Low compression in a few cylinders
#1
03-09-2019, 07:23 PM
#2
03-09-2019, 07:40 PM
#4
03-09-2019, 08:28 PM
I put the truck on my "automatic charger" 10a 12v and let it sit for a few hours before the test. All plugs out but I didn't do anything with the carb or air cleaner, I just left it. So I guess choke closed. Engine was cold as I did this all while I've got my manifolds off for a gasket change.
Although the battery was charged it still doesn't act like it wants to turn over very well.
Although the battery was charged it still doesn't act like it wants to turn over very well.
#5
03-09-2019, 09:23 PM
#6
03-09-2019, 09:41 PM
I'm with Tedster and Chris. Those numbers don't look good, but I've always been taught not to condemn an engine based solely on a cranking compression test. If the other way around and the numbers had looked good on a cranking compression test, then yep, that's fine to say the engine is okay.
But when a cranking compression test comes in low? There are just so many variables. Battery health and state of charge? Tired starter? Marginal battery cables? Those are just a few examples. All your results mean is you should run a leakdown test for confirmation. More specific, and less chance for error.
In addition to what Ted mentioned, not only should the battery be fully charged, but I even keep it on a charger as I move the tester between cylinders. You don't need misleading readings as the battery slowly runs down as each cylinder is tested.
Another thing I like to do before starting the test is to disconnect and cap the fuel line at the carb inlet. Then I run the engine until the carb runs dry. My intent is to prevent any fuel from being drawn into the cylinders and washing oil from the walls. Probably a moot point, as there is virtually no suction with all the plugs removed, but I like to give a tired engine the best odds for an accurate test.
But when a cranking compression test comes in low? There are just so many variables. Battery health and state of charge? Tired starter? Marginal battery cables? Those are just a few examples. All your results mean is you should run a leakdown test for confirmation. More specific, and less chance for error.
In addition to what Ted mentioned, not only should the battery be fully charged, but I even keep it on a charger as I move the tester between cylinders. You don't need misleading readings as the battery slowly runs down as each cylinder is tested.
Another thing I like to do before starting the test is to disconnect and cap the fuel line at the carb inlet. Then I run the engine until the carb runs dry. My intent is to prevent any fuel from being drawn into the cylinders and washing oil from the walls. Probably a moot point, as there is virtually no suction with all the plugs removed, but I like to give a tired engine the best odds for an accurate test.
#7
03-10-2019, 10:35 AM
The compression # itself is important but whether they are even #s across every cylinder maybe more so.
Another thing to watch when doing the test itself is how many "puffs" it takes to reach the highest pressure, and how fast, ideally it will reach the maximum value very quickly, maybe after the 2nd something like that. Can kind of get a feel that way after doing a few different engines, when they are "lazy" and slow to get to the highest # that isn't good either.
Another thing to watch when doing the test itself is how many "puffs" it takes to reach the highest pressure, and how fast, ideally it will reach the maximum value very quickly, maybe after the 2nd something like that. Can kind of get a feel that way after doing a few different engines, when they are "lazy" and slow to get to the highest # that isn't good either.
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#8
03-10-2019, 10:41 AM
Please see my previous comments about the suspect reliability of cranking compression test results.
Strike 1: The closed choke limits the airflow into the cylinders. With less air to compress, pressure can't build up as normal. Definitely secure the choke plate AND the throttle butterfly(s) fully open.
Strike 2: The closed choke MIGHT be letting raw fuel reach the cylinder walls, washing off oil and reducing compression. I say MIGHT as there probably isn't a lot of suction with 7 of the 8 cylinders open, but who knows. It's likely overkill, but as previously mentioned, I like to block the fuel supply and run the carb dry before a cranking compression test.
Strike 3: An engine makes less compression when cold. The test needs to be done with the engine warm.
