There is no need to spend money for a leakdown tester because you know there is a problem. All you need is a hose to screw into the spark plug opening and a compressor. Bring the piston TDC on the compression stroke (both valve closed). I used about 100 PSI.You can pick up your own leakdown tester at just about any parts store. The inexpensive ones usually run around $50-$80. All you need is access to a compressor.
Another advantage to doing a leak down test is you can almost always hear where the leak is coming from. If it is getting by the intake valve, you should be able to hear it reversing back into the intake manfold. You could open the throttle blase to listen even better.
There is no need to spend money for a leakdown tester because you know there is a problem. All you need is a hose to screw into the spark plug opening and a compressor. Bring the piston TDC on the compression stroke (both valve closed). I used about 100 PSI.
Well if you think 30 psi difference is only "suspects there is a problem" I do not want to see what you call problems. If a compression tested showed all cyclinders the same than I would do a leak down test to see how close they were. To me a compression test is like a rule and the leak down test is like a micrometer.He suspects there is a problem, he does not know for certain. Just because two cylinders show a 30 PSI lower reading than the other 6 does not automatically mean there is a problem especially since the 160 PSI is within Ford spec. That's why the leak down test shold always follow a static compression check if there are suspect cylinders. If is a more thorough test than a static compression test because it eleiminates many of the variables associated with performing a SC test.
You can listen to where the leak is coming from by the procedure in which you described, but then you are missing out on a crucial part of the leak down test....the leak down.
You can also piece together your own leak down tester for less than a new one, and a leak down tester is a must for every car guy to have in his toolbox anyway.
The thing about doing a compression test is that there are many factors that can possibly and adversely affect the outcome of each cylinder....battery condition, coolant system pressure, motor temp, carbon buildup on the deck of the piston and/or combustion chamber, position of the throttle blade, etc. etc. The leakdown test eliminates all of those influential factors and focuses on the differential of leakdown amongst the different cylinders. It is not a comparison of which test is more finite (compression test or LD test)....they are two completely different tests.
Everybody always automatically assumes that the 2 numbers that are "off" from the others are the low numbers. You have to keep a comprehensive perspective when doing engine diagnostics, and consider the possibility that the other numbers are possibly "high." And in my experience, 190 PSI on a stock 4.6 2V motor with close to 100k miles is a tad higher than the average similar motor.
He already mentioned that he ran the car for a few weeks with 2 known bad coils. If there were significant carbon buildup on either the piston and/or the combustion chamber, that could have skewed the PSI readings across the board except for the two cylinders in question which could have had a significant portion of their carbon buildup "washed down", especially if there was little to no spark on those two cyilnders for two weeks.
The compression check only shows you how much pressure the cylinder can develop under cranking conditions at the dwell of it's TDC stroke. That only tests to see how much pressure it can hold for a few tenths of a second. It's a rudimentary test, but can be useful as a rough preliminary diagnosis if taken for what it is.
You don't just slap on a compression tester, find two cylinders that have a 30 PSI differential, and go tearing into the motor. You exhaust every possible diagnostic that you can so as to find the problem, if any, as accurately and definitively as possible. That's exactly where the LD test takes over. Then IF you have to go into the motor, you know precisely what needs to be done.
The thing about doing a compression test is that there are many factors that can possibly and adversely affect the outcome of each cylinder....battery condition, coolant system pressure, motor temp, carbon buildup on the deck of the piston and/or combustion chamber, position of the throttle blade, etc. etc. The leakdown test eliminates all of those influential factors and focuses on the differential of leakdown amongst the different cylinders. It is not a comparison of which test is more finite (compression test or LD test)....they are two completely different tests.
Everybody always automatically assumes that the 2 numbers that are "off" from the others are the low numbers. You have to keep a comprehensive perspective when doing engine diagnostics, and consider the possibility that the other numbers are possibly "high." And in my experience, 190 PSI on a stock 4.6 2V motor with close to 100k miles is a tad higher than the average similar motor.
He already mentioned that he ran the car for a few weeks with 2 known bad coils. If there were significant carbon buildup on either the piston and/or the combustion chamber, that could have skewed the PSI readings across the board except for the two cylinders in question which could have had a significant portion of their carbon buildup "washed down", especially if there was little to no spark on those two cyilnders for two weeks.
The compression check only shows you how much pressure the cylinder can develop under cranking conditions at the dwell of it's TDC stroke. That only tests to see how much pressure it can hold for a few tenths of a second. It's a rudimentary test, but can be useful as a rough preliminary diagnosis if taken for what it is.
You don't just slap on a compression tester, find two cylinders that have a 30 PSI differential, and go tearing into the motor. You exhaust every possible diagnostic that you can so as to find the problem, if any, as accurately and definitively as possible. That's exactly where the LD test takes over. Then IF you have to go into the motor, you know precisely what needs to be done.
The only thing that the leak down test will give him that just blowing air into the cylinder will not is the % of air leaking. Also just for your information wear on the intake lobe cause cause a higher cranking compression reading. Cam phasing changes (centerline movement) also will cause changes in cranking compression. That is why you degree in your cam and may advance or retard it to change dynamic compresson ratio.
Well if you think 30 psi difference is only "suspects there is a problem" I do not want to see what you call problems. If a compression tested showed all cyclinders the same than I would do a leak down test to see how close they were. To me a compression test is like a rule and the leak down test is like a micrometer.
Compression test was done after the coils were replaced. I had 200 miles of driving w/ a misfire. And yes, they are the cyclinders that are low.
...and sorry, the g/f pic stays small.
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