low compression on all cylinders?
Thread Starter
Heel & Toe
Joined: Jun 2010
Posts: 22
Likes: 0
low compression on all cylinders?
hi, i just got my first vette and im just starting to fix it up.... Runs alright but im on a problem i dont know to much about... I ran a dry compression test on all the cylinders and was surprised when i saw the numbers.... they were all at about 105psi to 110psi and one cylinders at 90...., there is a popping noise coming from that cylinders too..... Is it just me or is all those really low? any help would be great
Jesse
Jesse
Drifting
Joined: Jun 2005
Posts: 1,523
Likes: 1
From: grand prairie texas
On the cylinder that is a little bit lower and popping I would take off the valve cover and do what is called staking the valves which is taking a hammer and tapping on the rocker arms to see if there is a little bit of carbon keeping the valve off of the seat and knock it free then do the compression check again. The 105-110 sounds about right these are not high compression engines
Race Director
Joined: Jul 2004
Posts: 12,696
Likes: 329
From: Scottsdale Arizona
Did you open the choke and throttle plates in the carb all the way before doing your test? This will affect the overall numbers a great deal. Usually you're looking for cylinders where the compression is different rather than the overall number in compression tests so unless there is a reason to worry about the engine, I wouldn't.
Advanced
Joined: Jun 2008
Posts: 67
Likes: 0
Are you using your compression gauge or one that you borrowed?
You may want to make sure someone has not replaced the schrader valve in the compression gauge or adapter hose with a tire schrader valve. Tire schrader valves usually start leaking somewhere around 100 psi.
I have seen some of the best professional technicians get bitten by this mistake.
Stormin
You may want to make sure someone has not replaced the schrader valve in the compression gauge or adapter hose with a tire schrader valve. Tire schrader valves usually start leaking somewhere around 100 psi.
I have seen some of the best professional technicians get bitten by this mistake.
Stormin
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Thread Starter
Heel & Toe
Joined: Jun 2010
Posts: 22
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I HOPE THERE IS STILL SOMEONE LISTENING, but i did the test where you put some oil directly into the cylender and it went up to 110 psi like the rest..... does that mean i have to pull apart the engine to fix this one piston ring sop the popping sound stops? and i was planning on putting new heads on soon, does the low compression have to be fix before that?
thanks for all the help!
Jesse
thanks for all the help!
Jesse
Pro
Joined: Jan 2010
Posts: 518
Likes: 12
From: Panhandle
Jesse,
You say you just got the Vette and are going to fix it up. Well you certainly have a problem with that cylinder. If my engine had a bad ring, I would pull it out of the car and do a complete engine rebuild. This way you know you have a good engine and can move on. Adding new heads is an exciting event and best done on a freshly rebuilt engine where together they can give you the best performance. Putting them on an engine with a problem is wasted effort as you will need to address the problem again sooner or later. Fight any temptation to just fix the one cylinder unless budget so dictates.
HOWEVER, if you are planning a major restoration, it may be wise to check out the car systems with the current engine so you know what needs repair. If the restoration lags for a year or so, why rebuild the engine now and have it sit while you restore the body or frame?
Whichever path you chose, take your time, enjoy it and be safe.
You say you just got the Vette and are going to fix it up. Well you certainly have a problem with that cylinder. If my engine had a bad ring, I would pull it out of the car and do a complete engine rebuild. This way you know you have a good engine and can move on. Adding new heads is an exciting event and best done on a freshly rebuilt engine where together they can give you the best performance. Putting them on an engine with a problem is wasted effort as you will need to address the problem again sooner or later. Fight any temptation to just fix the one cylinder unless budget so dictates.
HOWEVER, if you are planning a major restoration, it may be wise to check out the car systems with the current engine so you know what needs repair. If the restoration lags for a year or so, why rebuild the engine now and have it sit while you restore the body or frame?
Whichever path you chose, take your time, enjoy it and be safe.
Pro
Joined: Jan 2010
Posts: 518
Likes: 12
From: Panhandle
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
If your not smoking chance are your rings and bottom end is good. Just varafy your oil pressure is good. It should be between 25 and 40psi on a fully warmed up engine with 10/40w oil.I've seen compression that low many times before. The mix of a super low compression mid 70s engine, leaky valves and if the cam was swapped for a hotter cam it will kill your cranking compression
Pro
Joined: Jan 2010
Posts: 518
Likes: 12
From: Panhandle
Let's Discuss the Poping sound!
1: What is the condition of the spark plug? Post picture if possible. Oily, carboned up, etc.
2. Do a visual inspection.
Take off the valve cover and check of the push rods and rocker arms for being bent or broken.
Disable the ignition and have someone turn starter and watch the action of the rocker in the suspect cylinder compared to the others.
3. Pull off the rocker arms and get tool to pull out the valve lifter. Check condition. Can show cam lobe damage.
2. Do a visual inspection.
Take off the valve cover and check of the push rods and rocker arms for being bent or broken.
Disable the ignition and have someone turn starter and watch the action of the rocker in the suspect cylinder compared to the others.
3. Pull off the rocker arms and get tool to pull out the valve lifter. Check condition. Can show cam lobe damage.
Burning Brakes
Joined: Apr 2009
Posts: 828
Likes: 2
There are 2 type of compression tests. One is a dry test and one is a wet test. Make sure that your dry test is first ran with a cold motor. It is only normal for the worst rings to raise their PSI in the cyl. that you put oil into. The cold oil acts like a seal for the wear space between the cylinder wall and the ring and then your psi raises. Bums use STP to sell cars with bad motors so it looks like you have good pressure.
I can keep on typing here but if you do a google search on engine compression testing wet and dry, it will help you sort it out. Make sure that your seal on your compression tester that closes your spark hole is good too. The max should be a 10% -15 % difference in all of your chambers. Any number in that 10-15% range is fine. 10% of 105 is 95.5 psi. 15% is 90PSI. Check the range first in your service manual and go from there too. You can also do a leak down test.
I can keep on typing here but if you do a google search on engine compression testing wet and dry, it will help you sort it out. Make sure that your seal on your compression tester that closes your spark hole is good too. The max should be a 10% -15 % difference in all of your chambers. Any number in that 10-15% range is fine. 10% of 105 is 95.5 psi. 15% is 90PSI. Check the range first in your service manual and go from there too. You can also do a leak down test.
