Compression test results?
Thread Starter
Racer
Joined: Oct 2014
Posts: 265
Likes: 42
Compression test results?
I have a 2012 A6 Grand Sport, cammed with an A&A Vortech Ti-trim, and car would not pull past 110 on last couple of races. Normally traps 137 at the track. Just installed new drag pack and after first pull the service active handling would come on and tranny seemed like it did it want to shift. Same for the next 2 runs. Anyway, long story short, decided to do a compression test. All the cylinders tested 190-195 except for #8, which tested 160. The car does not idle rough nor does it blow any smoke whatsoever! Put stock wheels back on yesterday and did several pulls from 3rd gear and car pulled and shifted like it should. Thinkin about doing a leakdown test. Any thoughts? I have received a quote on rebuilding motor and tranny but sure don't want to if not needed. Thanks in Advance
Team Owner
Joined: Jun 2005
Posts: 46,295
Likes: 2,596
From: Northern, VA
St. Jude Donor '15
"In honor of jpee"
You already know all this, but it doesn't hurt to do the oil in low cylinder first imo.
"If one cylinder reads significantly below others, add about a tablespoon of SAE 30 motor oil to the cylinder and repeat the test, which is called a “wet” compression test, to see if compression improves. Here’s what it means:
One cylinder low usually indicates either bad piston rings or leaking valves. If adding oil caused compression to increase, the piston rings are at fault. If adding oil did not increase compression, the valves are leaking.
One cylinder low can also indicate that the head gasket is ruptured—a “blown head gasket”—affecting only that cylinder. If the vehicle has been losing coolant, that diagnosis is likely confirmed, but a cooling system pressure test will provide a sure answer.
Two adjacent cylinders with a low reading suggests a blown head gasket between those two cylinders.
Because head gaskets also seal off coolant passages, you should check for any indication that coolant is getting into the engine oil whenever you suspect a head gasket may have blown. If you find it, it confirms the diagnosis. Coolant mixed with oil takes on a brown, muddy color.
A cylinder that ultimately achieves a reading within tolerances of the other cylinders, but which takes an unusually large number of engine strokes to do so, is likely to have worn piston rings. A healthy cylinder should reach its maximum compression reading in the first two compression strokes. You can verify this by performing a wet compression test on the cylinder.
Compression that is above specifications is generally considered an indication of carbon build-up in the cylinder, cylinder heads, and piston top. It can also indicate that fluid, either coolant or oil, is leaking into the cylinder. In that case, however, you should see smoke from the exhaust when the engine is running."
fr.: https://www.cartechbooks.com/techtip...ressionteting/
"If one cylinder reads significantly below others, add about a tablespoon of SAE 30 motor oil to the cylinder and repeat the test, which is called a “wet” compression test, to see if compression improves. Here’s what it means:
One cylinder low usually indicates either bad piston rings or leaking valves. If adding oil caused compression to increase, the piston rings are at fault. If adding oil did not increase compression, the valves are leaking.
One cylinder low can also indicate that the head gasket is ruptured—a “blown head gasket”—affecting only that cylinder. If the vehicle has been losing coolant, that diagnosis is likely confirmed, but a cooling system pressure test will provide a sure answer.
Two adjacent cylinders with a low reading suggests a blown head gasket between those two cylinders.
Because head gaskets also seal off coolant passages, you should check for any indication that coolant is getting into the engine oil whenever you suspect a head gasket may have blown. If you find it, it confirms the diagnosis. Coolant mixed with oil takes on a brown, muddy color.
A cylinder that ultimately achieves a reading within tolerances of the other cylinders, but which takes an unusually large number of engine strokes to do so, is likely to have worn piston rings. A healthy cylinder should reach its maximum compression reading in the first two compression strokes. You can verify this by performing a wet compression test on the cylinder.
Compression that is above specifications is generally considered an indication of carbon build-up in the cylinder, cylinder heads, and piston top. It can also indicate that fluid, either coolant or oil, is leaking into the cylinder. In that case, however, you should see smoke from the exhaust when the engine is running."
fr.: https://www.cartechbooks.com/techtip...ressionteting/
Thread Starter
Racer
Joined: Oct 2014
Posts: 265
Likes: 42
Thanks for the reply! I will try that as well as a leak down test and see what I can find out!
"If one cylinder reads significantly below others, add about a tablespoon of SAE 30 motor oil to the cylinder and repeat the test, which is called a “wet” compression test, to see if compression improves. Here’s what it means:
One cylinder low usually indicates either bad piston rings or leaking valves. If adding oil caused compression to increase, the piston rings are at fault. If adding oil did not increase compression, the valves are leaking.
One cylinder low can also indicate that the head gasket is ruptured—a “blown head gasket”—affecting only that cylinder. If the vehicle has been losing coolant, that diagnosis is likely confirmed, but a cooling system pressure test will provide a sure answer.
Two adjacent cylinders with a low reading suggests a blown head gasket between those two cylinders.
Because head gaskets also seal off coolant passages, you should check for any indication that coolant is getting into the engine oil whenever you suspect a head gasket may have blown. If you find it, it confirms the diagnosis. Coolant mixed with oil takes on a brown, muddy color.
A cylinder that ultimately achieves a reading within tolerances of the other cylinders, but which takes an unusually large number of engine strokes to do so, is likely to have worn piston rings. A healthy cylinder should reach its maximum compression reading in the first two compression strokes. You can verify this by performing a wet compression test on the cylinder.
