2 cylinders @ 160 psi, others @190... how long will it last?

[Stan Weiss]

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.

 

[LaserRed01GT]

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.

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.

 

[Rickey_Bobby]

I'm always looking for a new excuse to tell the girlfriend when I wont buy her something shiny. "But baby, I needed a leakdown tester ..."

Just like the "But baby, I needed the drag radials"

 

[Stan Weiss]

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.

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.

 

[the98stang]

Bottom line is, 2 cylinders have low compression, and the problem needs to be fixed.

Leakdown test or not.....

 

[LaserRed01GT]

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.

 

[jstreet0204]

^^^ I agree with LaserRed. I will also add that doing a leak down test may also help diagnose a problem that may not be easily found once you start tearing it down. If a valve is leaking or the rings are bad, and you are looking for a head gasket. You may end up replacing a head gasket, putting it all back together only to find out you still have the problem. The leak down will point you in the right direction.

 

[Rickey_Bobby]

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.

Couldn't have said it better myself. I like knowing what I have to fix before I start a project so it can be done in the most efficient way.

BTW, In referance to the carbon issue, I just put some Seafoam through the motor and killed some mosquitos, Gunna do another compression test, and hopefully this week get a leakdown tester.

On thing that hasn't been addressed yet if the Vac I'm pulling at cruise....
I'm pulling about 10 in/Hg @ 70 mph w/ the A/C off. Now I know there are a lot of factors to this, but just for ****s and giggles, does anyone with a centuri blower of any sort know what they pull for vac @ 70 mph? And is 14mpg through town and a little highway need for concern? My driving style is the same and I used to get about 16mpg before the coils died and left me with this cluster **** of little problems.

 

[Stan Weiss]

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.

 

[LaserRed01GT]

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.

You are correct that a worn intake camshaft lobe will skew the cranking compression test. Yet another reason to perform a leakdown test.

Since the nose radius of the lobe is effectively made larger, the effective lift is lessened; and if the camshaft lobe is worn all the way around to the opening and closing ramps and flanks, then it is possible a little bit of duration will suffer as well. Either way, the cranking compression will be less than normal due to the intake valve not being held open as long and/or as far off the seat.

The one thing that will not be affected is ICL. The intake valve will reach it's maximum lift at a set point in crankshaft degrees after top dead center (ATDC). That is the ICL, which refers to where the camshaft has been installed (degreed) in the engine in relation to the crankshaft. The absolute only thing that can affect the ICL is re-degreeing the camshaft, having a timing chain/cam chain jump a tooth, or a severely damaged lobe which would cause immediate failure of the motor anyway.

 

[Mike97gt]

I don't have time to read every post since I have limited time on here and I moderate large parts of this site, I have only skimmed the posts so I may be missing somthing here.

was the two bad coils and misfire on the two cylinders that is "down" a little..

If so that can be a big factor due to washed down rings and the unburned fuel can remove some carbon buildup if driven like that long enough..

How long after the misfire was repaired was this compression test run?

 

[Mike97gt]

Ok now I read most of the posts.

pretty much do everything laserred01gt said..

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.

the 30 PSI difference is well within spec, I used the ford PC/ED manual for reference, I personally don't like to see more than a 10 percent difference but if this car was in for a warrenty repair we would not be able to use the compression results for a reason to replace the motor since it's in spec..

 

[FalconGuy016]

This is a very imformative thread that I have learned a lot from.

Rickey_Bobby, I just need one more piece of information though. Can you post a large picture of your avatar

 

[Rickey_Bobby]

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.

 

[Mike97gt]

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.

How long after the coils were replaced.. right after, two days later, 100 miles later????

If you did it RIGHT after then yeah you could be down that much from fuel wash

 

[Rickey_Bobby]

I had 2 weeks of light driving with bad coils total of 200 miles or so. Then the coils were replaced.

Pretty sure I washed the cylinders. Is this damage going to get worse now? Or just stay the same 30 psi lower?

 

[Stan Weiss]

What have you found? Have you done a leak down test?

 

[Rickey_Bobby]

Yup, sure enough the piston rings must be worn as I can hear air going into the block. Still makes 160psi on each. So I plan on driving it like I stole it till I see signs of it getting worse. Taking it to the track this thursday to test the rear suspension I made. Wish me luck!

 

[Stan Weiss]

Good lucky at the track. Let us know how you make out. It a good idea to recheck it now and then. I thing you should be OK and will just be down a little HP from the extra leakage in those two cylinders.

 

[Stan Weiss]

Hey Rickey_Bobby! If you got to the track on Thursday, how did you do?