No compression after changing valve seals
Thread Starter
Drifting
Joined: Nov 2001
Posts: 1,783
Likes: 2
From: Latham NY
No compression after changing valve seals
I have read the forum for some time and it has been helpful in rehabilitating my Vette. I hope you can help with this problem:
I took the plenum and runners off to clean them (they were dirty and never done before). I also changed the valve seals which were dried and cracked. When I reassembled everything, the car wouldn't start. I have spark and fuel and the distributor is in correctly. I checked compression and it is 0 even after adding oil to the cyclinders. I used compressed air to hold the valves in place and I heard air leaking inside the engine. It made a sound like blowing air into an empty bottle. This was the same for all cyclinders. Could the air pressure have blown the head gasket? It doesn't look like it has ever been changed and the car has 135000 miles. Is there anything else I should do before I disassemble again to change the head gasket?
I took the plenum and runners off to clean them (they were dirty and never done before). I also changed the valve seals which were dried and cracked. When I reassembled everything, the car wouldn't start. I have spark and fuel and the distributor is in correctly. I checked compression and it is 0 even after adding oil to the cyclinders. I used compressed air to hold the valves in place and I heard air leaking inside the engine. It made a sound like blowing air into an empty bottle. This was the same for all cyclinders. Could the air pressure have blown the head gasket? It doesn't look like it has ever been changed and the car has 135000 miles. Is there anything else I should do before I disassemble again to change the head gasket?
Melting Slicks
Joined: Jun 1999
Posts: 2,872
Likes: 3
From: Wilmington NC
Re: No compression after changing valve seals (Romeo Barrera)
same thing i was thinking! i am wondering if you set the rockers according to spec or just tightened them down... if so, your valves are not closed and this is most likely your problem! good luck with it.. a blown head gasket would give 0 compression on all 8!
chris :smash:
chris :smash:
Thread Starter
Drifting
Joined: Nov 2001
Posts: 1,783
Likes: 2
From: Latham NY
Re: No compression after changing valve seals (lcvette)
I did adjust the valves according to the manual. If I overtighten the valves, I hear air leaving thru the intake and exhaust. The sound with the valves closed is different so I think it is the head gasket.
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Re: No compression after changing valve seals (Bluevette85)
I would double check those valves again. In fact you might take the rockers completely off of one cylinder and see if you make compression.
I don't know if you ever read the thread about my friend's 396 Z-28. He put that thing back together and it would not start, tried everything with no love. He finally checked the compression to find it to be ZERO. He didn't even think about the valves because he has adjusted many a sets and they always worked fine.
$200 later from another mechanic the valves were adjusted and it runs like a dream. I know it sucks to think about but what would it hurt to try it? It could save you a lot of trouble. BTW, that would have to be a major head gasket problem, cars blow them all the time and still run, maybe not well, but at least they run.
I don't know if you ever read the thread about my friend's 396 Z-28. He put that thing back together and it would not start, tried everything with no love. He finally checked the compression to find it to be ZERO. He didn't even think about the valves because he has adjusted many a sets and they always worked fine.
$200 later from another mechanic the valves were adjusted and it runs like a dream. I know it sucks to think about but what would it hurt to try it? It could save you a lot of trouble. BTW, that would have to be a major head gasket problem, cars blow them all the time and still run, maybe not well, but at least they run.
Burning Brakes
Joined: Nov 2000
Posts: 1,117
Likes: 0
From: DFW TX
Re: No compression after changing valve seals (Bluevette85)
just how much pressure did you have on that compressor when hooked up to hold the valves up anyway? I don't have any other suggestions other than the ones above, but I couldn't imagine a compressor blowing the head gasket to the point of zero compression. But then, I could be wrong. My money is still on the rockers too tight. How much did you tighten after zero lash?
[Modified by 92corvette, 9:31 AM 11/26/2001]
[Modified by 92corvette, 9:31 AM 11/26/2001]
Burning Brakes
Joined: Dec 2000
Posts: 1,023
Likes: 0
From: Marietta, South Carolina, USA
Re: No compression after changing valve seals (Bluevette85)
Nathan Plemons and Icvette are right it is the valves.
You got them to tight, they are so tight that they are not closing, you cant mess up the head gasket with the air out of a air compressur and if it did it would not take them all out.
Do like Nathan is saying take off the rocker arms and then you will see that you will have compression.
I dont know how you justed the valves but you change all the seals and then you unhook the air and then you just them because if you have the air in the cyclinder it will push the piston to the bottom.
Hope this helps
Jay
You got them to tight, they are so tight that they are not closing, you cant mess up the head gasket with the air out of a air compressur and if it did it would not take them all out.
Do like Nathan is saying take off the rocker arms and then you will see that you will have compression.
I dont know how you justed the valves but you change all the seals and then you unhook the air and then you just them because if you have the air in the cyclinder it will push the piston to the bottom.
Hope this helps
Jay
Le Mans Master
Joined: Oct 1999
Posts: 9,713
Likes: 3
From: Sachse Texas
Cruise-In I Veteran
Cruise-In II Veteran
Re: No compression after changing valve seals (Bluevette85)
Just to make sure, you are rotating the engine so that the valves are closed on the cylinder you are checking...right? Otherwise, they will all come up "0".
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
Pro
Joined: Mar 2001
Posts: 602
Likes: 1
From: Hacienda Heights, Ca. USA
Re: No compression after changing valve seals (Vette92)
I agree with all of the above. Your valves are incorrectly adjusted. It was running before you took on this task, so it should run after provided the repairs were done correctly. I'll give you the adjustment procedure in just two crank rotations. With the motor on the compression stroke (TDC) on the number one cyl, (the timing pointer will be on 0), adjust: Exhaust 1-3-4-8, Intake 1-2-5-7. Then crank the engine one more complete revolution which will bring the no. 6 cyl. to the firing position (timing mark back on 0) and adjust: Exhaust 2-5-6-7, Intake 3-4-6-8. You should attain zero lash and then tighten 1 full turn. If you're using roller tip or full roller rocker go 1/2 turn past zero lash. Hope this helps.
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Re: No compression after changing valve seals (Bluevette85)
Guess I'll join the parade and agree that it sounds very much like the valves are being held open because the rocker adjusting nut(s) is too tight.
I just sent you a file on the procedure I recommend for properly setting lifter preload. The "manual way" often leads to the problem you're having because it's easy to become cunfused, set the wrong lifter, causing this problem.
Keep us posted.
Jake
I just sent you a file on the procedure I recommend for properly setting lifter preload. The "manual way" often leads to the problem you're having because it's easy to become cunfused, set the wrong lifter, causing this problem.
Keep us posted.
Jake
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Re: No compression after changing valve seals (Bluevette85)
Two more cents worth. I certainly HOPE 150 lbs. of shop air pressure didn't blow the head gaskets! If I thought it could, I wouldn't dare start my engine. I've got more cranking pressure than that on my 140,000 mile engine. If all else fails, back them all off, for just a little bit of clearence, according to proceedures outlined above. Clip some oil stoppers over the push rod end of the rocker arms. Start the engine and adjust each rocker arm until it just stops clicking. Then go back, and one at a time, loosen each rocker until it clicks again. Tighten it until it just quiets up and then adjust it the additional 1/4, 1/2, or full turn that you want. There is no special order required, because the engine is running. As you tighten each adjustment the engine may stumble and "want" to die. Wait a few seconds while the lifter adjust itself to the new running position. It'll smooth out. Possibly, you missed the zero lash point and over tightened the valve train. Then, when subjected to oil presure the lifters filled up and caused the valves not to seat. Good luck.
Le Mans Master
Joined: Feb 1999
Posts: 6,172
Likes: 11
From: somers, ny
Re: No compression after changing valve seals (Bluevette85)
you will not get much compression by removing the rockers completely. With the intake valve not opening...you will not be allowing air to fill the cyl (it will create a vacuum). Then during the compression stroke, it will releave the vacuum, with little or no compression.
However I do agree with everyone else...your rockers are too tight.
However I do agree with everyone else...your rockers are too tight.
Le Mans Master
Joined: Feb 1999
Posts: 6,172
Likes: 11
From: somers, ny
Re: No compression after changing valve seals (Nathan Plemons)
ah, but that is why you take the rockers off and leave the guage off until the cylinder is at the bottom, then hook it up and you WILL get compression.
OK, that would work.....................LOL
Safety Car
Joined: Dec 2000
Posts: 3,767
Likes: 5
From: Jacksonville Fl
Re: No compression after changing valve seals (ralph)
WHen I changed my guide seals...I found I could NOT adjust the valves. If I went any further than 1/8 turn past zero, I lost compression and ran rough.
My whole problem was worn out valve springs. I replaced them with heavy springs and it made the car run like a different engine all around. I was then able to adjust the valves correctly ( 1/4-1/2 turn past play with engine still running smooth at idle in gear ). My Upper RPM's were like new again...the car just ran smoother and pulled harder all around.
Dude...do yourself a favor and replace the springs. Don't even think "if"...just do it!
"If" an L98 engine has enough miles to need new seals...it needs new springs.
PERIOD.
My whole problem was worn out valve springs. I replaced them with heavy springs and it made the car run like a different engine all around. I was then able to adjust the valves correctly ( 1/4-1/2 turn past play with engine still running smooth at idle in gear ). My Upper RPM's were like new again...the car just ran smoother and pulled harder all around.
Dude...do yourself a favor and replace the springs. Don't even think "if"...just do it!
"If" an L98 engine has enough miles to need new seals...it needs new springs.
PERIOD.
Thread Starter
Drifting
Joined: Nov 2001
Posts: 1,783
Likes: 2
From: Latham NY
Re: No compression after changing valve seals (BBA)
Problem Solved!!
I had it way too tight and the valves were left open. I followed the method by Jake to make adjustments and compression was back (190's) and car started on the first try. In addition to the valve seals, I had already changed the springs(old ones were compressed) and rocker arms (worn out where the ball pivots). I just have to replace the valve covers (which I stripped and refinished) and set the timing.
Thanks to all who replied because you found the problem and saved me from tearing apart the engine for nothing.
Walt
I had it way too tight and the valves were left open. I followed the method by Jake to make adjustments and compression was back (190's) and car started on the first try. In addition to the valve seals, I had already changed the springs(old ones were compressed) and rocker arms (worn out where the ball pivots). I just have to replace the valve covers (which I stripped and refinished) and set the timing.
Thanks to all who replied because you found the problem and saved me from tearing apart the engine for nothing.
Walt
