Low compression on all cylinders

fourwheelinj1

Well-Known Member
Joined
Apr 11, 2011
Location
Raleigh
I finally got my project scrambler to crank over last night so I can do a compression test and I got confusing results. Supposedly the PO parked it 12 years ago for a bad clutch (clutch and trans were removed) so it has been sitting for 12 years. I put the bellhousing and starter on and crank it over and got 63-70 psi on all 6 cylinders. I then added a little oil to the cylinders and the readings went up a couple of pounds but nothing much. If the rings or cylinder walls are the problem then wouldn't the wet compression test have shown a much better improvement then just a couple pounds? I am wondering if I have a timing/valve problem. Any ideas to help me figure this out without spending a bunch of money on a motor that might turn out to be crap?
 
~If oil seals it up it is bad rings and/or scored clyd. wall issues.
~If oil dosent seal it up, it is a valve sealing issue or a head gasket, but most likely it is the valves not sealing.
~A leak down test will tell you right off. Pump 80-100 psi into the plug hole with the valves closed and listen for where it is going.
~Test the springs first and eliminate them from the catagory.
~The valves could be all carboned up and stuck open.
~If they are just stuck in the guides, you can use PB Blaster down the stem and take a loaded hammer and tap them. It may or may not free them.
~ If it were timing you would still have most, if not all valves closed at TDC (top dead center) and still have better compression than what you are showing.
~Timing is easy to check, just to make sure.
~Valve springs can be shot from age, previous wear and sitting in the compressed condition for years.
~ Valve spring testers are readily available to you that can be used while still on the engine, you just have to pull the rocker arms off to gain access.
~You can also change springs on the engine. If needed I can tell you how.
 
could also be a gauge issue?
 
Throw some gas in it & fire it up. I'll bet after it runs a bit it seals it self up. Fyi, it's gonna smoke like a freight train for a bit.
 
Thanks for all the replies. Guage is brand new and so is starter so I think I can rule them out. NCTom thanks for all the info. I don't have a leakdown tester so I put a cylinder at tdc and then hooked up 90 psi from my compressor directly and the motor rotates counter clockwise a little then starts leaking out the intake. I guess I need to hold the crank still somehow so the piston doesn't move so I can get a more accurate result. So I guess I need to remove the valve cover and check and see if the valves are moving properly first and the either try to free them if stuck or remove rockers and test springs? Is that what you are suggesting? Sorry I am a little ingnorant on this I have never worked on a head before. It's time to learn I guess.
 
No problem. Plenty of good folks on here that will lend you a hand or bust your chops when you need it!
Buy a Haynes manual for the thing. It is your best friend and it gives you a trouble shooting guide, assembly, tourqe specs, etc... You may even get it online, but I like a book on the bench myself.
Wedge a pull handle or ratchet against the frame to keep the crank from turning when you hit it with air. Dont use your hand. Good way to bleed a lot and break small bones.
Air coming out the intake when it is at TDC on that clyd. obviously means valve seal isnt happening. Try the tricks to free them up, if that dont cut it, you are headed for a valve job. When you pull the head you can also check the bores for wear, rust, etc... At that point you havent spent any money and you know where you are with the rest of the engine.
Be sure and drop the old oil and prime it with fresh oil if you are going to try and crank it.
Good luck with it.
 
Thanks again for the input. I have a haynes and chilton manual so I will look over them tonight. I am concerned that it might be valves and worn cylinder/rings. If I end up pulling the head is there a way to get an idea of the condition of the rings/cylinder other than just visual? I haven't really seen to many cylinders to know what is normal. I would hate to have done a valve job and reinstall the head to find out I have 30 or 40 less psi on a cylinder due to worn rings/cylinder.
 
Yep, visual is about it with limited tools. General rule is, if you can feel a scratch in the clyd. wall with your fingernail it is too deep. The ridge at the top of the clyd. is going to be about half carbon and half steel clyd. wall. You can sand on it with some coarse paper to get it down to pure metal. Then you will know how worn the walls are.
 
spin it over enough that you get good oil pressure on a wet gauge, don't sweat the compression figures just yet.

If it has been sitting that long, the lifters are probably partially collapsed due to not having oil pressure in so long. the valves won't open near as much and you won't be getting as much air volume, especially if your only cranking on battery.

if it makes oil pressure on a gauge, start it and run it til its operating temp. let it run for a bit, let it cool, THEN try a compression test. if it aleast runs, you know you have something to work with, then you can go from there.

BTW, are you pulling ALL plugs out when you are trying to get the reading ? you're fighting the compression on the other cyl's if they still have plugs in them.
 
All the plugs were out during the test. I will perform a leak down test on it tomorrow and hopefully that will give me an idea of the engine condition. If it is lack of oil pressure in the lifter not allowing enough air in then that shouldn't cause a problem with the leak down test right? Again thanks for all the info I am already learning a lot.
 
Blkvoodoo has a good point. I'd prime the oil pump and try the test again.

As un-scientific as it sounds, Jeff B is right. I put some ATF in the cylinders of an old Willys that hadn't run in over 15 years and let it sit. A couple days later I changed the oil and fired it up. It smoked like crazy for a few hours but cleared up and the compression was back to an acceptable level. Worked on a '70 Chevy 454 too.
 
Well I did a leak down test yesterday and got some mediocre results. I got between 55-65% leakdown on all cylinders which is considered moderate on the gauge. I heard some air leaking in the crank case but it was not very noticeable. It seemed that even at TDC it leaked out of the intake a little and also the gauge leaked some too (harbor frieght tool). So since it was only moderate and I think if the gauge and intake weren't leaking it would have been a lot better. I think I will replace the carb with a rebuilt motorcraft 2100 that i have in my shop ans try to fire it up and see what happens. Again thanks for all the replies, I want to do a 4.0 swap I just hope it can wait a year or so.
 
Back
Top