What could be causing my exhaust to suck in intermittently - JeepForum.com
 1Likes
Reply
 
LinkBack Thread Tools
post #1 of 25 Old 07-20-2021, 05:41 PM Thread Starter
Rylxmp
Registered User
2003 WJ 
 
Join Date: Dec 2020
Posts: 34
What could be causing my exhaust to suck in intermittently

Hi, I have a jeep gc with the 4.7 HO and 180k miles on it. I was having a minor rough idle and decided to try a trick i saw which was placing a paper over the exhaust to see if it sucks back in. It says that this trick shows you if you are having valve issues but i dont know how credible it is. When i decided to try it out, the paper would get somewhat sucked towards the exhaust after every 10 to 15ish seconds. I dont have any other issues that would lead me to believe that its a burnt valve or leaking valve seal and my gas mileage is great with also no CEL coming on. I also stay on top of maintenance and have replaced mainly everything on its interval ever since owning the vehicle. My question is, what causes the paper to get somewhat pulled back inconsistently ? thanks

Rylxmp is offline  
Sponsored Links
Advertisement
 
post #2 of 25 Old 07-21-2021, 06:37 AM
Delta0
Registered User
 
Join Date: May 2018
Posts: 827
Quote:
Originally Posted by Rylxmp View Post
Hi, I have a jeep gc with the 4.7 HO and 180k miles on it. I was having a minor rough idle and decided to try a trick i saw which was placing a paper over the exhaust to see if it sucks back in. It says that this trick shows you if you are having valve issues but i dont know how credible it is. When i decided to try it out, the paper would get somewhat sucked towards the exhaust after every 10 to 15ish seconds. I dont have any other issues that would lead me to believe that its a burnt valve or leaking valve seal and my gas mileage is great with also no CEL coming on. I also stay on top of maintenance and have replaced mainly everything on its interval ever since owning the vehicle. My question is, what causes the paper to get somewhat pulled back inconsistently ? thanks
It's an old trick used by unscrupulous garages to scam punters into valve jobs.

A compression test is the only reliable way I know to check the top end of your engine.

As the fat lady advised, if it ain't broke, don't fornicate it up.
Bigrigr likes this.
Delta0 is offline  
post #3 of 25 Old 07-21-2021, 06:55 AM
CJREX
Registered User
 
CJREX's Avatar
2004 WJ 
 
Join Date: May 2005
Posts: 344
If it was truly an indicator of valvetrain issues, it would happen a LOT more often than every 10 to 15 seconds since the engine is performing 700-1000 full revolutions per minute at idle.

2004 GC Overland 4.7 HO
Powerful Wanderer (wife named it after driving it)
265/70/17 MT, 2" BDS lift
CJREX is online now  
 
post #4 of 25 Old 07-21-2021, 06:56 AM
jtec
Registered User
 
jtec's Avatar
2014 JK Wrangler 
 
Join Date: Oct 2017
Posts: 10,462
Agree if it ain;t broke don't F.. with it.

No cel? - Always SCAN, the cel can be inop* and there are codes that will not command the CEL on.

*the CEL should come on at every start for @2seconds then go out.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
jtec is offline  
post #5 of 25 Old 07-21-2021, 01:09 PM
Delta0
Registered User
 
Join Date: May 2018
Posts: 827
Quote:
Originally Posted by Rylxmp View Post
Hi, I have a jeep gc with the 4.7 HO and 180k miles on it. I was having a minor rough idle and decided to try a trick i saw which was placing a paper over the exhaust to see if it sucks back in. It says that this trick shows you if you are having valve issues but i dont know how credible it is. When i decided to try it out, the paper would get somewhat sucked towards the exhaust after every 10 to 15ish seconds. I dont have any other issues that would lead me to believe that its a burnt valve or leaking valve seal and my gas mileage is great with also no CEL coming on. I also stay on top of maintenance and have replaced mainly everything on its interval ever since owning the vehicle. My question is, what causes the paper to get somewhat pulled back inconsistently ? thanks
There is a possibility that interference between waves of exhaust coming down one pipe to the Y interferes with waves coming the other pipe to cause high and low pressure in the pipe after the union.

I recall looking down at an "arroyo" that ended just before the trough on which the catamaran in which I was on watch.

Infront, the wave towered some 35ft above us.
To my right there was the trough in which we sailed.
To my left, there was nothing until I looked down.

Happily, we went up the next wave, leaving the double trough behind.

Last edited by Delta0; 07-21-2021 at 02:27 PM.
Delta0 is offline  
post #6 of 25 Old 07-21-2021, 04:52 PM
fracturedapple
Registered User
 
Join Date: Jan 2018
Posts: 113
Garage
Its a quick and dirty test to see if a car is misfiring only.

I've seen this put holes in paper and you will hear it.

Doesn't mean you have valve issues.

In your case maybe your mistaking the rebound of the paper from the exhaust pulse as the suck? Maybe a little windy? Not sure

As everyone is saying if you have no drive ability issues don't worry about it.

Sent from my SM-G965U using Tapatalk
fracturedapple is offline  
post #7 of 25 Old 07-22-2021, 10:20 AM
Bigrigr
Registered User
 
Bigrigr's Avatar
2000 WJ 
 
Join Date: Jan 2020
Location: Near SLC
Posts: 2,641
Garage
Throw the paper in the trash and add some fuel injector cleaner to the gas tank. You will be further ahead....

