Low Cylinder Compression. Why? - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 07-16-2010, 02:31 PM
Wanna-be custom upholsterer
 
Join Date: Mar 2004
Location: Tulsa
Age: 46
Posts: 48
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Low Cylinder Compression. Why?

I have a 99 Chevy Suburban with the 5.7 Vortec (not LS style) with 140K on the Motor. Was getting a P0303 code (#3cyl miss-fire) and a rough idle but good driving performance. I had already done a Major TuneUp using all ACDelco Parts at 130K and I noticed #3 plug was Blacker w/burnt oil on it, but it was running fine. Started problem about 500mi ago and Intake Gasket was leaking coolant so that was all changed with Felpro PermaDry gaskets, Upper and Lower w/new intake bolts. Thought the injector/Poppet was plugged so I ran BG44 with an improvement in idle but not perfect. New Delco Plat plug is getting the same black/burnt oil on it now.

I did a compression test on the warmed up engine last night. Cyl#1=180 Cyl#3=90 Cyl#5=175. Squirted oil in the #3 cylinder and ran compression check 3 more timed. All 3 times, the Initial crank was a major jump and gradual increase for the next 4 cranks but only went to 95-100psi, so not the rings. I then put it back together and checked vacuum at the Power Brake booster port. 17.5 in/Hg and constant, no fluctuation at all. Hit the throttle wide open and snapped it shut. Vacuum went instantly to 0 in/hg then straight to 23 in/hg then settled solid at 17.5 in/hg, so valves should be good.

What the H%## am i missing here? Was thinking of running GM "Top Engine Cleaner" through the vacuum port to knock out any carbon as the Intake had a lot of EGR/PCV oil and carbon, and Lifter Valley had dry, hard carbon on the bottom of the Valley by the lifters that i had to scrape out. But I'm afraid of clogging the 2 CatConv and 3 O2 sensors on the truck.

Thanks for ANY help I can get!!!

Robert

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 07-16-2010, 02:40 PM
Registered User
 
Join Date: Jul 2010
Location: Thunder Bay
Posts: 66
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
what I would do is get a crankcase blowby gauge and check that. you said you put oil in the cylinders and it was the same comp. so maybe your blowing compression else where..(well obviously right).

try this, install a rad cap blowby unit, or make your own, and see what your reading there, maybe your puttin some compression in the cooling systerm.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 07-16-2010, 04:24 PM
406 bug's Avatar
9.82 @ 139mph
 
Last wiki edit: Finding vacuum leaks Last photo:
Join Date: Nov 2007
Location: phoenix, arizona
Posts: 581
Wiki Edits: 1

Thanks: 0
Thanked 1 Time in 1 Post
I would bet it is a result of your previous coolant leak. Cracked/warped head. I would do a leak down test. Good luck.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 07-16-2010, 05:34 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,688
Wiki Edits: 0

Thanks: 3
Thanked 403 Times in 348 Posts
Quote:
Originally Posted by bobbyv


I did a compression test on the warmed up engine last night. Cyl#1=180 Cyl#3=90 Cyl#5=175. Squirted oil in the #3 cylinder and ran compression check 3 more timed. All 3 times, the Initial crank was a major jump and gradual increase for the next 4 cranks but only went to 95-100psi, so not the rings.

Robert
I would disagree with your conclusion of this test. 90 psi warm and "dry of oil" versus a major increase followed by 95-100 psi "wet with oil" is a pretty strong indicator that there is a cylinder wall sealing problem. A leakdown test was suggested, that's a good idea to narrow this down.

A side note, most intake to head sealing problems with the Gen I Vortec is the result of overheating the engine or a failure of the DexCool coolant. Did this happen?

Bogie
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 07-16-2010, 05:39 PM
Wanna-be custom upholsterer
 
Join Date: Mar 2004
Location: Tulsa
Age: 46
Posts: 48
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Compression on cyls 1&5 are 175psi, no coolant in oil or oil in coolant either. Another board suggested a sticking valve. What do you guys think?
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 07-16-2010, 06:03 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,688
Wiki Edits: 0

Thanks: 3
Thanked 403 Times in 348 Posts
Quote:
Originally Posted by bobbyv
Compression on cyls 1&5 are 175psi, no coolant in oil or oil in coolant either. Another board suggested a sticking valve. What do you guys think?
A sticking or leaking valve will show on a vacuum test as a rhythmic drop in the vacuum every-time that cylinder comes around.

Your vacuum test also tends to support low compression. The dropped throttle test should come back to 26-29 inches with the mild cam in a Vortec; instead you're getting a 23 inches which is in the range requiring further investigation of the ring seal.

Bogie
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 07-16-2010, 08:22 PM
T-bucket23's Avatar
Hotrodders.com Moderator
 
Last wiki edit: Engine basic condition - how to check Last photo:
Join Date: Aug 2005
Location: Massachusetts
Age: 56
Posts: 5,089
Wiki Edits: 26

Thanks: 7
Thanked 102 Times in 87 Posts
I would be willing to bet you have a burnt exhaust valve. It will cause low compression and will not effect the vacuum gauge that bad unless it is really bad. Where you have 100 lbs of compression, it cant be real bad.
Put some air pressure in that cylinder and see where it is leaking from
__________________
Artificial Intelligence is no match for Natural Stupidity



Chet
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 07-16-2010, 09:21 PM
BOBCRMAN@aol.com's Avatar
Member
 
Last photo:
Join Date: May 2002
Location: Holly, michigan
Posts: 8,084
Wiki Edits: 0

Thanks: 23
Thanked 258 Times in 242 Posts
I get a few of these per year. The over heat has caused the no. 3 intake valve to pull into the seat/port. Making a "Tulip" style looking valve that slowly leaks compression.

