Diagnose engine compression issues like a pro with this Redline Update


The Hagerty employee-restored 1969 Chevrolet Camaro is not running well. It has racked up miles driving from sea to shining sea, but the latest trip has brought it to the Redline Rebuild garage for Davin to diagnose excessive oil consumption and poor running. That calls for a compression test, but Davin takes it one step further and completes a leakdown test as well—showing you exactly how to do it yourself along the way.

This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2019/04/17/diagnose-engine-compression-issues-like-a-pro

Is there really a Hagerty Redline Rebuild Garage? In Traverse City??
I’m a Hagerty Drivers Club Member, own a '64 Chevelle SS, 327, last year I noticed a decline in performance. I’m thinking the Holley 4bbl isn’t opening up all the way.
Will be rolling it out soon.
Any suggestions?
Thank you,


Jim, if you suspect your Holley, I highly recommend the ‘paper clip’ test to see if your secondaries are opening. If you search/Google this test it is well explained on several sites and YouTube videos. I find a bobby pin works better, but regardless, a safe, fast confirmation of the secondaries operation, or lack thereof.


I’ve tried the paper clip, it works. However, I still don’t believe the thrust from gas gets there.
The carb metering block was loaded with goey substance resulting from ethanol gas sitting too long. I’ve cleaned it, still not getting the thrust it should have.


A “Leakdown Test”, also know as a “Differential Compression Test”.
Practically never seen in automotive repair shops, this test is the only compression test you’ll see in an aircraft repair shop.
Does our narrator fully understand the leakdown test? Correctly performed the engine should be at TDC. On true TDC it won’t spin. The test can be conducted at BDC as in the clip, but leakage pressure reading will not be as accurate. (Some aircraft engine manufacturers publish a minimum pressure for a cylinder to be deemed “serviceable”.) But more importantly interpretation. In the “ideal” cylinder there is no leakage - and thus BOTH gauges will read the same. However, there is no “ideal” cylinder, so the second (“leakage”) gauge will always show a lower pressure - the difference in pressure between the gauges being proportional to the amount of leakage. The supply gauge (nearest the regulator) feeds through a calibrated orifice allowing it to show a set feed pressure (in the clip 90 psi was used, in aviation 80 psi is standard). As leakage increases the cylinder can not maintain pressure and the pressure on the leakage gauge drops - more leakage, lower pressure on the gauge. In the extreme, say a broken valve, the second gauge may never read any pressure (0 psi). A good tight cylinder may likely read within 20 psi of the supply gauge pressure.

The narrator opens citing 2 sources for an oil leak. But, as noted later in the clip, there is a third source of oil leakage into the cylinder - the gaskets. On GM engines and similar engines that use the intake manifold to seal the valve valley, as well as engines which combine the valve valley cover with the intake manifold gasket, an intake gasket leak can contribute oil. This leak will also contribute a vacuum leak resulting in an excessively lean mixture. All those gray deposits on the spark plugs? But note there is also a pressure port bringing oil to the head to lubricate the valves. A head gasket leak may spray oil into the nearest cylinder.

Running the engine down to 2 quarts oil! And knowing it burns oil! Someone needs to be whipped with a wet noodle and deprived of ALL driving privileges!


Referring to the paper clip, when you say ‘it works’, do you mean the secondaries are opening?


Yes. The clip was pushed down, so without seeing the secondaries actually opening. The clip devise indicates the secondaries did open.
My thoughts are something is still blocking the free flow of gas to the secondaries. Am I thinking correctly? I have a picture of the metering block prior to cleaning it up


Yup, secondaries are working then. My thinking is if the ethanol gummed up the metering circuits that badly, there could be further issues both in the bowls and jets. Ethanol is very corrosive. Unfortunately, it may need a rebuild. I had a friend needed to rebuild all 6 carbs on his outboard Mercury due to ethanol damage -$1200 later it ran like a charm.


I thought 396 was down on power when I drove it last summer. At least compared to my recently rebuilt 383! :wink: