Rookie needs guidance for compression test

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gdizzle

gdizzle

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OK, just got a compression tester from Oreillys loaner program (free) for 2 days.
I watched some YouTube and read a couple posts here.
My main questions,
1: Do I have to remove the valve cover?
2:Do I have to remove the sparkplug tubes?
3: Do I have to disconnect the coil?
4: How tight does the compression hose need to be in the plug hole?

5:Now if I am able to do that part, what kind of compression # should I be looking for?

Now I also see Wet vs Dry. Since this is Slant 6, how is it possible to coat the entire head of the piston with oil? And once the oil is in there, will it just burn out when I reconnect the plugs? Is that safe?

thanks in advance
66 dart 225 slant 270 3.7
 
gdizzle said:
OK, just got a compression tester from Oreillys loaner program (free) for 2 days.
I watched some YouTube and read a couple posts here.
My main questions,

1: Do I have to remove the valve cover?

No.

2: Do I have to remove the sparkplug tubes?

What type of tester do you have, screw in type? I don't think you do.

3: Do I have to disconnect the coil?

Yes


4: How tight does the compression hose need to be in the plug hole?

If it's the one that's pointy rubber, just enough pressure so it doesn't leak out. See question above.

5:Now if I am able to do that part, what kind of compression # should I be looking for?

Not sure on a /6.

Now I also see Wet vs Dry. Since this is Slant 6, how is it possible to coat the entire head of the piston with oil? And once the oil is in there, will it just burn out when I reconnect the plugs? Is that safe?

thanks in advance
66 dart 225 slant 270 3.7
Click to expand...


First and foremost, I would make sure the valves are adjusted up properly before you start.

I would run the engine and let it warm up, let it cool to the point you can work on it.

Remove all of the plugs, pull all of the wires (keep track of which one goes where), unplug the coil wire, use a screwdriver to hold the carb open.

Start with one cylinder and just go from front to back in order WRITE down your numbers. You have just done a dry test.

To do the wet test, a squirt of oil in the cylinder and repeat the process, WRITE down the numbers.

You will want to let the engine turn over about three times for each test on each cylinder, WRITE DOWN the highest reading.

Put a battery charger on the battery so you have even cranking throughout the process.
 
What you are looking for is uniformity between all cylinders. That is, the same number for each cylinder +/- 10%. This percentage figure varies by mechanic, maintenance book, and engine type. So if all six were in a range of 125 - 135 psi generally that would be a good engine but so would a test range of 90 - 100 psi. The smaller the variation between the best and worst cylinder, the better your engine condition is generally speaking. You don't need much oil in a cylinder for a wet test. You expect a wet test on an engine to show higher numbers that a dry test (again not by much). This is caused by the oil helping to seal the upper compression ring. Whatever change you see should be close across all six cylinders. If during a wet test you find a cylinder value that doesn't change you typically have a valve problem in that cylinder but there are others causes as well.
 
Low variation is good but there is a minimum that you want to see for /6 cylinder compression numbers. The desirable number is in the 110 or better range. 100 psi is getting a bit low, and 90 or less is showing some significant wear somewhwere.

Just squirt 3-4 squirts of medium weight motor oil into the spark plug holes for the wet test, and then crank the engine over a few times. The oil will spread out around the top rings, and yes, it will burn out when you start the car; no danger or problems.

Remove all the plugs before any testing, so the engine will spin easily; that wil be easy on the battery. And, as you test each cylinder, crank each cylinder the same number of times; you can hear it when the cylinder under test compresses. I usually crank 5 times per cylinder.
 
Don't pour too much oil or you could "hydro-lock" the cylinder, which could bend a valve or break a tooth on the starter wheel. Also, it would be cheating the test since displacing air volume. As people say, just enough oil to coat the rings. Sure the cylinders slant over, but so do a V-6 or V-8. Even my 4 cyl Dodge Aries engine slanted back a bit. Indeed, it was derived from the slant 6.

I agree that >100 psig is a minimum. I had a 70 psig cylinder in my 1969 slant once. The spark plug was always pristine since flushed w/ raw gas that wasn't burning. When I pulled the head, that cylinder was shiny clean metal inside and the walls showed deep scuffs from the rings.

The reason there is no exact value is that the camshaft and valve timing make a big difference. Some high rpm cams may show low cranking compression pressure because they only work well at high rpm and/or have much valve overlap for that lopey "race-car" idle that many desire, though runs like crap at low speeds w/ noxious fumes. Other cams may show >200 psig cranking pressure, which is bad because the starter must fight it. For a factory slant cam, 145 psig would probably be the "as new" pressure. It is for my 65 273 engine and I vaguely recall for a slant too.

