Low Compression / First time start up
Thread Starter
Racer
Joined: Sep 2014
Posts: 421
Likes: 34
From: Troy Michigan
Low Compression / First time start up
Hello all,
Wonderful forum members who answer many of my questions... I come to you again with a new problem...
I am working on a first time start up of an old 400 engine... (don't ask my why I dare comment about a 400 on a corvette forum, but you guys are wonderful)
Anyhow,
I rented an engine compression tester, put a battery on the thing, changed the oil... took the spark plugs out and got to testing,
Below are my results... in the order I tested them,
4- 95/100
2- 130/135
6- 125/130
8 -110
1 - 100
3 - 95
5 - 65
7 - 150
Welp! not good news right? that is low compression... 100% well, atleast the numbers are all over the place!
anything dumb I should be checking before considering sending her out for a rebuild....
Estimated thoughts on a cost for a "stock" rebuild, I am not crazy about doing anything new / nuts, just want the original engine and to have an original engine running well...
Should I consider starting the thing anyhow? Could me not allowing any warm up.... be hurting me here? I think not, but...?
Best,
Kevin
Wonderful forum members who answer many of my questions... I come to you again with a new problem...
I am working on a first time start up of an old 400 engine... (don't ask my why I dare comment about a 400 on a corvette forum, but you guys are wonderful)
Anyhow,
I rented an engine compression tester, put a battery on the thing, changed the oil... took the spark plugs out and got to testing,
Below are my results... in the order I tested them,
4- 95/100
2- 130/135
6- 125/130
8 -110
1 - 100
3 - 95
5 - 65
7 - 150
Welp! not good news right? that is low compression... 100% well, atleast the numbers are all over the place!
anything dumb I should be checking before considering sending her out for a rebuild....
Estimated thoughts on a cost for a "stock" rebuild, I am not crazy about doing anything new / nuts, just want the original engine and to have an original engine running well...
Should I consider starting the thing anyhow? Could me not allowing any warm up.... be hurting me here? I think not, but...?
Best,
Kevin
Is my vette stock?? HAHA
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From: Im not allowed to tell you
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Startt amd run it, maybe you can get those sticking rings to pop out and seat. I would see how it runs before tearing into it. Maybe try some marvels mystery oil in therre to see if that loosens up the rings. Cant hurt it any. coul e bad valvves as well. did you whack the top of the valve springs with a wooden or rubber mallet to get them to seat?
Last edited by Rescue Rogers; Sep 22, 2021 at 09:04 PM.
2026 Loser of the Year
Joined: Sep 2013
Posts: 36,597
Likes: 7,046
From: New Or-leens Loo-z-anna
Crank the engine without the spark plugs to see if fresh oil is getting to the rocker arms (may need to remove a valve cover OR remove the oil filler cap to see a few rocker arms.
Install the spark plugs and Start the engine and make sure the oil pressure is at least 25-30 psi.
It may take a minute or two for the lifters to quieten down.
IF there are no knocking noises and the engine doesnt overheat, let the engine warm up to 180-200 degrees.
Wear gloves and remove the spark plugs....retest each cylinder for compression.
Tell us the new readings.
Install the spark plugs and Start the engine and make sure the oil pressure is at least 25-30 psi.
It may take a minute or two for the lifters to quieten down.
IF there are no knocking noises and the engine doesnt overheat, let the engine warm up to 180-200 degrees.
Wear gloves and remove the spark plugs....retest each cylinder for compression.
Tell us the new readings.
Racer
Joined: Feb 2002
Posts: 378
Likes: 19
The motor has been sitting around for awhile? If so, the rings may be sticking and is throwing off the compression readings?
I would remove the valve covers to get an idea on whether scheduled oil changes were performed that's based on how clean it looks. If its sludged up, then there would be concerned about the condition of the short block.
If it looks clean, I would hook-up a fitting to the low #5 cylinder and listen for air escaping past the rings. Try the same test in one of the high reading cylinders for comparison purposes.
I would remove the valve covers to get an idea on whether scheduled oil changes were performed that's based on how clean it looks. If its sludged up, then there would be concerned about the condition of the short block.
If it looks clean, I would hook-up a fitting to the low #5 cylinder and listen for air escaping past the rings. Try the same test in one of the high reading cylinders for comparison purposes.
Thread Starter
Racer
Joined: Sep 2014
Posts: 421
Likes: 34
From: Troy Michigan
The motor has been sitting for awhile indeed, possibly 5 years...
If the rings are sticking, what is my best bet to free them without damaging them? I did add some engine oil into the cylinders and spent sometime turning the engine over by hand before going forward.
I also had the valve covers and intake off and things looked pretty decent...
I suppose I'll continue prep to start the old beast and see what happens, Maybe if I can get my hands on that engine oil you recommend I'll add that. Possibly warming the engine up will help prevent these rings from sticking... surely they are old...
Best,
Kevin
If the rings are sticking, what is my best bet to free them without damaging them? I did add some engine oil into the cylinders and spent sometime turning the engine over by hand before going forward.
I also had the valve covers and intake off and things looked pretty decent...
I suppose I'll continue prep to start the old beast and see what happens, Maybe if I can get my hands on that engine oil you recommend I'll add that. Possibly warming the engine up will help prevent these rings from sticking... surely they are old...
Best,
Kevin
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Thread Starter
Racer
Joined: Sep 2014
Posts: 421
Likes: 34
From: Troy Michigan
What I'll be doing tonight is adding the mystery oil and letting it sit over the weekend. I am waiting on a few parts to start the car anyhow....
Then, well then I guess I'll see if I can get her started and report back with new compression numbers!
Best,
Kevin
Melting Slicks
Joined: Feb 2017
Posts: 3,232
Likes: 1,232
From: PHX AZ
My issue with 400 SBC is they are NOT the good 'ol 350. or even a 383. They have Siamesed (no real water jacket between the cyls). Paired cyls. They tend to plug up the PALTRY cooling passages between those paired cylinders with rust, overheat and pop the headgaskets. In a severe case the very thin web between the cylinders can crack.
I'm guessing= 'used engine' = headgaskets.
Unkahal
I'm guessing= 'used engine' = headgaskets.
Unkahal
Thread Starter
Racer
Joined: Sep 2014
Posts: 421
Likes: 34
From: Troy Michigan
Update!
Hey guys,
I've continued work and I’ve made headway. At this point I’ve been able to fire her up when connected to a gas can. Still having some issues indeed, however! I am moving along.
I had a question for you guys. The oil pressure gauge, not sure if it works... and I do NOT want to run this thing for any amount of time with low compression.
I did add mystery oil into the cylinders a few days before starting it so, I probably haven’t caused any damage “yet”...
Now,
the question.
the valve cover on passenger side was free of oil and fairly dry when I started the car... I checked again, there is now oil under that valve cover. That implies there is oil pressure correct?
Best!
I've continued work and I’ve made headway. At this point I’ve been able to fire her up when connected to a gas can. Still having some issues indeed, however! I am moving along.
I had a question for you guys. The oil pressure gauge, not sure if it works... and I do NOT want to run this thing for any amount of time with low compression.
I did add mystery oil into the cylinders a few days before starting it so, I probably haven’t caused any damage “yet”...
Now,
the question.
the valve cover on passenger side was free of oil and fairly dry when I started the car... I checked again, there is now oil under that valve cover. That implies there is oil pressure correct?
Best!
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
Run it. If rings free up and/or junk blows off valve seats so they seal better, good. If not you are no worse off than if you tear it down now. We already know it is a P-bomb. What car is it? I had 65-68 GTO’s. And 68 bird convert. Wish I coulda kept the firebird and 65 GTO convert.
Last edited by derekderek; Oct 7, 2021 at 07:18 AM.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,212
Likes: 9,352
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Get er done Bandit!!!!