Leakdown test

[freedom_in_4low]

Back in March I did a compression test while troubleshooting a crank-no start. It was unrelated to the issue but I did find that cyl 4 only had 90 psi and cyl 5 had 60. All the others had 120 (note that the test was done at 7300' elevation, so I'm not worried about the 120 at all). My first thought was a head gasket leak between those cylinders.

Today I did a leakdown test (first one I've ever done) on just cylinders 4 and 5 (also 1 for a baseline) and the results were... Confusing.

Cylinder 1 held 84 out of 90 psi and I only detected air to the crankcase.

I tested both 4 and 5 while 1 was at TDC, so 4 should have been on power stroke and 5 on compression but couldn't feel anything definitive in the spark plug hole.

At TDC, cylinder 4 was holding about 80-82 psi out of 90 and I could feel some air leaving the tailpipe. Cylinder 5 was holding 80 out of 90 and I could baaaarely feel some air coming back through the intake (had to block the throttle body almost completely with my palm to force it into a small jet). In both cases I couldn't for sure say I felt anything in the spark plug hole.

So this could be an intake valve issue on 5 AND an exhaust valve on 4...or it really is the leak between cylinders and behaving this way because cyl 5 has an open intake valve when cylinder 4 is at TDC, and when 5 is at TDC, 4 has an open exhaust valve. The air out the exhaust is faint enough, like the intake, that I have to cover it with my palm to reduce it to a jet about the size of a pencil to feel anything. But I still feel like if that much was making it through the cat and muffler I should feel SOMETHING at the spark plug hole.

Also, is a 10psi/11% drop in the leakdown enough to explain 25-50% drop in compression? Or could I also be dealing with a couple of dead lifters or ruined cam lobes? I have not repeated the compression test at operating temp, so the lifters would only have gotten oil pressure from cranking, and they're usually noisy for a couple minutes after startup.

I guess in the end it doesn't really matter, because the head comes off for any of these issues. Just hate not knowing.

 

[freedom_in_4low]

I spent some time reading and youtubing last night and decided that my leakdown test result overall is "good". The difference between cylinders is small and none are leaking above what seems to be reasonable for the engines mileage.

I also found that leaks that correspond with my observed compression loss should be large on the leakdown gauge and obvious to feel... Not the barely discernible whisps I found.

So my conclusion so far is that I probably don't have a head gasket or damaged valve problem, and my poor compression may have been deflated lazy lifters due to a compression test run cold. My next move will be a hot compression test and if it doesn't improve I'll pull the valve cover and put a dial indicator on the valves.

 

[delagem]

Are you using a known good compression gauge? Known good Leakdown Gauge? Do you clearly understand the procedure for a proper compression test? (Wide open throttle, good battery or better yet, jumpers to another running vehicle). Harbor Freight and the like are famous for erroneous readings, different readings back-to-back, etc. The HF Leakdown Gauge is an absolute disgrace and in no way compliant with the FAA's guidance on leakdown testing (yes, the FAA created the "Leakdown Test"!)

60 psi is a terrible compression reading, but could be affected by a collapsed lifter. On the other hand, a collapsed lifter will not affect a leakdown in any way. You must do the leakdown test at TDC Compression of each cylinder to have any meaningful results. You don't care that your ring fits fine in the bore at BDC, it needs to be at spec at TDC.

I suspect the top of your bore is worn way beyond spec in 5, and close to worn out on 4 as well. You'll have to re-test at TDC to be sure.

 

[freedom_in_4low]

Are you using a known good compression gauge? Known good Leakdown Gauge? Do you clearly understand the procedure for a proper compression test? (Wide open throttle, good battery or better yet, jumpers to another running vehicle). Harbor Freight and the like are famous for erroneous readings, different readings back-to-back, etc. The HF Leakdown Gauge is an absolute disgrace and in no way compliant with the FAA's guidance on leakdown testing (yes, the FAA created the "Leakdown Test"!)

60 psi is a terrible compression reading, but could be affected by a collapsed lifter. On the other hand, a collapsed lifter will not affect a leakdown in any way. You must do the leakdown test at TDC Compression of each cylinder to have any meaningful results. You don't care that your ring fits fine in the bore at BDC, it needs to be at spec at TDC.

I suspect the top of your bore is worn way beyond spec in 5, and close to worn out on 4 as well. You'll have to re-test at TDC to be sure.

Followed the factory service manual for both procedures, the only difference is I ran them both cold since the compression test was done when the engine wouldn't start, and my atmospheric pressure is only 11.2psia so even a brand new engine isn't going to give compression numbers that people would be used to seeing at sea level. Plugs out, freshly charged brand new battery, WOT with fuel and ignition disabled. Compression gauge is a good one, leakdown is the $85 HF but I checked the gauges against a known good gauge and they were accurate with zero leakage and I ran it full open to see what the gauge read with total leakage. It just has 2 pressure gauges instead of the second gauge being percentage that you calibrate to zero like I'm seeing on YouTube, but the service manual talks both percentage and psi. My biggest complaint was the threads weren't long enough to reach the threads in the 4.0 spark plug holes so I had to use the adapter from the compression tester.

The numbers I posted were with the cylinders at TDC. Compressor regulated to 100psi, putting 90 to the cylinder and holding 80-84 depending on the cylinder. I only ran the test at mid stroke to try to track the leak with all the valves closed.

As you pointed out, a collapsed lifter would produce the results I'm seeing and with as much lifter noise I hear for the first minute after startup I'm leaning that direction.

 

[Goatman]

—-This will cause an uproar-—


Try adding some Seafoam (I prefer diesel) to the crankcase and run it for a good 5-10 miles before oil change. The detergent action should help both rings and lifters to improved function and then try retesting again. Don't cruise around. Get that engine running hard. Some call it an Italian tune up. It can help clear carbon deposits throughout the engine and remove (the new catch word) 'stickshin' in the rings, valve guides and lifters.

 

[freedom_in_4low]

—-This will cause an uproar-—


Try adding some Seafoam (I prefer diesel) to the crankcase and run it for a good 5-10 miles before oil change. The detergent action should help both rings and lifters to improved function and then try retesting again. Don't cruise around. Get that engine running hard. Some call it an Italian tune up. It can help clear carbon deposits throughout the engine and remove (the new catch word) 'stickshin' in the rings, valve guides and lifters.

hey, I'm up for anything that keeps me from tearing this thing apart (or swapping in a junkyard motor over a weekend that will probably have a collapsed lifter, too). Part of what's so frustrating about this is there's at least a half decent chance that there isn't even anything broken or worn out, just that some jackwad in this motors history didn't change his oil and clogged a passage or something.

I ran marvel mystery oil in it for 1000 miles leading up to an oil change, and it did nothing. I suppose seafoam is the next logical step. I'll go with that over diesel simply because all my fuel containers are currently full of either gasoline or 2 stroke mix so the seafoam is easier.

What volume of oil do you typically replace with seafoam? A full 16oz can?

I live semi-rural so I've got plenty of room to get on it. Hills, too, so I can run it hard without necessarily going that fast, haha. The road I used to break in my black magic pads will be perfect.

 

[freedom_in_4low]

—-This will cause an uproar-—

I'm gonna be a little disappointed if this uproar doesn't materialize.

 

[Goatman]

I add a quart. Not replace with, but on top of. Won't hurt for just a few miles. Try and stay away from fuel additives without confirming it's safe for catalytic converters. If you ran a qt of Marvel's in the oil then suscess is less likely with the Seafoam, but still worth a shot. These are durable engines. If still nothing improves a full compression and leak down test would be the next step to figuring out what has been damaged.