| Dolman | -- 09-18-2011 @ 8:15 AM |
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How sick is my engine? (34) Compressions are 58 to 76 at 72 degrees with throttle wide open and cranking 4 revolutions. Maybe I should run it until it's hot and check it again? It needs to be confessed that my pressure gauge is 40 years old, but I put new O-rings on it.
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| supereal | -- 09-18-2011 @ 11:24 AM |
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That is fairly low, but not unusual for engines that have lots of miles on them. It is always best to do a "dry/wet" test. With all the plugs out, and the throttle propped open, run the starter long enough for the gauge to peak, and note the reading. Then, squirt some motor oil into the cyinder just tested, and repeat the test. Compare the results with the dry test. If there is a marked increase in compression, the piston rings are leaking. If the compression stays about the same, the valves are leaking. In either case, if the compression is far below 100, or varies widely between cylinders, the time to have work done is approaching. At our shop, we use the "blow down" method. This is where we inject compressed air into the cylinder with a device and gauge that screws into the plug hole, then we put the piston at top dead center and apply air. The time it takes for the pressure to drop, after the air is shut off, gives an accurate picture of condition that is generally more reliable than cranking the engine. From the readings you mention, I suspect you are close to a rebuild. Compression testers, old and new, commonly work well enough to do the job.
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| ford38v8 | -- 09-18-2011 @ 11:31 AM |
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Bob, I'm sure it was just an oversight, but I think you would also advise to warm up the engine before the compression test. Alan
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| Stroker | -- 09-18-2011 @ 12:38 PM |
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I would also suggest that if you do opt for the "blow-down" method, that you secure the crank from rotating using a helper and a socket and breaker-bar on the crank pully nut to ensure that the piston on the cylinder being tested doesn't move from top-dead-center on the compression stroke when the air is applied. The "blow-down" method is particularly good at isolating the cause of low compression. When the air is applied, you may hear it escaping. If the sound comes from the engine oil filler/breather: it indicates ring blow-by. If it comes from the carburetor intake it is a leaking intake valve. Noise coming from the tailpipe (you'l need a 3rd "helper") would indicate a leaking exhaust valve.
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| Dolman | -- 09-18-2011 @ 1:09 PM |
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It's interesting that it starts easily, and except for the intermitent misfire, runs like a scared rabbit and doesn't smoke. I'll test again after I warm it up and see what I get. However I am thinking, I will have to open it up and replace some stuff. It's spooky that 20 or so years ago while going through my junk, I found a ring compressor I used in replacing rings and valves in a marine Ford V8 flat head 40 years ago. Many times I started to throw it away but then I thought I might need it some day. That's why I have so much junk. What do you want bet that if I dig in all of the boxes, I'll find a spring compressor, the stick with the rubber cup and a cylinder hone. Wish I would have had the same thought when I sold the cherry picker and engine stand.
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| Dolman | -- 09-18-2011 @ 2:08 PM |
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Tested only #1 while hot and got a reading of 58. Squirted oil in and got a reading of 75. No point in testing the others. Looks like I need to find a reliable flathead mechanic for a re-build. Not inclined to do something difficult myself that I can pay someone else to do. Good for the economy.
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| Stroker | -- 09-18-2011 @ 2:59 PM |
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Dolman: I'm not sure what is causing your intermittent miss, but if it were me, I'd try to sort that out before you jerk the motor out. Your little truck motor isn't new, but it doesn't have a lot of weight to pull, unless you are using it to haul heavy loads. Of course I know your winters are long and dark, so perhaps that factors-in. Remember this: every existing 221 inch vintage block has only so many re-builds left in its' iron innards. If you are happy with the performance and if you could solve the "miss", why not just enjoy your truck. The truck may be simply experiencing "culture-shock" having been teleported from Florida to Alaska.
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| Dolman | -- 09-18-2011 @ 5:32 PM |
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I took a nap and woke up with the same thought that you have. As usual your counsel carries a lot of credibility. The only load the old pickup hauls is itself and me and I am far from being a heavyweight. As I previously said, it starts on 1 revolution, (thanks to Skip Haney), runs well, doesn't smoke, and the oil didn't turn black after 100 miles on an oil change So I'm going to focus on the misfire. If I can't fix that I'll take a retroactive F in engineering. As far as culture shock goes, even though we are "Blue Eyed Arabs" we use the US Dollar for currency and speak a passable version of English - some of us as our first language! Old Henry, as I call him, survived -40 on the trip back from Florida but complained silently went I went to back him out of the trailer. Put my airplane pre-heater in there an hour or so and he sprang to life. Speaking of long and dark, do you know what the speed of dark is?
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| shogun1940 | -- 09-18-2011 @ 5:47 PM |
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iI would take the advice of super real and do a complete leak down test of all the cylinders, its nice to know what is wrong before a rebuild. The speed of dark is 186,000miles per second ,same as light unless your in the twilite zone.
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| Dolman | -- 09-18-2011 @ 6:42 PM |
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Here I always thought the speed of dark was the reciprocal of the speed of light, ie, a dark hole where I am convinced my money goes. Thanks for the illumination. Now to serious stuff. I have decided that I will drive it as-is and postpone a rebuild until it starts thrashing and hope it isn't raining when that happens. But out of curiosity, I'm going to complete the test on all cylinders so that I will have 8 things to worry about. Kind of like a child with a monster under his bed.
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| supereal | -- 09-18-2011 @ 8:50 PM |
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It will make some difference if the engine is hot, but it does make the job a bit more difficult, and chances are the engine will be cool before you are done, affecting the readings somewhat. If you are doing the blowdown test, the reason for putting the cylinder at TDC is to be sure both valves are closed. Just be sure you are on the compression stroke. While you can block the crank from turning with a wrench, we just set the parking brake and put the vehicle in gear. I assumed most non pros wouldn't use the blowdown test, so I didn't go into lots of detail.
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