has already been said.

The way you use a vacuum gauge to see if you have a plugged up exhaust is to watch the gauge and if the vacuum starts to get lower as you run the motor

Dave is still less annoying than Helirich. Guy made a thread asking for help but it sounds like he just wants people to tell him hes right.

[dn010]

No, I don't agree. I repeat, the vacuum will appear normal upon start up but the needle will soon drop to 0 if you have a plugged exhaust system. Using a vacuum gauge as a diagnostic tool isn't really common anymore, I work on engines from the 40's & 50's+ so the gauge comes out from time to time but you can't just hook it up, run to the computer asking what 16 means and then ask others to pull out their rulers and compare - you need a chart to know what the values/needle bounce means. You also diagnose before you start taking things apart and while Dave does constantly repeat what is already posted, I do agree that the exhaust getting plugged would be a rare thing. Even when mine was full of the converter pellets it still ran fine.

I did an engine swap so I don't run that engine anymore but it did have 70K+ on it. Sorry, I've seen multiple times where you mentioned you have a ton of tools so I assumed a bore scope would be one of them, I use that more than the vacuum gauge.


In my research, they said a weak vacuum at idle is an indicator of restricted exhaust. If I remove the exhaust and get a reading and then install it and the reading is much weaker, it's plugged. Do you not agree? I attached the same place as you. I did plug the ports. It would appear your motor is in much better condition then mine. (Mine has almost a 100,000 on it) I don't own a bore scope.

I did resurface the manifold when I had it off.

[dn010]

The way you use a vacuum gauge to see if you have a plugged up exhaust is to watch the gauge and if the vacuum starts to get lower as you run the motor

Dave is still less annoying than Helirich. Guy made a thread asking for help but it sounds like he just wants people to tell him hes right.

:cheers:

[Helirich]

When I googled it, I found this.

https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge/


In it says this

"Normal manifold vacuum at idle for an engine in good condition is about 18 to 22 in.-Hg. Manufacturers used to publish vacuum specs in service manuals, but this isn't as common as it was years ago. Still, the physics of internal combustion haven't changed in a hundred years, so the guidelines given here are a good starting point for vacuum gauge troubleshooting. Your best analysis based on vacuum readings will come from your own experience, however. As you use a vacuum gauge on different engines, you'll learn what's typical for one model compared to another. Some engines have reputations as low-vacuum motors; others are unusually higher than average. Experience is your best teacher."

I have used a vacuum guage one other time in my life. (Not for this purpose) So I have no experince to draw on. The first reading I got was 16, so I wasn't keen on possibly blowing the other gasket. That's why I pulled the muffler. If the reading goes up, (it did) I assume it is somewhat restricted.

In the artical, it also said this,

"Low vacuum also can be an immediate clue to a plugged exhaust. To check further, run the engine at about 2500 rpm for about 15 seconds. If vacuum drops during this period and does not increase when you close the throttle, you're almost certainly looking at a restricted exhaust."

Do you disagree with this?

I use to own a small factory. I took many of the tools when I sold it. I did not take the bore scope. I've cut open some mufflers and found multiple layers of heavy screen. A bore scope wouldn't be much help with one like that. Have you scoped one of ours and found it to be an open can?


Sorry for irritating questions.
Not sure how to ask a question that's not irritating. This is a forum. The purpose is to ask questions and open discussion. If you can't be bothered, don't read/answer.

[David T]

You're almost at 8,000 posts of constant reposting of what has already been said.

Actually you are mostly correct. I find many questions recurring. When a newbie gets his Delorean, instead of searching, they jump right to the keyboard and start asking questions. Many things are also seasonal, like when everyone starts up their car for the Spring or when it gets hot and there are lots of questions about A/C. It is getting rare to have to answer a really novel question anymore. As for my number of posts, I am on other forums and for whatever reason no matter which forum I post to, the total of all of them gets shown. I don't have that many posts on DMCTALK. Because I can't physically help every owner this is the best I can do, posting and answering their questions. My hope is to get as many Deloreans on the road as I can and in the process educate them so they can be successful and maybe help others.

[dn010]

"Low vacuum also can be an immediate clue to a plugged exhaust. To check further, run the engine at about 2500 rpm for about 15 seconds. If vacuum drops during this period and does not increase when you close the throttle, you're almost certainly looking at a restricted exhaust."

Do you disagree with this?



I am going to just jump to this part - did you do the test as they instruct? You also don't know what your initial vacuum was because you didn't check it in the same conditions that you did with the exhaust removed - engine warmed up, etc. so it is hard to have a comparison. If you have THAT much back pressure that you're going to blow an exhaust gasket, I don't think you'd see 16in HG with a cold engine, the value would have dropped and then you'd find yourself in trouble.

I never scoped my muffler, I cut the curved inlet pipe off and looked inside but this was nearly 20 years ago so I can't exactly remember what it looked like other than seeing the pellets in it.

[Helirich]

I am going to just jump to this part - did you do the test as they instruct? You also don't know what your initial vacuum was because you didn't check it in the same conditions that you did with the exhaust removed - engine warmed up, etc. so it is hard to have a comparison. If you have THAT much back pressure that you're going to blow an exhaust gasket, I don't think you'd see 16in HG with a cold engine, the value would have dropped and then you'd find yourself in trouble.

I never scoped my muffler, I cut the curved inlet pipe off and looked inside but this was nearly 20 years ago so I can't exactly remember what it looked like other than seeing the pellets in it.

I will do the test as they say when I reinstall the muffler. But at this point, I figure why not get the car running right without the muffler? It completely takes it out of the equation. I just took the car on another test ride. I'm going to start another thread asking for ideas.