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Leak Down Testing?

7K views 20 replies 12 participants last post by  Philthy 
#1 ·
I've just been given a leak down testing kit & I've never used one before.:confused:

Does any one know how much air pressure I should apply? I was thinking around 20psi, is this enough?

Thanks Guy's.:vrod:
 
#2 ·
Paul I usually use 100psi or just line pressure. After all we know the rings are going to leak and all your looking for is the time they leak. I take a time reading between cylinders to compare. If I find one cylinder that leaks much faster it will usually have lower compression doing a regular test. I also remove the dipstick (oil fill) itself, hate to blow seals out of the engine if it can't vent fast enough.
 
#3 ·
Max, I have done this test, but have never fully understood it. Does this test show you that valves are seating more than ring issues. Because don't many compression and oil rings expand upon detination actually sealing the cylinder better. I just didn't know what this test really told you over a compression test?
 
#5 ·
Ray the leakdown test has some big advantages. If you use pressurized air in the 90-100psi range the engine is not turning so you can hear were the air is escaping. So you know if it's rings or valves. You can also remove the radiator cap and check for bubbles in the cooling system to check the heads which a standard compression test cannot do. I think there are different leak down gauges as well that determine differently the flow. Mine is one from kentmoore (alias cost more) and you leave the air on all the time and it gives a reading of air being lost. Then you can open the throttle bores and listen for air and go to the tail pipe and listen for air as well. You can also listen to the crankcase and feel the air being lost on some engines.
 
#6 ·
One thing I almost forgot Paul. Make sure that you use something to lock the engine down and you really need to turn each piston to tdc when you put the pressure to it. You want to lock the engine so it won't turn because of course it would push the piston back down but also because it could turn the engine backwards and it sounds like your tool didn't come with good instructions.
 
#7 ·
Thanks cowboy and Max, so really a compression tester is just a quick reference for overall individual cylinder condition, and the leak down is the same only with more diagnostic trouble shooting capabilities?
 
#8 ·
Yes Ray but the leak down takes longer to setup and run per cylinder as well. You have to run each piston to tdc, lock the engine down and then take time to analyze each cylinder but when real performance is the issue the leak down test is much better.
 
#9 ·
vrodcustoms said:
Thanks cowboy and Max, so really a compression tester is just a quick reference for overall individual cylinder condition, and the leak down is the same only with more diagnostic trouble shooting capabilities?
Yes and no. Two different tests actually. The compression test gives the cylinder three or four revolutions to “pump up” – and since you don’t know what the psi should be (many factors – cam timing, duration, compression ratio, state of charge in the battery, etc) it’s a good indicator to make sure all of the cylinders are equal – usually within 10%.

If you find a low one or two, adding a touch of oil will tell you if it’s the rings…

The leak down test is shows how well the cylinder actually seals. The Moroso tester I have shows the percentage of leak. The more leak down you have, the less efficient that cylinder is and less power that it will make.

HTH,

Dave
 
#10 ·
Dave your exactly right that you really need both numbers for real performance. I was simply using it in a diagnostic stand point. Sorry that wasn't clear.
 
#11 ·
Max said:
One thing I almost forgot Paul. Make sure that you use something to lock the engine down and you really need to turn each piston to tdc when you put the pressure to it. You want to lock the engine so it won't turn because of course it would push the piston back down but also because it could turn the engine backwards and it sounds like your tool didn't come with good instructions.
Thanks Max, reckon that pretty much covers it mate! Unfortunately I got no instructions as the kit was given to me & it's not new.
I won't have any problems now though, thanks again.;)
 
#12 ·
Doesnt the V have some sort of pressure relief during startup using the cams to hold the valve open or something? Is that only during start cranking or what? Never understood that very well.
 
#13 ·
Yes Sam the cam has a centrifugal weight ACR (automatic compression realease) mechanisim that is activted at start, and deactivated by centrifugal force around 900 rpms.
 
#14 ·
Ray you mentioned a magic number of 900 rpms, isn't that the same rpm the engine will stall at when a small load is applied without adding rpm. It seems like we did some playing with stalling the engine and I was surprised to see that it stalled almost like the key was shut off at or about 900rpm. Any chance that some of the early stall complaints came from this compression release valve?
 
#15 ·
This thread appears to be the most relevant to my problem. We've got some incredibly knowledgeable people on this forum, and I need some advice.

