Compression varies between cylinders on new engine?

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OregonRedRam

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I am re-building a 5.9 magnum gas. Have built engines before and never ran across this. The engine won't start. A compression check of all cylinders varies between 75 and 170. Engine was bored .030 by a NAPA shop. New flat top .030 pistons & rings installed. The decks are level. Cylinder heads are new and flat, not warped. I installed a new cam and lifters. Cam was degree'd and installed straight up. I pulled the timing cover off to check that the timing marks do line up. What am I missing? Is it possible the new rings are not sealing? Can I test this by putting oil in the plug holes as if its a old engine?
Is it possible the new cam is just totally off (comp cam) ? The new heads use the stock pedestal rocker arms so they can't be mis-adjusted, right? If the cam was out of phase, wouldn't the compression still be the same in all cylinders, excpet maybe high or low? WHAT AM i MISSING?
 

SYKRAMMAN

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Wow, that’s a huge jump in compression. Maybe bad valve install on new heads, not seating all the way. Yea you can use a table spoon of oil per cylinder to check rings, that should give you an idea of what the deal is. Also, do a leak down test, that’ll tell ya if it’s valves or whatnot. Good luck bud.
 

Yeret

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Dropping some oil into the cylinders could be very revealing. If a little oil causes the pressure to spike, you're definitely looking at something wrong with the rings or, heaven forbid, the cylinder roundness, which shouldn't be an issue if the block was machined but hey, stuff does happen...

Otherwise, a leak-down test seems well in order. Where did you get your heads? Could well be a bad valve job. Honestly, I've never heard of someone machining a block, rebuilding the engine and then having bad compression, unless the rings weren't seated right (which obviously doesn't seem to be your case).
 

MADDOG

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I wonder if the builder staggered the ring gaps properly.
 

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As a sanity check I think I would take the valve covers off and roll it through the firing order ensuring the valves are closed at each cylinders TDC compression stroke.
 

NCRaineman

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Wow, that’s a huge jump in compression. Maybe bad valve install on new heads, not seating all the way.

Valve not seating completely would definitely do it.
 

Hero6

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A compression check of all cylinders varies between 75 and 170.

It should still start and run with those numbers if the COMPRESSION was the only problem. Just did a compression check on mine yesterday, 2 cylinders down to 70 and runs pretty good still (going to rebuilt it). I would look more toward the valves, cam, timing.
What cylinders have what compression?
Let us know what you find.
 
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OregonRedRam

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I did drop some oil down 1 cylinder and the compression immediately spiked to 200 (maybe a little too much oil!).
I am thinking the rings are not sealing. I am the builder and I did rotate the ring gaps. I question now weather I have some rings on upside down or ring #2 on top, etc? I did not have any trouble installing any piston so I'm sure none are broken. I did blow it on one item...I didn't measure the cylinders after the block came back. I just trusted the shop. The pistons seamed to fit properly but who can really eyeball the proper size.
Let me throw out this question. The motor never ran so the rings would not have had a chance to set. I have never tried checking compression on a brand new fresh engine. Maybe this is normal until its run for 30 minutes?
As to the heads, I have pulled the valve covers and the valves are closed. I can't say for sure that they are sealing. Guess I need to rent a leak down tester. I also double checked the head bolt torque and they are all good.
 

MADDOG

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Yeah, I'd say a leak down test would be next. It would help nail down where the air is escaping from on those low pressure cylinders.

Not having the rings sealing before you run the engine is an interesting question and one that has merit. You do have to run the engine some for them to seal properly but I can't say that the difference between the cylinders makes any sense, either. The gap seems too wide.
 

crazzywolfie

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a leakdown test would definitely tell you if there was an issue with valve or rings but i think you almost need to get it to fire before really trusting the compression readings since aren't you generally suppose to do a compression test on a warm engine?

did you remove the crank position sensor before pulling the engine? have you also tried doing the key dance to see if the computer is complaining about anything?
 

ouch1011

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Since the engine has never run, I wouldn't fully trust the compression numbers. As someone else said, the engine should still run with those compression numbers, even if it did run rough. Have you checked to make sure you're getting spark at the plugs and good fuel/pressure? If the truck has been sitting for a while, the fuel could be bad as well.
 

Gr8bawana

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So you put rings on the pistons without ever checking the ring end gap?
A friend of mine did just that, said he could eyeball it. He ended up having to tear it back apart when it smoked like crazy.
 

Khoover1988

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Hate to say it..... find a loner vehicle and re tear down the engine and do ring end gap clearance. If too close, when engine warms up they can kind of buckle seaze or break from comming together and touching, or if too far away, excessive blow by and low power/compression. If all you tests come back ok or iffy. Better to be safe that sorry and waste all that hard earned money. Good luck buddy keep us updated!
 
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OregonRedRam

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Update: 1st chance I've had to touch the truck. Did a leak down test and virtually no leaking from any cylinder. Yea!!! Bad news...I'm right back where I started. Either the compresion test is bogus or the cam has issues. Here are the original compression numbers:
cyl 1 - 180 Cyl 2 - 100
cyl 3 - 180 Cyl 4 - 110
cyl 5 - 110 cyl 6 - 95
cyl 7 - 115 cyl 8 - 110

could a out of phase cam prouduce wacked numbers? If so I would think they would all be off the same amount.


I will put the degree wheel back on tonight and measure timing events for several cylinders.

If the compression readings are just flat out wrong I'm going to be pissed at myself.
 

crazzywolfie

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i would see about getting the engine running first before trusting a compression tester. do you have fuel pressure and spark while cranking the engine over?
 
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