'00LMTD-4.7l,242hd,D30,D44A,3.73's,True-trac front,Spartan rear,4" short arm lift,JK'sW/32's
Bigrigr is offline  
post #8 of 25 Old 07-22-2021, 03:12 PM
Double E
Web Wheeler
 
Double E's Avatar
2004 WJ 
 
Join Date: Oct 2003
Location: Raleigh, NC
Posts: 6,596
Agree that the paper thing is not a valid diagnostic tool.
You would be better off doing the maintenance items mentioned like a bottle of FI cleaner.
I would also swap out the TPS (super easy), clean the back of the throttle body and verify the battery connections are clean and tight.
Beyond that, swap spark plugs if you do not know when they were last done.

That you got to180K is super. You're paying attention to the fragile 4.7 HO. Keep it cool, change the oil regularly and you should easily get another 80K out of it before big and expensive stuff happens.

"We have it totally under control. Itís one person coming in from China. We have it under control. Itís going to be just fine." ~D. Trump, Jan 22, 2020.
Double E is offline  
post #9 of 25 Old 07-22-2021, 03:48 PM
2oldjeeps
Registered User
 
2oldjeeps's Avatar
2000 XJ Cherokee 
 
Join Date: Aug 2017
Posts: 2,116
what on earth could cause vacuum in exhaust.
stuk ex valve during intake stroke.
anything else??

99wj, 150,000 miles
00xj ,189,000 miles
10hummer3,76,000 miles
98xj, 133,000 miles
2oldjeeps is offline  
post #10 of 25 Old 07-22-2021, 04:00 PM
fracturedapple
Registered User
 
Join Date: Jan 2018
Posts: 113
Garage
Misfire means no power stroke no pressure no heat.

Instead you get decompression. When the piston reaches bottom Dead center the cylinder is in a vacuum.

Then exhaust valve opens while that cylinder is under vacuum. = vacuum in exhaust.

If your really curious john Thornton has some interesting videos about in cylinder pressure testing that have found their way to YouTube

Sent from my SM-G965U using Tapatalk
fracturedapple is offline  
post #11 of 25 Old 07-22-2021, 08:09 PM
2oldjeeps
Registered User
 
2oldjeeps's Avatar
2000 XJ Cherokee 
 
Join Date: Aug 2017
Posts: 2,116
Instead you get decompression.....???why??
the cylinder is in a vacuum.?? why??

99wj, 150,000 miles
00xj ,189,000 miles
10hummer3,76,000 miles
98xj, 133,000 miles
2oldjeeps is offline  
post #12 of 25 Old 07-22-2021, 09:55 PM
Boojo35
Registered User
1987 YJ Wrangler 
 
Join Date: Jul 2007
Location: Beer City
Posts: 3,216
Garage
It is not a worthy test. Do not waste your time says the mechanic that is older that most T- rex's. We have compression tests, leakdown tests, timing tests, etc. Some paper over an exhaust???? Find it one time in a manual and then bring it back to how it is a legit test. This is the worst internet lore ever. What a joke.

If you cannot fix it with a hammer then it has to be an electrical problem.
Boojo35 is online now  
post #13 of 25 Old 07-23-2021, 12:48 AM
Delta0
Registered User
 
Join Date: May 2018
Posts: 827
Think about a wave pool.

In the beginning.
A little dubry at one end, lobs a little wave at a doofer at the other end.
The doofer lobs the little wave back ai the dubry.
Somewhere about the middle, the new wave from the dubry collides with the wave from the doofer forming a big wave, and a big trough.

Now transfer that picture to you exhaust.
Each pop in the engine sends a little wave down the front pipe into the silencer / muffler.
The back end of the silencer lobs the little wave back to meet the next little wave from a cylinder.
Every so often a big wave and a big trough form in the silencer.

The big wave pushes the paper away.
The big trough sucks the paper in.

It's the way the world goes round.

Last edited by Delta0; 07-23-2021 at 06:45 AM.
Delta0 is offline  
post #14 of 25 Old 07-23-2021, 05:52 AM
CJREX
Registered User
 
CJREX's Avatar
2004 WJ 
 
Join Date: May 2005
Posts: 344
Quote:
Originally Posted by 2oldjeeps View Post
Instead you get decompression.....???why??
the cylinder is in a vacuum.?? why??
When the fuel/air mixture is pulled into the cylinder, the spark plug ignites it and it rapidly expands, pushing the piston down to create a power pulse.

If the mixture doesn't get ignited (misfire), it doesn't expand and the piston creates a vacuum as it moves down.

When the exhaust valve opens, air rushes in to fill the vacuum and the only source of air at that point is pulling it back through the exhaust system.

2004 GC Overland 4.7 HO
Powerful Wanderer (wife named it after driving it)
265/70/17 MT, 2" BDS lift
CJREX is online now  
post #15 of 25 Old 07-23-2021, 07:12 AM
Delta0
Registered User
 
Join Date: May 2018
Posts: 827
Quote:
Originally Posted by 2oldjeeps View Post
what on earth could cause vacuum in exhaust. Have you ever watch the flapper valve on vertical exhaust pipe 2Old?

stuk ex valve during intake stroke.
anything else??
Delta0 is offline  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the JeepForum.com forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid e-mail address for yourself.



Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome