Leak Down Test Results
Thread Starter
5th Gear
Joined: Feb 2023
Posts: 5
Likes: 0
Leak Down Test Results
I just picked up a 78 from my grandfather. He said his cousins drove the car for a year until they gave it back. I bet I know why they gave it back. We just did a leak down test today because it is backfiring through the carburetor. We were trying to rule out valves before we went the wiped cam route. Our findings had 78% on one, 40% on another and 36% on another blowing air out the fill hole in the valve cover and the dipstick tube. Are there any other alternatives it could be other than bad rings? Just wanted to make sure we aren't overlooking something before we get into a rebuild stage. Thanks for any insight! Also if anyone has any insight on the backfiring I will take that too. There were no leaks through the intake or the exhaust. It runs smooth as silk idling and starts to backfire once revved up to higher RPMs.
Race Director
Joined: Nov 2006
Posts: 13,193
Likes: 7,793
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Those are really bad leak down test results.
sounds like more than just rings. Most likely rust damage to the bores.
With results like that I would just start taking it apart.
sounds like more than just rings. Most likely rust damage to the bores.
With results like that I would just start taking it apart.
2026 Loser of the Year
Joined: Sep 2013
Posts: 36,573
Likes: 7,017
From: New Or-leens Loo-z-anna
I had a 383 engine that jumped timing so much that it idled well but had no power.
It took the mechanic an hour of troubleshooting to solve the problem/plastic timing gear teeth crumbled.
The good news was...no bent valves.
1 in 100,000,000, but it happened
Race Director
Joined: Nov 2006
Posts: 13,193
Likes: 7,793
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Jumped cam timing wouldn't cause leak down.
So no. Absolutely not the issue
So no. Absolutely not the issue
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
rtdice94,
Before tearing into this engine, you could do a quick test.
With the valve covers off, have a helper sit behind the wheel. You will be checking for any valve that is not closing completely. Look at the first cylinders rockers. When both valves are closed, there should be a slight movement of the rocker. Have your helper jog the IGN key and move right down the line grabbing rockers.
Valve lash set too tight will allow air to blow out the valve cover ports.
Valve lash set too tight will give horrible Leak-Down results.
Before tearing into this engine, you could do a quick test.
With the valve covers off, have a helper sit behind the wheel. You will be checking for any valve that is not closing completely. Look at the first cylinders rockers. When both valves are closed, there should be a slight movement of the rocker. Have your helper jog the IGN key and move right down the line grabbing rockers.
Valve lash set too tight will allow air to blow out the valve cover ports.
Valve lash set too tight will give horrible Leak-Down results.
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,040
Likes: 4,395
From: Marlton NJ
Well leak-down should be 90%+.
So you are leaking a lot of air...somewhere.
While you have the leakdown gear hooked up, you should listen to both the exhaust pipe, the valve cover oil fill hole, and the carb.
All that air-leak has to be going out one of those places.
Then you will have more clues as to where the problem lies. You may even rule-out one or two.
So you are leaking a lot of air...somewhere.
While you have the leakdown gear hooked up, you should listen to both the exhaust pipe, the valve cover oil fill hole, and the carb.
All that air-leak has to be going out one of those places.
Then you will have more clues as to where the problem lies. You may even rule-out one or two.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
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
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,040
Likes: 4,395
From: Marlton NJ
Yep no intake valve or exhaust valve sealing problem.
So it has to be leaking past the rings.
Next step I would do is stick a borescope down a sparkplug hole and see how the cylinder walls look.
It could be rust, or sticking rings. For the low leak down readings.
The popping could be a lot of things.....
So it has to be leaking past the rings.
Next step I would do is stick a borescope down a sparkplug hole and see how the cylinder walls look.
It could be rust, or sticking rings. For the low leak down readings.
The popping could be a lot of things.....
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I might even question such bad leak down numbers because I wasn't there during the test. It take a higher HP air compressor to supply a constant 100 psi. It's not going to happen with a Harbor Freight 3.5 HP with a 20 gallon tank.
You have to be right at TDC because it will rotate the motor.
The only time in my life that I ever saw horrifically bad leak down numbers like that was when I installed Keith Black Hypereutectic in my L-82 355 ci. They smeared aluminum down through the rings locking them in the lands. The motor sounded like it had terrible rod knock from piston slap. No ring sealing at all. Cracked or detonation burnt through pistons from NO2, But those kind of motors smoke horribly out the exhaust because they are just burning oil.
You have to be right at TDC because it will rotate the motor.
The only time in my life that I ever saw horrifically bad leak down numbers like that was when I installed Keith Black Hypereutectic in my L-82 355 ci. They smeared aluminum down through the rings locking them in the lands. The motor sounded like it had terrible rod knock from piston slap. No ring sealing at all. Cracked or detonation burnt through pistons from NO2, But those kind of motors smoke horribly out the exhaust because they are just burning oil.
Thread Starter
5th Gear
Joined: Feb 2023
Posts: 5
Likes: 0
rtdice94,
Before tearing into this engine, you could do a quick test.
With the valve covers off, have a helper sit behind the wheel. You will be checking for any valve that is not closing completely. Look at the first cylinders rockers. When both valves are closed, there should be a slight movement of the rocker. Have your helper jog the IGN key and move right down the line grabbing rockers.
Valve lash set too tight will allow air to blow out the valve cover ports.
Valve lash set too tight will give horrible Leak-Down results.
Before tearing into this engine, you could do a quick test.
With the valve covers off, have a helper sit behind the wheel. You will be checking for any valve that is not closing completely. Look at the first cylinders rockers. When both valves are closed, there should be a slight movement of the rocker. Have your helper jog the IGN key and move right down the line grabbing rockers.
Valve lash set too tight will allow air to blow out the valve cover ports.
Valve lash set too tight will give horrible Leak-Down results.
Thread Starter
5th Gear
Joined: Feb 2023
Posts: 5
Likes: 0
I might even question such bad leak down numbers because I wasn't there during the test. It take a higher HP air compressor to supply a constant 100 psi. It's not going to happen with a Harbor Freight 3.5 HP with a 20 gallon tank.
You have to be right at TDC because it will rotate the motor.
The only time in my life that I ever saw horrifically bad leak down numbers like that was when I installed Keith Black Hypereutectic in my L-82 355 ci. They smeared aluminum down through the rings locking them in the lands. The motor sounded like it had terrible rod knock from piston slap. No ring sealing at all. Cracked or detonation burnt through pistons from NO2, But those kind of motors smoke horribly out the exhaust because they are just burning oil.
You have to be right at TDC because it will rotate the motor.
The only time in my life that I ever saw horrifically bad leak down numbers like that was when I installed Keith Black Hypereutectic in my L-82 355 ci. They smeared aluminum down through the rings locking them in the lands. The motor sounded like it had terrible rod knock from piston slap. No ring sealing at all. Cracked or detonation burnt through pistons from NO2, But those kind of motors smoke horribly out the exhaust because they are just burning oil.
Race Director
Joined: Nov 2006
Posts: 13,193
Likes: 7,793
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
I'd say, back up a bit. Basics.
What do all 8 plugs look like?
Have you run a compression test?
High speed miss could very well be a ignition system component. Like a failing coil or ign. module.
What do all 8 plugs look like?
Have you run a compression test?
High speed miss could very well be a ignition system component. Like a failing coil or ign. module.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
A wiped cam lobe does however, affect the compression testing because the valvetrain is now put into action for this test.
There are some products on the market that claim to repair cylinder wall scratches. Supposedly, it fills in the voids in the walls caused by ring end gap damage.
I assume these products are introduced through all eight sparkplug holes. Then spin it over.
Off hand, I can't remember the name of the product. Maybe someone else has some thoughts on if it works or not.
And you really need to check that all 16 valves are truly closing all the way. Wiggle those rockers.
Thread Starter
5th Gear
Joined: Feb 2023
Posts: 5
Likes: 0
Most looked good. 2 or 3 had pretty weak spark. The number 3 cylinder with the 78% leak spark plug had a questionable oil smell to it and weak spark. The compression test was next but we are headed out of town and my buddies are headed to the racetrack so we didn't get to it. Recommend both wet and dry? Or just a dry would tell you enough. We didn't anticipate the leak down results that we got because as we ran it and drove it like I said before it didn't smoke, knock, ping, or hesitate so how the motor performs doesn't quite line up with the leak down results so I wanted to see what we could be overlooking before having to go through a rebuild. We were only trying the leak down test to see if a valve was bad or if the camshaft was bad and ended up with those results lol.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Pot metal austempered cams from Comp Cams started out around the year 1998? (but don't quote me on that) Big companies like comp make cam cores and the smaller vendors grind them to their specs. I had a flat fuel pump lobe on my CC extreme H-roller grind. I was so pissed about the whole deal that I went to billet steel cams. I would rather pay a little more now than a wheel barrel full later with a trashed motor from metal in the oil.
That is when the proliferation of lobe ware in a short period of time started. If you have an original GM cam from pre 2000 you are not going to have flat lobes unless you use some substandard oil with rare oil changes.
That is when the proliferation of lobe ware in a short period of time started. If you have an original GM cam from pre 2000 you are not going to have flat lobes unless you use some substandard oil with rare oil changes.
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,040
Likes: 4,395
From: Marlton NJ
If your comment above means NO hissing thru the intake or the exhaust, DURING the leakdown test, then you have NO valve sealing problems. If so, stop worrying about the valve lash. The valves are seated. Your leak must be the ring/bore seal. It needs a borescope or to come apart.
You could however still have a wiped cam, and only getting half of the normal lift, but that is another issue entirely. But with the above problem, it's irrelevant.
You could however still have a wiped cam, and only getting half of the normal lift, but that is another issue entirely. But with the above problem, it's irrelevant.
Team Owner
Joined: Mar 2002
Posts: 44,644
Likes: 1,827
From: Huntsville AL
The car doesn't smoke at all and runs very smooth at idle. It pulls strong through the power band even once it gets up to higher RPMs but that is when it starts to backfire. No loss of power, smoking, hesitating, knocking etc. That is why we are trying to cover out bases because if it leaks past the rings this bad we would have thought it would smoke real bad. Anything you can think of that we are overlooking/not doing properly?