Compression that is above specifications is generally considered an indication of carbon build-up in the cylinder, cylinder heads, and piston top. It can also indicate that fluid, either coolant or oil, is leaking into the cylinder. In that case, however, you should see smoke from the exhaust when the engine is running."
fr.: https://www.cartechbooks.com/techtip...ressionteting/
One cylinder low usually indicates either bad piston rings or leaking valves. If adding oil caused compression to increase, the piston rings are at fault. If adding oil did not increase compression, the valves are leaking.
One cylinder low can also indicate that the head gasket is ruptured—a “blown head gasket”—affecting only that cylinder. If the vehicle has been losing coolant, that diagnosis is likely confirmed, but a cooling system pressure test will provide a sure answer.
Two adjacent cylinders with a low reading suggests a blown head gasket between those two cylinders.
Because head gaskets also seal off coolant passages, you should check for any indication that coolant is getting into the engine oil whenever you suspect a head gasket may have blown. If you find it, it confirms the diagnosis. Coolant mixed with oil takes on a brown, muddy color.
A cylinder that ultimately achieves a reading within tolerances of the other cylinders, but which takes an unusually large number of engine strokes to do so, is likely to have worn piston rings. A healthy cylinder should reach its maximum compression reading in the first two compression strokes. You can verify this by performing a wet compression test on the cylinder.
Compression that is above specifications is generally considered an indication of carbon build-up in the cylinder, cylinder heads, and piston top. It can also indicate that fluid, either coolant or oil, is leaking into the cylinder. In that case, however, you should see smoke from the exhaust when the engine is running."
fr.: https://www.cartechbooks.com/techtip...ressionteting/
Drifting
Joined: Jun 2016
Posts: 1,291
Likes: 222
From: Temecula, CA
If your car ran normally with your stock wheel set up, you have a separate issue beyond the compression. Did you try making runs with your drag pack set up with TC/AH off? Nannies may be kicking in due to non-stock height tires and different front to rear tire height ratios.
Thread Starter
Racer
Joined: Oct 2014
Posts: 265
Likes: 42
If your car ran normally with your stock wheel set up, you have a separate issue beyond the compression. Did you try making runs with your drag pack set up with TC/AH off? Nannies may be kicking in due to non-stock height tires and different front to rear tire height ratios.
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Thread Starter
Racer
Joined: Oct 2014
Posts: 265
Likes: 42
Melting Slicks
Joined: Jul 2010
Posts: 3,006
Likes: 245
From: Surprise, Az
I think you found the low cylinder by accident and as mentioned, are looking at a size issue.
You could have the cylinder in questioned scoped to see if there is any scoring and if not, sounds like a valve issue the leak-down test should have helped identify. OTOH, if it maintains the same pressures over time, not much to worry about.
You could have the cylinder in questioned scoped to see if there is any scoring and if not, sounds like a valve issue the leak-down test should have helped identify. OTOH, if it maintains the same pressures over time, not much to worry about.
Drifting
Joined: Feb 2014
Posts: 1,418
Likes: 267
From: Pontiac Quebec
happened to a few of my buddies cars when going with another wheel setup. the tpms system can cause the car to go into a "limp mode" or "reduced power" My tuner was able to work passed that. We also made a small tube with pvc piping and connected a tire valve to it. we put all 4 tpms sensors in it and have it at 32psi. the car then sees all 4 tires with same values and that light stays off regardless what tire set up you have on.
Melting Slicks
Joined: Jul 2005
Posts: 2,651
Likes: 864
When you say "I had a leakdown performed and it was good." - what exactly does that mean ???
If it means the leakdown was something less than 5 psi (with 100 psi input air), and all cylinders were even, that's GREAT news !!
But if the leakdown on the one cylinder was higher than the others but still in the supposedly "OK" range of less than 10% - you would want to know more. One of the most useful things about the leakdown test is that you can listen for where the air is coming from (Intake, Exhaust, Oil filler, bubbles in the radiator, etc.) - which tells you a lot about what the problem may be.
If the leakdown numbers are consistent and low - it's probably safe to assume the engine is fine ... Hope it was the PCM was killing power based on the TPMS - and you might want to see if you can replicate it by going back to the other tires/wheels and seeing if it happens again.
If it means the leakdown was something less than 5 psi (with 100 psi input air), and all cylinders were even, that's GREAT news !!
But if the leakdown on the one cylinder was higher than the others but still in the supposedly "OK" range of less than 10% - you would want to know more. One of the most useful things about the leakdown test is that you can listen for where the air is coming from (Intake, Exhaust, Oil filler, bubbles in the radiator, etc.) - which tells you a lot about what the problem may be.
If the leakdown numbers are consistent and low - it's probably safe to assume the engine is fine ... Hope it was the PCM was killing power based on the TPMS - and you might want to see if you can replicate it by going back to the other tires/wheels and seeing if it happens again.
Thread Starter
Racer
Joined: Oct 2014
Posts: 265
Likes: 42
**UPDATE**
Just received a call from GPI on my car. Good news the motor is fine. The car was running really lean at 13.1 and good bit of belt slip. Waiting on them to take a look at the service active handling issue . So far going to go ahead and move forward with stall converter and a flex fuel system for E-85.
Just received a call from GPI on my car. Good news the motor is fine. The car was running really lean at 13.1 and good bit of belt slip. Waiting on them to take a look at the service active handling issue . So far going to go ahead and move forward with stall converter and a flex fuel system for E-85.