Or an eroded/cracked exhaust seat. Usually 3 or 5..exhaust...
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 07-16-2010, 09:51 PM
Wanna-be custom upholsterer
 
Join Date: Mar 2004
Location: Tulsa
Age: 46
Posts: 48
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by oldbogie
I would disagree with your conclusion of this test. 90 psi warm and "dry of oil" versus a major increase followed by 95-100 psi "wet with oil" is a pretty strong indicator that there is a cylinder wall sealing problem. A leakdown test was suggested, that's a good idea to narrow this down.

A side note, most intake to head sealing problems with the Gen I Vortec is the result of overheating the engine or a failure of the DexCool coolant. Did this happen?

Bogie
I will run a leakdown test, but when i said "Major Jump" i was using that phrase so no one thought each stroke was a slow build. It built compression the same way with or without oil in the cylinder and only varied 5psi, so i felt the rings were sealing OK.

The intake coolant leak was from the DexCool destroying the factory gaskets just like the 3.4L in my 2000 Impala did. The area of the intake gasket that sealed the rear coolant passage had almost turned to jelly. Thanks GM
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 07-16-2010, 09:56 PM
Wanna-be custom upholsterer
 
Join Date: Mar 2004
Location: Tulsa
Age: 46
Posts: 48
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by BOBCRMAN@aol.com
I get a few of these per year. The over heat has caused the no. 3 intake valve to pull into the seat/port. Making a "Tulip" style looking valve that slowly leaks compression.

Or an eroded/cracked exhaust seat. Usually 3 or 5..exhaust...
What would this look like on a vacuum gauge? And would the rough idle randomly get better or worse on it's own? Lastly, if it is the valves, would I be better off getting New Vortec heads instead of rebuilding mine. I know the new factory heads are pretty damn cheap.
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 07-21-2010, 09:03 AM
Wanna-be custom upholsterer
 
Join Date: Mar 2004
Location: Tulsa
Age: 46
Posts: 48
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Did a Leak Down Test last night... Looks like an Exhaust Valve is leaking. Going to run some Goodwrench Top Engine Cleaner through it and see if its just carbon. Will keep you all posted.
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 07-21-2010, 10:54 AM
adantessr's Avatar
'23 T-Bucket Pickup
 
Last wiki edit: Troubleshooting ignition Last photo:
Join Date: Jun 2010
Location: South Charleston , WV
Age: 61
Posts: 1,549
Wiki Edits: 3

Thanks: 43
Thanked 17 Times in 15 Posts
Quote:
Originally Posted by oldbogie
I would disagree with your conclusion of this test. 90 psi warm and "dry of oil" versus a major increase followed by 95-100 psi "wet with oil" is a pretty strong indicator that there is a cylinder wall sealing problem. A leakdown test was suggested, that's a good idea to narrow this down.

A side note, most intake to head sealing problems with the Gen I Vortec is the result of overheating the engine or a failure of the DexCool coolant. Did this happen?

Bogie
What ? Help me here . I didn't know Dex-cool caused gasket failure . I was going to put 150,000 mi coolant in my T-bucket after I flush the cooling system . Should I just use the old type permanent anti-freeze / coolant instead ?
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 07-21-2010, 04:07 PM
Wanna-be custom upholsterer
 
Join Date: Mar 2004
Location: Tulsa
Age: 46
Posts: 48
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by adantessr
What ? Help me here . I didn't know Dex-cool caused gasket failure . I was going to put 150,000 mi coolant in my T-bucket after I flush the cooling system . Should I just use the old type permanent anti-freeze / coolant instead ?
GM had problems in the late 90's early 2000's with DexCool causing Intake Gaskets to fail. My 2000 3.4L Impala started Leaking at 60K. GM new it was a Defect because they Redesigned the Intake gasket and sold them to repair the leak. They didn't however Warranty the defect and wanted $780.00 to replace it. (did it myself w/$50 in parts). Common Problem on Gen2 SBC truck engines too. Just use FelPro gaskets and don't worry about it. Oh... watch for sludge.
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 07-21-2010, 04:47 PM
LATECH's Avatar
Hotrodders.com Moderator
 

Last journal entry: Motor - vator
Last photo:
Join Date: Jul 2010
Location: Northeast Pennsylvania
Posts: 5,167
Wiki Edits: 0

Thanks: 199
Thanked 249 Times in 231 Posts
.

T bucket has it right. Burned exhaust valve wont cause fluctuation in engine vacuum as it has little effect on the intake side of the cylinder. Putting oil in the cylinder in question is standard diagnostic routine and from the sounds of it proved rings ok thus with that to be absoluteley sure pull the valve cover and remove the rocker arms for #3 cylinder and charge it with air. ( leak down test). That will reveal how much the exhaust is leaking if needed pull the rockers off of a known good cylinder next to #3 and do the same so you have something to compare the leakage to .To perform a compression test properly ( although your presumptive results are fine in this case) one should pull all the plugs first and be sure to crank each cylinder an equal amount of times. Doing it on a cold engine is more precise to determine engine wear as when warmed up tolerances are a little tighter.Plus you wont have to use your best cuss words cause you fried your arm on a hot manifold.
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Right cam for a 383? atomicrockerdude Engine 23 11-02-2009 12:13 AM
Low Compression KoTT Engine 7 06-25-2009 05:53 PM
What diff. does the compression make from high and low. wild_man84 Engine 1 05-17-2009 09:39 AM
351 windsor running rich ???? ve3ham Engine 19 08-08-2005 12:48 PM
how to keep low compression 67bird Engine 14 02-08-2004 07:10 PM


All times are GMT -6. The time now is 06:51 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.