Failed Compression Test
Thread Starter
Safety Car
Joined: Dec 2001
Posts: 4,295
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From: Indianapolis Indiana
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Failed Compression Test
Well, I did a compression test last night and the numbers were okay-ish with 170-195 after 6 cycles in each, but #7 lagged behind at 90. Crap, it was 90 every time. After 1 cycle they were all 110-130, though #3 was 150.
I still need to put some oil in #7 and check, as well as do a leakdown, but I'm pretty certain something is wrong, I just need to follow up on it.
Surprisingly the car still pulls strong. I'm hoping I've caught this before things really get torn up. How much work do you think I'm looking at here? I assume all new rings will be in order, but I'm just not sure if that can be done with the engine in the car or not, and if I've likely already sustained more damage.
I'll be doing more tests tonight and post them up.
I still need to put some oil in #7 and check, as well as do a leakdown, but I'm pretty certain something is wrong, I just need to follow up on it.
Surprisingly the car still pulls strong. I'm hoping I've caught this before things really get torn up. How much work do you think I'm looking at here? I assume all new rings will be in order, but I'm just not sure if that can be done with the engine in the car or not, and if I've likely already sustained more damage.
I'll be doing more tests tonight and post them up.
Melting Slicks
Joined: May 2005
Posts: 3,394
Likes: 12
From: Leon Iowa
St. Jude Donor '06-'07-'08-'09
It may not be the rings at all, it could be a valve leaking from a couple things. Could be bent just a little or even have some carbon on it. I would run the Seafoam through it like people do. I believe most people suck it in through the PCV. Then let the car sit for a bit and fire it up and watch the smoke. If it's carbon this should clean it all up. i would do this before tearing it down..
Last edited by mvvette97; Mar 2, 2008 at 03:40 PM. Reason: spelling
Thread Starter
Safety Car
Joined: Dec 2001
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From: Indianapolis Indiana
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I'll be injecting oil and also doing a leakdown tonight, so we'll certainly have a better diagnosis soon. My gut feeling is rings, but I can't give any quantifiable data yet.
I don't think I've had any wild over-revs since I got new valves with the H/C, but I know things have been hot/lean a few times and I spin it to 7k at the track regularly (real track, not the drag strip).
I seafoam it every 10k miles.
I don't think I've had any wild over-revs since I got new valves with the H/C, but I know things have been hot/lean a few times and I spin it to 7k at the track regularly (real track, not the drag strip).
I seafoam it every 10k miles.
Melting Slicks
Joined: May 2005
Posts: 3,394
Likes: 12
From: Leon Iowa
St. Jude Donor '06-'07-'08-'09
I see, well if it comes down to having to pull the engine down I think I would start by pulling the head that has the cylinder with low compression. I am working on an LS that's in a Z28 right now and when I tore it down it has very small eye lash morks in a couple of the pistons. He tells me that he hasn't missed a gear and over reved it but ya never know.
Just a thought though valve float can cause the piston to tap the valve also. I hope your lucky and maybe it's just a leaking valve. Good luck .
Just a thought though valve float can cause the piston to tap the valve also. I hope your lucky and maybe it's just a leaking valve. Good luck .
Race Director
Joined: Feb 2006
Posts: 10,336
Likes: 14
From: Bellville OH
St. Jude Donor '06
#7 is also notorious for failures do to the lack of cooling to the cylinder if the Sea foam and leak down doesn't provide any help, I would pull the heads and take a look at 'ol #7. There are plenty of thread on here about this issue, from what I understand the fix is a better water pump.
Thread Starter
Safety Car
Joined: Dec 2001
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From: Indianapolis Indiana
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Well, the hot compression test tonight reaffirmed the 90psi in #7. The first cycle would be 50-60psi and then peak at 90psi after 5 cycles of the ignition. Adding oil really didn't do much other than make the first cycle read closer to 70, but it still peaked at 90psi after 5 cycles.
We then did a leakdown on #7 and air blew out the dipstick tube. It looks like the piston or rings is the culprit at this point.
I had the luxury of doing this at a friend's house who has a fried LS6 in his garage. #3,5, and 7 were burnt in increasingly severe fashion. #7 in his had zero compression. My friend noted that he had similar compression numbers in #3, and the cylinder walls seemed to be in pretty good shape. I'm hoping I'll be in the same boat and just toss some new rings and an OEM piston in and get this back on the road. What do you guys think? I plan to just drop the cradle/oil pan and get this done in short order if the cylinder walls are okay. If not we're pulling it and spending serious money.
We then did a leakdown on #7 and air blew out the dipstick tube. It looks like the piston or rings is the culprit at this point.