My bike is a 2003 VRSCA - bought it used with 12,000 miles - now has 12,500. The bike was owned by a guy who was about 60 - I doubt that he was really beating on it, and the bike's appearance and maintenace records were quite good.

Bike is pretty much stock - except for KN air filter and Screaming Eagle pipes.

I have a problem with an engine miss. At a constant 45-50 MPH, it will fail to fire about 20-40 times per mile traveled. Occasionally, it will have a giant hiccup and will decelerate noticeably. There's never a backfire, so I don't suspect the valves being faulty.

I've taken it to the HD dealer twice now. On the first trip, they told me that the ECU was programmed for a 30-tooth sprocket (apparently, I have a 28-tooth), and I spent $250 to get it re-programmed. They told me that this should fix it.

No joy...no change in the problem. Worse than that, during the check-in process today, there were three people involved - person #3 said that the only thing the reprogramming would do is to re-calibrate the speedometer.

I did not hear any offer forthcoming about a refund due to their misdiagnosis.

Today's diagnosis said that they ran a compression check - observed 160 PSI - and that the range should be 203 - 247 PSI.

They recommend new rings and cylinder liners...estimated cost $2280.

In all fairness, the rubber tubes leading from the crankcase to the intake manifold are slobbering. The bottom side of the air butterfly was wet with an oily residue. I do not find this oily stuff in the exhaust pipes.

I questioned the mechanic about the lack of oil in the exhaust and the lack of oil fouling of the plugs. He said that it burns off. Question: if it burns off, what causes the failure to fire?

I have not driven enough miles to observe if the oil level in the crankcase is dropping noticeably.

After their first costly mis-diagnosis, I have lost some of my trust in them. After spending a few hours this afternoon reading the advice contained on this forum, I find that because of the compression release feature, the compression test might be sort of iffy, and that a leak down test is perhaps the better way to determine the fault.

Misdiagnoses and loss of trust in your dealer is not a good thing. Having a forum like this - with no one having a financial interest - is a much better thing.

Any advice from you guys?

Jim
 
#16 ·
Holy thread revival
The first common problem on this model year was the Fuel Flange it was a TSB from the Moco. Look in the fuel tank while the engine is running and look for misting or a spray. It is easier if the tank is 1/2 or less full of fuel. If you see misting or spraying, the fuel flange is defective. Do a search on fuel flange, there are plenty of topics and the TSB.
 
#18 ·
being a dealer tech i hate to bad mouth another dealer but this does sound bad.. the change in calibration for the different pulley should cost maximum one hr charge. and no way would that cause the problem you describe... i wouldn`t go for the rebuild until you have completely checked out all the other possible problems ,i haven`t seen a stock v-rod wear out the rings and liners yet and we have a bunch of high mileage v-rods in our area.. as far as leak down a v-rod will show higher leak down figures than other motors.. harley allows a lot higher leak down on a v-rod than a big twin before they will pay for going in the motor..i think the number they will accept is up to 16%.. as an example my race kawasaki`s were around 4%.. i think that the liners in a v-rod don`t stay as straight as they do in some other motor designs..
 
#20 ·
Jimrat, listen to these guys. They know what they are talking about and can walk you through diagnosing almost anything on the vrod.

As Hal64hd said, the first and easiest thing to check is the fuel flange fitting. Start the bike, remove the fuel cap and look inside. You may need a flashlight. If you see any kind of misting or sraying, that will be your problem.

Let us know what you find. And oh yea, stay away from that dealer!
 
#21 ·
According to the Service Manual, the compression of the front cylinder should be between 113 and 137 psi while the compression of the rear cylinder should be between 203 and 247 psi. The difference is due to the compression release on the front cylinder and is normal.
For a good compression or leak down test, the engine should be at operating temperature. by the way. A cold or cool engine won't give an accurate reading with either test.

The manual mentions checking and adjusting valve lash for both tests if leaking valves are suspected.

I tore my hair out earlier this year with a similar problem on a BMW of mine. All the cylinders had 170 psi compression and leakdown was 0 to 2%. Turned out the problem was low fuel pressure due to a faulty fuel pressure regulator. Instead of a nice atomized spray I was getting droplets. I would look for a fuel problem on such a low mileage bike. These engines do not wear out rings and valve with such low miles as yours.
 
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