It sounds like something is seriously wrong with your starter system. With 7 of the 8 cylinders open to the atmosphere, it doesn't take much effort to furiously spin the crankshaft. Get that fixed before trying again. Details here:
https://www.ford-trucks.com/forums/1...w-starter.html
At this point, I would suggest purchasing a small bottle of lighter fluid, a small metal box, and some 5 minute epoxy. These supplieswould be very handy in a situation like this.I
Take the paper with your previous test results and soak it with in lighter fluid. Set the wet paper on a surface you're not worried about damaging, such as a concrete patio or the hood of a Prius. From a safe distance, toss a match (lit, in case I have to be specific) at the paper. Once the ashes cool, sweep them into the metal box. Seal the lid with the epoxy. Find an abandoned well or vertical mine shaft, preferably at least several hundred feet deep. Making sure your footing is secure, toss the sealed box into the abyss. Return home, get a fresh sheet of paper, and try the test again, only this time following the advice from this thread. I think you will be much happier with the results.
#9
03-10-2019, 07:59 PM
Did a leak down test today. I calibrated the tester before each cylinder, and each cylinder was at TDC. I would get the %, then hook up shop air to find where it was leaking.
1 - 50% leak down, air out of pcv hole in valve cover
2 - 40% leak down, air out of pcv and radiator
3 - 40% leak down, difficult to find where it's leaking
4 - 50% leak down, air out exhaust
5 - 43% leak down, air out pcv
6 - 38% leak down, difficult to find where it's leaking
7 - 45% leak down, air out pcv
8 - 40% leak down, air out exhaust
1 - 50% leak down, air out of pcv hole in valve cover
2 - 40% leak down, air out of pcv and radiator
3 - 40% leak down, difficult to find where it's leaking
4 - 50% leak down, air out exhaust
5 - 43% leak down, air out pcv
6 - 38% leak down, difficult to find where it's leaking
7 - 45% leak down, air out pcv
8 - 40% leak down, air out exhaust
#10
03-10-2019, 08:15 PM
#12
03-10-2019, 08:23 PM
#14
03-11-2019, 10:57 AM
You can limp along with it if you take care of it. I got my 1980 f150 for $300 years ago because I had moved and needed 4x4. The engine smoked like crazy and used a quart of oil to go to work which was only 30 miles. So I never drove it unless the weather was bad and I needed 4x4. And I always took oil with me. While that was going on I got a free 302 engine out of a Lincoln Mark IV. It had been run out of oil and needed a crank shaft, but the cylinder bores looked good. Got a crankshaft kit from the store, got the heads rebuilt, found out I had some cracked pistons so new pistons, honed the standard bores, put it all together, and had a nice little engine that lasted 8 years till the crankshaft broke. I blame that on the crankshaft kit I got.
Then I found a complete 1978 Mercury Cougar that ran with a 351w for $200. Bought the whole car, pull the engine out. Had to find a oil pan for it. Dropped that in place. A few other things would not fit because the 351w was taller and wider, but got that worked out and that engine was in it till I sold the truck.
Then I found a complete 1978 Mercury Cougar that ran with a 351w for $200. Bought the whole car, pull the engine out. Had to find a oil pan for it. Dropped that in place. A few other things would not fit because the 351w was taller and wider, but got that worked out and that engine was in it till I sold the truck.
#15
03-11-2019, 11:43 AM
You can limp along with it if you take care of it. I got my 1980 f150 for $300 years ago because I had moved and needed 4x4. The engine smoked like crazy and used a quart of oil to go to work which was only 30 miles. So I never drove it unless the weather was bad and I needed 4x4. And I always took oil with me. While that was going on I got a free 302 engine out of a Lincoln Mark IV. It had been run out of oil and needed a crank shaft, but the cylinder bores looked good. Got a crankshaft kit from the store, got the heads rebuilt, found out I had some cracked pistons so new pistons, honed the standard bores, put it all together, and had a nice little engine that lasted 8 years till the crankshaft broke. I blame that on the crankshaft kit I got.
Then I found a complete 1978 Mercury Cougar that ran with a 351w for $200. Bought the whole car, pull the engine out. Had to find a oil pan for it. Dropped that in place. A few other things would not fit because the 351w was taller and wider, but got that worked out and that engine was in it till I sold the truck.
Then I found a complete 1978 Mercury Cougar that ran with a 351w for $200. Bought the whole car, pull the engine out. Had to find a oil pan for it. Dropped that in place. A few other things would not fit because the 351w was taller and wider, but got that worked out and that engine was in it till I sold the truck.
Here is what I'm thinking.
https://www.summitracing.com/parts/c...31-3/overview/
https://www.summitracing.com/parts/f...-000/overview/