Question to those who are experienced in the subject- what are acceptable leak down numbers before you should be concerned? I've heard it can vary depending on the engine/manufacturer and I've searched on here and the interwebs to try and find some reference number without any luck. Any info/advice would be appreciated.
What are acceptable cylinder leak down numbers?
- Thread starter Billy06
- Start date
IF you do a test for leak down of the cylinders using compressed air; ensure the tested cylinders valves are shut.
IF you hear the air coming out of the throttle body then that cylinder intake valve that is leaking by.
IF you hear air coming from the PCV grommet you have piston ring(s) that are leaking by (normal to have a little).
IF you hear air coming from the exhaust pipe that cylinders exhaust valve is leaking by.
IF you hear the air coming out of the throttle body then that cylinder intake valve that is leaking by.
IF you hear air coming from the PCV grommet you have piston ring(s) that are leaking by (normal to have a little).
IF you hear air coming from the exhaust pipe that cylinders exhaust valve is leaking by.
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The leakdown test is best used as a comparative test. dont put a spec on it. Leakdown the entire engine and compare each cylinder. Temperature changes will effect the results of this test.
Every engine varies in what is acceptable. And also depends on compression ratio. Obviously an engine with 9.5:1 would have a higher number than an engine with 8.25:1 would have. It is better to have a slightly lower number that is nearly the same on all cylinders than to have 1 or 2 cylinders testing significantly lower than the others. On an engine with 100,000 miles on it 20% (in my experience) is pretty decent. But, as others have said, 25% is getting into the range where efficiency of combustion is getting poor.
Okay, thanks for the responses. Reason if the question is I'm trying to track down a P0301 (Cyl 1 Misfire code). I bought this Jeep back in August and got a heck of a deal from a buddy. Part of the reason for the deal is it had this code already and he didn't want to deal with fixing it. It's a 2006 Rubicon with 200,000 miles on the original 4.0 engine. I've put about 1500 miles on it since I bought it and hasn't used a drop of oil and runs well, minus the slight misfire, which is usually only noticeable at idle. I have eliminated plugs, coil pack, and injectors as the reason for the code. After doing that I did a compression test and yesterday did the leak down test. The leakage was coming from the crankcase, which I've read means it could be a ring problem. I've also read you will usually hear air escaping through the crankcase as they will never seal 100%, so that's why I was curious what the acceptable leakage % was. This was my first time doing compression and leak down test, so there's a chance my numbers may be off a tad one way or the other. Please let me know if these numbers should be alarming. This is a third vehicle for me (2nd Jeep) and bought it as a long term project Jeep and probably wouldn't see more than 5-6000 miles per year. I was hoping I could get another couple years out of this motor, then do a Stroker or hemi swap. Like I mentioned, it runs fairly well, and only reason I'm currently trying to get the code resolved is because I need to get it smog tested here in California.
Compression Leak Down
Cyl1- 130 18%
Cyl2- 145 10%
Cy3- 140 15%
Cyl4- 145 10%
Cyl5- 140 15%
Cyl6- 145 13%
Compression Leak Down
Cyl1- 130 18%
Cyl2- 145 10%
Cy3- 140 15%
Cyl4- 145 10%
Cyl5- 140 15%
Cyl6- 145 13%
For an engine with 200,000+ miles on it... those are respectable numbers.
I chased a misfire code for quite a while. It was usually one cylinder, although at times it would change cylinders, or show more than one cylinder misfiring. Mostly, it would be just one. Cylinder #3.
I tried many different things to eliminate the issue, I cleaned the idle air control valve, replaced the MAP sensor, replaced the throttle position sensor, did new plugs, even changed the coil, but still had the issue. I even wondered if the issue was the fuel injectors.
Then I asked the forum what told the computer that a misfire was occurring. One of the members told me it was the crankshaft position sensor. It monitors that function. I replaced that sensor, and have not had the misfire code since.
The sensor's not cheap, about a hundred IIRC. And it's a pain to get to, on the bell housing, drivers side, barely within reach from underneath the Jeep.
And I'll also say that if you consider replacing the CPS, only use a Mopar sensor. I found when replacing mine the previous owner had used a cheapie aftermarket one when he replaced it. It looked new, but did not function correctly.
Don't know if that will work for you, but that is how I resolved the issue in my 2002 4.0 6 cyl.
I tried many different things to eliminate the issue, I cleaned the idle air control valve, replaced the MAP sensor, replaced the throttle position sensor, did new plugs, even changed the coil, but still had the issue. I even wondered if the issue was the fuel injectors.
Then I asked the forum what told the computer that a misfire was occurring. One of the members told me it was the crankshaft position sensor. It monitors that function. I replaced that sensor, and have not had the misfire code since.
The sensor's not cheap, about a hundred IIRC. And it's a pain to get to, on the bell housing, drivers side, barely within reach from underneath the Jeep.
And I'll also say that if you consider replacing the CPS, only use a Mopar sensor. I found when replacing mine the previous owner had used a cheapie aftermarket one when he replaced it. It looked new, but did not function correctly.
Don't know if that will work for you, but that is how I resolved the issue in my 2002 4.0 6 cyl.
When I was receiving numerous persistent Misfire codes and I couldn't resolve the problem I brought the Jeep up to a friend that is a 30 year Jeep dealership technician. He connected a factory diagnostic tool to the OBD port and accessed the computer readings.
I was receiving more than 200 misfires in a 200 second window of time. We replaced the double platinum spark plugs with the factory recommended NGK spark plugs, performed a combustion chamber cleaning with factory Mopar cleaner letting it soak for 15 minutes and upon restart there was a tremendous plume of smoke coming from the exhaust pipe.
After this the engine misfires went to down to nothing with an occasional reoccurrence of a few misfires every month or so...
When these misfires occur I believe they are caused by carbon build up that is still remaining.
Every month or when I think about it; I put Seafoam in the gas tank which appears to resolve the problem.
I was receiving more than 200 misfires in a 200 second window of time. We replaced the double platinum spark plugs with the factory recommended NGK spark plugs, performed a combustion chamber cleaning with factory Mopar cleaner letting it soak for 15 minutes and upon restart there was a tremendous plume of smoke coming from the exhaust pipe.
After this the engine misfires went to down to nothing with an occasional reoccurrence of a few misfires every month or so...
When these misfires occur I believe they are caused by carbon build up that is still remaining.
Every month or when I think about it; I put Seafoam in the gas tank which appears to resolve the problem.
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if your misfire is only at idle and only on cylinder #1, you may want to check for a leak at the intake manifold.
I did the Seafoam down the throttle body and never really saw the typical plume of smoke. I came back later and did the 1oz/qt recommendation of Seafoam in the crankcase. Noticed a little white smoke after doing that and honestly the Jeep did seem to run a bit smoother after doing that, but the misfire is still there.
Is there any good way of checking the manifold without taking the fender off or removing the manifold? Seems to be packed under there and would be hard to spot a potential leak point.
Okay, haven't had time to much of anything on the Jeep until today. Got the driver side fender off so I could see and work easier, hooked the shop vac up to the exhaust pipe, and sprayed everything with soapy water. Found a leak at the base of one of the O2 sensors. I tightened it up and it all but stopped leaking. Also found each cat had a leak at the weld, right below the flange going into the manifold. Seems to be pinhole leaks, as I can't see any cracking. Took a video of it, but apparently it's too large to post. Could these leaks be my P0301 code problem? Also, any suggestions on fixing it? Not sure if it's best to replace it all, or try and have it welded. Everything is in good shape. This rig has been in the desert southwest it's whole life, so no rust anywhere to be found.
So, I spent some more time trying to find a reason for the PO301 code with no luck. Decided to take it to a recommended shop to see if they could find something I was missing. They started with the basics, like I did, and eventually got around to running a compression and leak down test. Their tests resulted in better numbers than my tests. Said they got 150 on each cylinder for the compression and only had 8% leakage on Cyl 1 for the leak down, where I had 18% leakage. They didn't go any farther than that before recommending an engine replacement due to the mileage. I paid them for their time, which wasn't much, and brought it back home today. At this point, I think I'm going to pull the head and and try and see if I can't get a better look at what's going on. Would love to just pull this motor out and put a stroker in, but a recent plumbing failure is forcing me into some unexpected renovations to the house. Had a little fund set aside for the stroker, but that money needs to go toward the house now.
I've used a 10% leak down as my rule of thumb. But alot of my work is on older engines pushing 6.5 to 1 compression or a bit higher. I'm thinking back to my old textbooks and seem to remember something about compression tests and leak down changes a bit with different compression ratios. Been a long time, and I'm pre-coffe. 
That's kind of what I'm trying to wrap my head around. I've heard of acceptable leak down numbers from 7-25%. Seems like a pretty big gap. The mechanic did mention how unfortunate it is, as the misfire is barely noticeable. Just enough to trip the code and keeping my from getting it smogged.I've used a 10% leak down as my rule of thumb. But alot of my work is on older engines pushing 6.5 to 1 compression or a bit higher. I'm thinking back to my old textbooks and seem to remember something about compression tests and leak down changes a bit with different compression ratios. Been a long time, and I'm pre-coffe.
The biggest thing for measuring piston health has always seemed to be consistency. As long as all cylinders are showing the same basic numbers it is considered to be ok. I have an old tractor with compression so poor that I can turn the engine over by turning the fan blade. And the fan isn't even part of the pump. And the belt is faarrr from tight. Yet it will fire right up and pull as good as when I got it back in the 80's when it was already over 30 years old! 
How does it start? Does it fire right up, or do you have to crank it a couple times? I had a mystery misfire, same as you. Same cylinder too. I could clear it, and it would come back, sometimes immediately, sometimes after a month. I tried all the normal things to solve it. I also had a bad backflow preventer in my fuel pump, which would allow the pressure to drop instantly, once the pump shut off. I replace the fuel pump and I haven't had a misfire code since.
I would have never tied the two together, because it ran so well. But...I can't argue with the result.
I would have never tied the two together, because it ran so well. But...I can't argue with the result.