I suggest you buy the Harbor Freight compression kit next time on sale (I got for ~$15.). Besides doing compression check, they have air fittings so you can easily connect to an air compressor. That lets you apply pressure at TDC and see how well the cylinder seals. If you hear a hissing leak, search for it. Past the rings blows out the PCV hole on the valve cover. Past the exhaust valves blows out the tail-pipe. Past the intake valves blows out the carburetor.

The poor-man's compression check is free and very instructive. Turn the engine over by hand, which you can do by tugging on the slant's fan belt (clock-wise as viewed from front). You should clearly feel 3 cylinders per rev. If you hear each one hiss down for >10 sec, the engine has excellent sealing. If you have ever kicked over a motorcycle, you will understand. I would also say tugged a lawn-mower, but many today have a "compression release" easy-start when tugging, for today's girly-man.
 
Ok help me out with these results.
I am not real sure how accurate this is, as I got towards the last several Wet tests, I was having to crank way more times than the earlier tests.

1st things: my spark plugs are basically 6 weeks old. and I havent really driven more than about 20 miles. and maybe 3 hours worth of idling in the driveway.
1-5 all seem to have a whitish grayish coating to them? 6 was basically dark brown, sorta oily. 6 seems to have oil on it?
I will post a picture of the plugs.

2nd thing:
Compression test,First I did all cylinders dry, I let each one crank for about 5 cranks.
1 = 125
2 = 130
3 = 138
4 = 132
5 = 102 (checked twice)
6 - 120


Then I went back to the beginning and squirted couple drops of oil in. I would squirt, test it, then go onto the next, squirt, test etc...

Wet test:
1 - 113
2 - 120
3 - 132
4 - 115 (had to let this crank way more than previous, longer to get needle to stop)
5 - 117 (had to crank even more than #4 to get it to stop)
6 - 132 (had to crank like #5)

I am not sure what is going on. If I didnt do this right or what?
I am thinking maybe I need to redo this test tomorrow?

I do not have a battery charger, but the battery is only a few months old.
ideas?
 

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#5 is a little weak but not terrible. It will still fire plenty good.
 
So it mainly will tell me that it could need a valve job, or a piston has a hole, or it needs new piston rings?
 
Sounds like the later wet tests 4-6 might not have had the tester well sealed in the plug threads. Sometimes you have to twist it in pretty firmly to get a good seal. A low test can reveal the things you say, as well as a head gasket leak. A hole in a piston is pretty dramatic in regular operation, BTW.

What symptoms are you having that made you do this test? And yes, #5 is on the weak side; but you could run it that way for quite a while if it held at that level.
 
We are assuming you have a screw-in compression gage. The rubber tipped "hold it tight" ones can give erratic results. I would remove all spark plug tubes and try again, in case it is leaking past that gap. Also, replace all the rubber tube seals since cheap and you don't want a "drool tube". If your measurements above are valid, #5 is a bit weak, probably leaking past the rings. You would likely find the cylinder walls scored a bit if you pulled the head, but no need for that yet.

Once it gets below ~70 psig, you might start finding the spark plug totally clean (washed w/ unburned gas). If a cheap bastard like me, you could then improve mpg by removing the lifters and push-rods on that cylinder (not possible for later "peanut" heads) so it doesn't flow fuel uselessly. After bragging about my "5 cyl slant" 1969 Dart to a co-worker he said that without the lifters it would lose oil pressure (he was a Chevy/Ford rodder), so I re-installed, adjusted the push-rods to not open and added weak springs to keep from rattling. I now know that is not needed for a slant since the lifter bores have no oil feed. I drove that way fine for a year then installed a rebuilt long block and rebuilt tranny. 4 months later the car was stolen and never recovered (license plate found on a stolen Jetta 7 yrs later). I hope your best efforts aren't similarly rewarded.
 
You properly did a dry test first. Typically if a low reading(s) is found in one or more cylinders then you squirt oil. The purpose of putting oil in the cylinders is to help seal the rings to the cylinder walls to determine if you have a ring problem or possibly something else. (like a burned valve or blown head gasket) Notice that the compression in #5 increased after squirting in the oil? That tells me that the piston rings in that hole are not sealing as well as the others. The oil temporarily helped seal the rings to the cylinder walls. You got quite a bit of variation between cylinders so your engine is not in the best of shape but it will run.
 
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