I had the luxury of doing this at a friend's house who has a fried LS6 in his garage. #3,5, and 7 were burnt in increasingly severe fashion. #7 in his had zero compression. My friend noted that he had similar compression numbers in #3, and the cylinder walls seemed to be in pretty good shape. I'm hoping I'll be in the same boat and just toss some new rings and an OEM piston in and get this back on the road. What do you guys think? I plan to just drop the cradle/oil pan and get this done in short order if the cylinder walls are okay. If not we're pulling it and spending serious money.
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Race Director
Joined: Feb 2006
Posts: 10,336
Likes: 14
From: Bellville OH
St. Jude Donor '06
Well, the hot compression test tonight reaffirmed the 90psi in #7. The first cycle would be 50-60psi and then peak at 90psi after 5 cycles of the ignition. Adding oil really didn't do much other than make the first cycle read closer to 70, but it still peaked at 90psi after 5 cycles.
We then did a leakdown on #7 and air blew out the dipstick tube. It looks like the piston or rings is the culprit at this point.
I had the luxury of doing this at a friend's house who has a fried LS6 in his garage. #3,5, and 7 were burnt in increasingly severe fashion. #7 in his had zero compression. My friend noted that he had similar compression numbers in #3, and the cylinder walls seemed to be in pretty good shape. I'm hoping I'll be in the same boat and just toss some new rings and an OEM piston in and get this back on the road. What do you guys think? I plan to just drop the cradle/oil pan and get this done in short order if the cylinder walls are okay. If not we're pulling it and spending serious money.
We then did a leakdown on #7 and air blew out the dipstick tube. It looks like the piston or rings is the culprit at this point.
I had the luxury of doing this at a friend's house who has a fried LS6 in his garage. #3,5, and 7 were burnt in increasingly severe fashion. #7 in his had zero compression. My friend noted that he had similar compression numbers in #3, and the cylinder walls seemed to be in pretty good shape. I'm hoping I'll be in the same boat and just toss some new rings and an OEM piston in and get this back on the road. What do you guys think? I plan to just drop the cradle/oil pan and get this done in short order if the cylinder walls are okay. If not we're pulling it and spending serious money.
Thread Starter
Safety Car
Joined: Dec 2001
Posts: 4,295
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From: Indianapolis Indiana
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I have zero interest in forced induction. Now if I got more safe rpms or quicker revs via new pistons I might be interested, but I'll likely go LS2 once I'm ready to get spendy, so the pistons now would be kinda useless. I might also be interested in anything that bumps reliability over the stock setup.
My tune getting off and life at 7k rpms on the track were the likely culprits.
I like your advice on the better water pump. Any recommendations before I start reading?
My tune getting off and life at 7k rpms on the track were the likely culprits.
I like your advice on the better water pump. Any recommendations before I start reading?
Pro
Joined: May 2003
Posts: 520
Likes: 0
From: Wilmington NC
Not to highjack the thread but I'm a noob to this and am curious!
You can pull rods/pistons without pulling the whole motor via the bottom of the motor?
I have an 01 and if its going to be that easy to do a piston and rod swap I may as well go ahead and do it!
Any good tech articles on this floating around?
Thanks!
You can pull rods/pistons without pulling the whole motor via the bottom of the motor?
I have an 01 and if its going to be that easy to do a piston and rod swap I may as well go ahead and do it!
Any good tech articles on this floating around?
Thanks!
Thread Starter
Safety Car
Joined: Dec 2001
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From: Indianapolis Indiana
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I have read you can pull the pistons out from the bottom if you remove the front cradle. Certainly not a small job, but it can be done.
Pro
Joined: May 2003
Posts: 520
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From: Wilmington NC
Sometimes and in some situations its much easier said than done to pull the whole motor and tear it down. I have access to a lift but don't have my own garage and cherry picker to pull the motor.
Trying to make due with what I've got. I will probably end up putting off the short block plans until I get out of this green machine anyway so I can do it right, as you've said.
Thanks
Trying to make due with what I've got. I will probably end up putting off the short block plans until I get out of this green machine anyway so I can do it right, as you've said.
Thanks
Burning Brakes
Joined: Aug 2000
Posts: 894
Likes: 1
From: Bethel Township Ohio
Sorry to hear that Brody, I hope it is not hurt to bad, if you need it I have a couple of my old engines at the machine shop waiting to be freshened up that I am not going to be using after I switch to the big dry sump setup.
Thread Starter
Safety Car
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From: Indianapolis Indiana
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Hi Dale, thanks for the heads-up. I still need to pull that piston out and see what the issue is. Pulling the head really didn't show me anything. My goal is to get this rolling again for your autox. You'll definitely be hearing from me if it looks ugly.