dead cylinder but only misses when hot

Reply

  #1  
Old 06-08-06, 06:43 AM
Member
Thread Starter
Join Date: Jun 2006
Posts: 4
Angry dead cylinder but only misses when hot

HI, I have an old (89) Suburban with a chev v8 (305 or 350 have not checked the numers it was a replacement by previous owner)

It had decreasing power and started missing even after a tuneup so I took a look and compression test shows #6 cyl= dead (others +/-120 but #6=20)

The funny thing about it is that even with that cylinder showing vastly lower compression than all the others, when you start it up cold it will hit on all 8 and run smooth until it warms up most of the time.

Possibilities I have thought of: broken valve spring, weak valve spring, burned valve, problem with lifter, compression ring problem?? CAnt be cam because I pulled the valve cover off and looked to see that both valves were opening and closing every revolution.

Im trying to determine which way to repair this as it is old and I only use it for hunting and fishing and dont want to spend alot of $

So my question is how to troubleshoot beyond this without pulling the valve train apart or just making a blind assumption that it is a top end problem and replacing parts without knowing for sure.

ANY HELP IS APPRECIATED!!!!
 
Sponsored Links
  #2  
Old 06-08-06, 07:45 AM
Member
Join Date: Sep 2004
Posts: 238
try this:

Chances are good that your compression tester can be threaded into your spark plug hole and the other end of the hose connected to an air hose. Blow air into the cyl when it is at tdc and listen for air at tail pipe, intake and oil filler cap. This should tell you if your leak is at intake valve, exhaust valve, or rings.

I wonder if the fast idle at cold start-up is masking your problem. Pull and ground the no. 6 plug wire and see if it still runs ok cold. If yes, than the cold running mystery is solved.

If you have solid (unlikely) lifters, they may be adjusted too tight. Maybe one of your lifters is so full of crud it acts as a solid lifter and as the pushrod gets hot and longer one of the valves does not close fully. I have never heard of this, but maybe it is possible.
 
  #3  
Old 06-08-06, 12:39 PM
Member
Join Date: Dec 2001
Posts: 2,538
would probably pull the valve cover and check the valve springs again on # 6 cylinder usually they will be broken if its causing an intermitant miss.
does it have good compression when its cold?
 
  #4  
Old 06-08-06, 05:31 PM
DaVeBoy's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Mar 2006
Posts: 1,665
Originally Posted by nmbigelk
..., when you start it up cold it will hit on all 8...
What are you basing this comment on? Sound? Have you tried pulling the plug wire off on #6 to see if the engine slows down? Engines, running with just a little gas, even under poor compression , especially an 8 cylinder, as opposed to a 4 cylinder, can act like it is running fairly smooth, when in essence there is not much fire power going on in one cylinder.
 
  #5  
Old 06-09-06, 03:11 PM
Member
Join Date: Aug 2003
Location: Barre,Vermont
Posts: 472
Fuel control and/or ignition system concerns are moot if it's only got twenty pounds compression in that cylinder. The test peabees said is a good one, what I would add is that you need to regulate down the air pressure when you pressure check it because typical shop air will push the piston down, with valves opening resultant, which of course makes your test data false. Use about 25 psi.
 
  #6  
Old 06-10-06, 06:15 AM
Member
Thread Starter
Join Date: Jun 2006
Posts: 4
dead cylinder but only misses when hot-comments..

BIGGUY: yes you are right...fuel and ignition are irrelevant with 20lbs pressure...there is a mechanical failure I believe....perhaps I could have worded my post better...anyway...

peabees/bigguy: so it sounds like the next step is pressure or "leakdown"test..(?)which should help determine if this is top or bottom end (or ex or intake) can do on the 25 lb pressure. yes I have a threaded comp gauge for air fitting...and peabees no way are these solid lifters hehehe! (this appears to be a 305 smogger the p.o. had installed...and additionally this is a tbi model...I would be surprised if there is even a solid cam grind that would idle with a GM TBI!)

will do test this weekend and repost thanks for advice...
 
  #7  
Old 06-10-06, 06:18 AM
Member
Thread Starter
Join Date: Jun 2006
Posts: 4
will look again but have aleady looked many times...

[QUOTE=bejay]would probably pull the valve cover and check the valve springs again on # 6 cylinder usually they will be broken if its causing an intermitant miss.

yeah I know...first thing I checked....no breaks or cracks visible....
 
  #8  
Old 06-10-06, 06:19 AM
Member
Thread Starter
Join Date: Jun 2006
Posts: 4
see post under bigguy

see post under bigguy
 
Reply

Posting Rules
You may not post new threads
You may not 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 Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'